| 1. |
|
|
| 2. |
- Dmitriev, Vladimir, et al.
(författare)
-
Pressure-temperature phase diagram of LiBH4: Synchrotron x-ray diffraction experiments and theoretical analysis
- 2008
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 77:17
-
Tidskriftsartikel (refereegranskat)abstract
- An in situ combined high-temperature high-pressure synchrotron radiation diffraction study has been carried out on LiBH4. The phase diagram of LiBH4 is mapped to 10 GPa and 500 K, and four phases are identified. The corresponding structural distortions are analyzed in terms of symmetry-breaking atomic position shifts and anion ordering. Group-theoretical and crystal-chemical considerations reveal a nontrivial layered structure of LiBH4. The layers and their deformations define the structural stability of the observed phases.
|
|
| 3. |
- Aavikko, R., et al.
(författare)
-
Clustering of vacancy defects in high-purity semi-insulating SiC
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:8, s. 085208-
-
Tidskriftsartikel (refereegranskat)abstract
- Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of four to five missing atoms and (ii) Si-vacancy-related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity in both as-grown and annealed materials. Our results suggest that Si-vacancy-related complexes electrically compensate the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity. © 2007 The American Physical Society.
|
|
| 4. |
- Abergel, David, et al.
(författare)
-
Infrared absorption of closely aligned heterostructures of monolayer and bilayer graphene with hexagonal boron nitride
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : AMER PHYSICAL SOC. - 1098-0121 .- 1550-235X. ; 92:11
-
Tidskriftsartikel (refereegranskat)abstract
- We model optical absorption of monolayer and bilayer graphene on hexagonal boron nitride for the case of closely aligned crystal lattices. We show that perturbations with different spatial symmetry can lead to similar absorption spectra. We suggest that a study of the absorption spectra as a function of the doping for an almost completely full first miniband is necessary to extract meaningful information about the moire characteristics from optical absorption measurements and to distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of electric-field-induced interlayer asymmetry might provide additional information about the moire parameters.
|
|
| 5. |
- Aboelfotoh, M. O., et al.
(författare)
-
Schottky-barrier behavior of metals on n- and p-type 6H-SiC
- 2003
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:7, s. 075312-
-
Tidskriftsartikel (refereegranskat)abstract
- The Schottky-barrier height of a number of metals (Ti, Ni, Cu, and Au) on n- and p-type Si-terminated 6H-SiC has been measured in the temperature range 150-500 K. It is found that the barrier height to n-type 6H-SiC does not exhibit a temperature dependence, while for p-type 6H-SiC the change in the barrier height with temperature follows very closely the change in the indirect energy gap in 6H-SiC. These results are inconsistent with models of Schottky-barrier formation based on the concept of a charge neutrality level. Furthermore, the present results cannot be reconciled with a defect pinning mechanism, contrary to the conclusions of earlier studies on III-V compound semiconductors. We suggest that chemical bonding at the metal-semiconductor interface plays an important role in determining the Schottky-barrier height.
|
|
| 6. |
- Abou-Hamad, E., et al.
(författare)
-
Structural properties of carbon nanotubes derived from (13)C NMR
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - College Park, Md. : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:16, s. 165417-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a detailed experimental and theoretical study on how structural properties of carbon nanotubes can be derived from 13C NMR investigations. Magic angle spinning solid state NMR experiments have been performed on single-and multiwalled carbon nanotubes with diameters in the range from 0.7 to 100 nm and with number of walls from 1 to 90. We provide models on how diameter and the number of nanotube walls influence NMR linewidth and line position. Both models are supported by theoretical calculations. Increasing the diameter D, from the smallest investigated nanotube, which in our study corresponds to the inner nanotube of a double-walled tube to the largest studied diameter, corresponding to large multiwalled nanotubes, leads to a 23.5 ppm diamagnetic shift of the isotropic NMR line position d. We show that the isotropic line follows the relation d = 18.3/D + 102.5 ppm, where D is the diameter of the tube and NMR line position d is relative to tetramethylsilane. The relation asymptotically tends to approach the line position expected in graphene. A characteristic broadening of the line shape is observed with the increasing number of walls. This feature can be rationalized by an isotropic shift distribution originating from different diamagnetic shielding of the encapsulated nanotubes together with a heterogeneity of the samples. Based on our results, NMR is shown to be a nondestructive spectroscopic method that can be used as a complementary method to, for example, transmission electron microscopy to obtain structural information for carbon nanotubes, especially bulk samples.
|
|
| 7. |
- Abrikosov, Igor, et al.
(författare)
-
Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 76:1, s. 014434-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.
|
|
| 8. |
- Adamovic, Dragan, 1973-, et al.
(författare)
-
Kinetic pathways leading to layer-by-layer growth from hyperthermal atoms : A Multibillion time step molecular dynamics study
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 76, s. 115418-115425
-
Tidskriftsartikel (refereegranskat)abstract
- We employ multibillion time step embedded-atom molecular dynamics simulations to investigate the homoepitaxial growth of Pt(111) from hyperthermal Pt atoms (EPt=0.2–50eV) using deposition fluxes approaching experimental conditions. Calculated antiphase diffraction intensity oscillations, based on adatom coverages as a function of time, reveal a transition from a three-dimensional multilayer growth mode with EPt<20eV to a layer-by-layer growth with EPt≥20eV. We isolate the effects of irradiation-induced processes and thermally activated mass transport during deposition in order to identify the mechanisms responsible for promoting layer-by-layer growth. Direct evidence is provided to show that the observed transition in growth modes is primarily due to irradiation-induced processes which occur during the 10ps following the arrival of each hyperthermal atom. The kinetic pathways leading to the transition involve both enhanced intralayer and interlayer adatom transport, direct incorporation of energetic atoms into clusters, and cluster disruption leading to increased terrace supersaturation.
|
|
| 9. |
- Adey, J., et al.
(författare)
-
Theory of boron-vacancy complexes in silicon
- 2005
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:16, s. 165211-
-
Tidskriftsartikel (refereegranskat)abstract
- The substitutional boron-vacancy BsV complex in silicon is investigated using the local density functional theory. These theoretical results give an explanation of the experimentally reported, well established metastability of the boron-related defect observed in p-type silicon irradiated at low temperature and of the two hole transitions that are observed to be associated with one of the configurations of the metastable defect. BsV is found to have several stable configurations, depending on charge state. In the positive charge state the second nearest neighbor configuration with C1 symmetry is almost degenerate with the second nearest neighbor configuration that has C1h symmetry since the bond reconstruction is weakened by the removal of electrons from the center. A third nearest neighbor configuration of BsV has the lowest energy in the negative charge state. An assignment of the three energy levels associated with BsV is made. The experimentally observed Ev+0.31 eV and Ev+0.37 eV levels are related to the donor levels of second nearest neighbor BsV with C1 and C1h symmetry respectively. The observed Ev+0.11 eV level is assigned to the vertical donor level of the third nearest neighbor configuration. The boron-divacancy complex BsV2 is also studied and is found to be stable with a binding energy between V2 and Bs of around 0.2 eV. Its energy levels lie close to those of the V2. However, the defect is likely to be an important defect only in heavily doped material.
|
|
| 10. |
- Adnane, Bouchaib, et al.
(författare)
-
Origin of photoresponse at 8-14 μm in stacks of self-assembled SiGe/Si quantum dots
- 2009
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A normal incidence photodetector operating at 8-14 μm is demonstrated using p-type δ-doped SiGe dot multilayer structures grown by molecular beam epitaxy on Si(001) substrates. Based on the experimental results of photoluminescence and photoluminescence excitation spectroscopies together with numerical analysis, the origin of the measured photocurrent was attributed to intersubband optical transitions between the heavy hole and light hole states of the valence band of the self-assembled SiGe dots and subsequent lateral transport of photo-excited carriers in the conduction channels formed by Ge wetting layers.
|
|
| 11. |
- Agterberg, Daniel F., et al.
(författare)
-
Checkerboard order in vortex cores from pair-density-wave superconductivity
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:10
-
Tidskriftsartikel (refereegranskat)abstract
- We consider competing pair-density-wave (PDW) and d-wave superconducting states in a magnetic field. We show that PDW order appears in the cores of d-wave vortices, driving checkerboard charge-density-wave (CDW) order in the vortex cores, which is consistent with experimental observations. Furthermore, we find an additional CDW order that appears on a ring outside the vortex cores. This CDW order varies with a period that is twice that of the checkerboard CDW and it only appears where both PDW and d-wave order coexist. The observation of this additional CDW order would provide strong evidence for PDW order in the pseudogap phase of the cuprates. We further argue that the CDW seen by nuclear magnetic resonance at high fields is due to a PDW state that emerges when a magnetic field is applied.
|
|
| 12. |
- Agterberg, Daniel F., et al.
(författare)
-
Microscopic prediction of skyrmion lattice state in clean interface superconductors
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:6, s. 064509-
-
Tidskriftsartikel (refereegranskat)abstract
- When an in-plane field is applied to a clean interface superconductor, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like phase is stabilized. This phase has a U(1)xU(1) symmetry and, in principle, this symmetry allows for flux carrying topological excitations different from Abrikosov vortices (which are the simplest defects associated with S-1 --> S-1 maps). However, in practice, largely due to electromagnetic and other intercomponent interactions, such topological excitations are very rare in superconducting systems. Here, we demonstrate that a realistic microscopic theory for interface superconductors, such as SrTiO3/LaAlO3, predicts an unconventional magnetic response where the flux-carrying objects are skyrmions, characterized by homotopy invariants of S-2 --> S-2 maps. Additionally, we show that this microscopic theory predicts that stable fractional vortices form near the boundary of these superconductors. It also predicts the appearance of type-1.5 superconductivity for some range of parameters. Central to these results is the assumption that the Rashba spin-orbit coupling is much larger than the superconducting gap.
|
|
| 13. |
- Agåker, Marcus, et al.
(författare)
-
Double core excitations in LiI : Evidence for multicenter coupling in resonant inelastic soft x-ray scattering spectra
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:24, s. 241105-
-
Tidskriftsartikel (refereegranskat)abstract
- Resonances in inelastic soft x-ray scattering spectra of LiI, in which final states with Li 1s as well as I 4d vacancies are populated have been observed. The intermediate states involve both Li 1s double core excitations and I parallel to 4p(-1)+4d(-2)4f > states. It is suggested that the spectra are influenced by multicenter coupling.
|
|
| 14. |
- Agåker, Marcus, et al.
(författare)
-
Double Core Excitations in Lithium Halides
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:4, s. 045112-
-
Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic x-ray scattering spectra of LiF , LiCl , LiBr , and LiI excited in the vicinity of the Li double core hole resonances are presented. All lithium halides show similar phenomenology, including scattering via states where both excited electrons are localized during the scattering process, as well as states where one electron delocalizes. Also transitions that involve additional band excitations are observed. A strong influence of the chemical surrounding is found, and it is discussed in terms of the ionic character of the chemical bond.
|
|
| 15. |
- Agåker, Marcus, et al.
(författare)
-
Resonant inelastic soft x-ray scattering at double core excitations in solid LiCl
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:24
-
Tidskriftsartikel (refereegranskat)abstract
- Inelastic soft x-ray scattering in LiCl, resonantly enhanced at states with two Li 1s vacancies, is investigated. States in which both excited electrons are localized during the double core hole lifetime, in which one of the electrons delocalize, as well as triply excited states in which the double core excitation is accompanied by a valence-to-conduction band excitation, contribute to the scattering. The angular momentum symmetry of the involved states and the vibronic coupling during the scattering process are reflected in the angular anisotropy. The effect on the local electronic structure of multiple core holes is theoretically studied by means of supercell band calculations.
|
|
| 16. |
- Ahlberg, Martina, et al.
(författare)
-
Influence of boundaries on magnetic ordering in Fe/V superlattices
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:21, s. 214429-
-
Tidskriftsartikel (refereegranskat)abstract
- We study the role of surface boundaries on the magnetic properties of [Fe/V](n) superlattice structures, with n=2-10. Using the magneto-optical Kerr effect and polarized neutron reflectivity measurements, we examine the evolution of both the total and the layer-resolved magnetizations as a function of temperature. By varying n, we observe a large shift in the transition temperatures T-c and a substantial change in the total magnetization critical exponent beta. In particular, the thicker samples exhibit nonuniversal exponent values. By resolving the magnetization as a function of position within the superlattice, we show that this behavior arises from contributions of the surfaces. Furthermore, we attribute the large shift in T-c to long-ranged interactions present in the superlattice.
|
|
| 17. |
- Ahlberg, Martina, et al.
(författare)
-
Reversed interface effects in amorphous FeZr/AlZr multilayers
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:18, s. 184403-
-
Tidskriftsartikel (refereegranskat)abstract
- We report an anomalous enhancement of the critical temperature (T-c) when the thickness (d) of the magnetic layer is decreased from 60 to 20 angstrom in amorphous FeZr/AlZr multilayers. Further reduction of the thickness causes a decrease of T-c, as expected by finite-size scaling, while the magnetic moment decreases monotonically for all values of d. The increase of the critical temperature is attributed to a reversed interface effect where local changes in the nearest-neighbor distance and coordination number gives a higher effective magnetic coupling at the interfaces compared to the interior of the layer. We have successfully described the results within a model where such interface effects are combined with finite-size scaling.
|
|
| 18. |
- Ahlberg, Martina, et al.
(författare)
-
Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:22, s. 224404-
-
Tidskriftsartikel (refereegranskat)abstract
- We present an experimental realization of a magnetic two-dimensional XY system using amorphous materials. The classification of the dimensionality is based on the critical behavior of amorphous Co68Fe24Zr8(d)/Al70O30 (20 angstrom) multilayers, where d = 11 - 16 angstrom. Analysis of the remanent magnetization, the magnetic isotherms, the initial susceptibility, and the magnetic correlation length shows that the magnetic phase transition can be described by the 2D XY model. The samples are not paramagnetic above the critical temperature but are characterized by local magnetic order manifested in the field and temperature dependence of the magnetization. Furthermore, an average spin-spin interaction length of 8.1 angstrom was estimated using the thickness dependence of the Curie temperature.
|
|
| 19. |
- Ahmed, T., et al.
(författare)
-
G W quasiparticle calculations with spin-orbit coupling for the light actinides
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:3, s. 035104-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the importance of GW self-energy corrections for the electronic structure of light actinides in the weak-to-intermediate coupling regime. Our study is based on calculations of the band structure and total density of states of Np, U, and Pu using a one-shot GW approximation that includes spin-orbit coupling within a full potential LAPW framework. We also present RPA screened effective Coulomb interactions for the f-electron orbitals for different lattice constants, and show that there is an increased contribution from electron-electron correlation in these systems for expanded lattices. We find a significant amount of electronic correlation in these highly localized electronic systems.
|
|
| 20. |
- Ahola-Tuomi, M., et al.
(författare)
-
Properties of self-assembled Bi nanolines on InAs(100) studied by core-level and valence-band photoemission, and first-principles calculations
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:24, s. 245401-
-
Tidskriftsartikel (refereegranskat)abstract
- We have studied self-assembled bismuth (Bi) nanolines on the Bi-terminated InAs(100) surface by core-level and valence-band photoelectron spectroscopy, and ab initio first-principles calculations. A structural model for this intriguing surface is suggested based on the comparison of the measured and calculated core-level shifts. Also, the atomic origins for the core-level shifts are proposed based on the calculations. A clear peak related to this surface was observed in the valence band 0.34 eV below the Fermi level, which can be used as a "fingerprint" of a well-ordered Bi/InAs(100) nanoline surface.
|
|
| 21. |
|
|
| 22. |
- Aichhorn, Markus, et al.
(författare)
-
Importance of electronic correlations for structural and magnetic properties of the iron pnictide superconductor LaFeAsO
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:5, s. 054529-
-
Tidskriftsartikel (refereegranskat)abstract
- We present calculations of structural and magnetic properties of the iron-pnictide superconductor LaFeAsO including electron-electron correlations. For this purpose we apply a fully charge self-consistent combination of density-functional theory with the dynamical mean-field theory, allowing for the calculation of total energies. We find that the inclusion of correlation effects gives a good agreement of the arsenic z position with experimental data even in the paramagnetic (high-temperature) phase. Going to low temperatures, we study the formation of the ordered moment in the striped spin-density-wave phase, yielding an ordered moment of about 0.60 mu(B), again in good agreement with experiments. This shows that the inclusion of correlation effects improves both structural and magnetic properties of LaFeAsO at the same time.
|
|
| 23. |
- Aikebaier, Faluke, et al.
(författare)
-
Effects of short-range electron-electron interactions in doped graphene
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:15
-
Tidskriftsartikel (refereegranskat)abstract
- We study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of substitutional impurities. Our computational approach is based on the π orbital tight-binding model for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. The finite impurity concentration is modeled using the supercell approach. We compare explicitly noninteracting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens up at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.
|
|
| 24. |
- Airiskallio, E., et al.
(författare)
-
Third element effect in the surface zone of Fe-Cr-Al alloys
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:3
-
Tidskriftsartikel (refereegranskat)abstract
- The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.
|
|
| 25. |
- Airiskallio, E., et al.
(författare)
-
Tuning the surface chemistry of Fe-Cr by V doping
- 2009
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:15
-
Tidskriftsartikel (refereegranskat)abstract
- The reversal of the magnitudes of the bulk and surface chemical-potential differences induces the outburst of Cr on the otherwise pure Fe surface of Fe-Cr alloys. This threshold value for the Cr content is about 10 at. %. It is found that vanadium addition to Fe-Cr shifts the Cr threshold to a substantially lower value suggesting V having a positive effect on the corrosion resistance of low Cr steels. The obtained shift in the Cr threshold is shown to be connected to the change in volume of the alloy.
|
|
| 26. |
- Al-Zoubi, Noura, et al.
(författare)
-
Completeness of the exact muffin-tin orbitals : application to hydrogenated alloys
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:4
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the basis set convergence of the exact muffin-tin orbitals by monitoring the equation of state for Al, Cu, and Rh calculated in the conventional face-centered-cubic lattice (str-I) and in a face-centered-cubic lattice with one atomic and three empty sites per primitive cell (str-II). We demonstrate that three (spd) muffin-tin orbitals are sufficient to describe Al in both structures, but for str-II Cu and Rh at least five (spdfg) orbitals are needed to get converged equilibrium Wigner-Seitz radius (within <= 0.8%) and bulk modulus (<= 3.3%). We ascribe this slow convergence to the nearly spherical densities localized around the Cu and Rh atoms, which create strongly asymmetric charge distributions within the nearest cells around the empty sites. The potential sphere radius dependence of the theoretical results for structure str-II is discussed. It is shown that a properly optimized overlapping muffin-tin potential in combination with the spdfg basis yields acceptable errors in the equilibrium bulk properties. The basis set convergence is also shown on hydrogenated Sc and Sc-based alloys.
|
|
| 27. |
- Al-Zoubi, Noura, et al.
(författare)
-
Tetragonality of carbon-doped ferromagnetic iron alloys : A first-principles study
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:1, s. 014112-
-
Tidskriftsartikel (refereegranskat)abstract
- Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.
|
|
| 28. |
- Alam, Aftab, et al.
(författare)
-
Effect of disorder on the electronic properties of graphene : A theoretical approach
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:8, s. 085454-
-
Tidskriftsartikel (refereegranskat)abstract
- In order to manipulate the properties of graphene, it is very important to understand the electronic structure in the presence of disorder. We investigate, within a tight-binding description, the effects of disorder in the on-site (diagonal disorder) term in the Hamiltonian as well as in the hopping integral (off-diagonal disorder) on the electronic dispersion and density of states by the augmented space recursion method. Extrinsic off-diagonal disorder is shown to have dramatic effects on the two-dimensional (2D) Dirac cone, including asymmetries in the band structures as well as the presence of discontinuous bands (because of resonances) in certain limits. Disorder-induced broadening, related to the scattering length (or lifetime) of Bloch electrons, is modified significantly with increasing strength of disorder. We propose that our methodology is suitable for the study of the effects of disorder in other 2D materials, such as a boron nitride monolayer.
|
|
| 29. |
- Alling, Björn, 1980-, et al.
(författare)
-
A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:10, s. 104203-
-
Tidskriftsartikel (refereegranskat)abstract
- We use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.
|
|
| 30. |
- Alling, Björn, et al.
(författare)
-
Configurational disorder effects on adatom mobilities on Ti1-xAlxN(001) surfaces from first principles
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : american physical society. - 1098-0121 .- 1550-235X. ; 85:24, s. 245422-
-
Tidskriftsartikel (refereegranskat)abstract
- We use metastable NaCl-structure Ti0.5Al0.5N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution during film growth. First-principles calculations were employed to obtain energy potential maps of Ti and Al adsorption on an ordered TiN(001) reference surface and a disordered Ti0.5Al0.5N(001) solid-solution surface. The energetics of adatom migration on these surfaces are determined and compared to isolate effects of configurational disorder. The results show that alloy surface disorder dramatically reduces Ti adatom mobilities. Al adatoms, in sharp contrast, experience only small disorder-induced differences in migration dynamics.
|
|
| 31. |
- Alling, Björn, et al.
(författare)
-
Dynamic and structural stability of cubic vanadium nitride
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 91:5, s. 054101-
-
Tidskriftsartikel (refereegranskat)abstract
- Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.
|
|
| 32. |
- Alling, Björn, 1980-, et al.
(författare)
-
Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American institute of physics. - 1098-0121 .- 1550-235X. ; 82, s. 184430-
-
Tidskriftsartikel (refereegranskat)abstract
- Two different methods for the modeling of a magnetically disordered CrN stateusing a supercell approach are investigated. They are found to give equivalentresults of the total energy, being also similar to results obtained with an effectivemedium approach. Furthermore, CrN is shown to be better described using aLDA+U framework for the treatment of electron-electron correlations as comparedto GGA or LDA calculations. Modeling the cubic paramagnetic phase with ourmodels for magnetic disorder and considering the strong electron correlations, thetemperature and pressure induced phase transitions in CrN can be explained.
|
|
| 33. |
- Alling, Björn, et al.
(författare)
-
Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys
- 2009
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:13
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1-xCuxMnSb alloys as a case study. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase T-C in Cu-rich samples.
|
|
| 34. |
- Alling, Björn
(författare)
-
Metal to semiconductor transition and phase stability of Ti1-xMgxNy alloys investigated by first-principles calculations
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 89:8, s. 085112-
-
Tidskriftsartikel (refereegranskat)abstract
- Titanium nitride based materials are applied in several technological applications owing to their stability at high temperatures, mechanical and optical properties, as well as good electrical conductivity. Here, I use theoretical first-principles methods to investigate the possibilities for a semiconducting state as well as the phase stability of Ti1-x MgxNy ternary alloys and compounds. I demonstrate that B1 (NaCl) solid solutions of Ti1-xMgxN with x less than= 0.5 are thermodynamically stable with respect to all previously reported phases in the Ti-Mg-N system. For the composition x = 0.5, an ordered TiMgN2 phase with an L1(1)-type order of Ti and Mg on the metal sublattice is predicted to be the configurational ground state at low temperatures. Other ordered phases present in neighboring materials systems are considered but are found to be unstable. The electronic origin of the stability of the B1 structure solid solutions is identified. A metal to semiconductor transition is observed as the Mg content is increased to x = 0.5. TiMgN2 as well as disordered Ti0.5Mg0.5N solid solution are investigated with hybrid functional calculations and predicted to be semiconductors with band gaps of 1.1 eV and around 1.3 eV, respectively, the latter depending on the details of the Ti and Mg configuration.
|
|
| 35. |
- Alling, Björn, et al.
(författare)
-
Metastability of fcc-related Si-N phases
- 2008
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:13, s. 132103-132103
-
Tidskriftsartikel (refereegranskat)abstract
- The phenomenon of superhardening in TiN/SiNx nanocomposites and the prediction of extreme hardness in bulk gamma-Si3N4 have attracted a large interest to this material system. Attempts to explain the experimental findings by means of first-principles calculations have so far been limited to static calculations. The dynamical stability of suggested structures of the SiNx tissue phase critical for the understanding of the nanocomposites is thus unknown. Here, we present a theoretical study of the phonon-dispersion relations of B1 and B3 SiN. We show that both phases previously considered as metastable are dynamically unstable. Instead, two pseudo-B3 Si3N4 phases derived from a L1(2)- or D0(22)-type distribution of Si vacancies are dynamically stable and might explain recent experimental findings of epitaxial SiNx in TiN/SiNx multilayers.
|
|
| 36. |
- Alling, Björn, 1980-, et al.
(författare)
-
Mixing and decomposition thermodynamics of c-Ti1-xAlxN from first-principles calculations
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:045123
-
Tidskriftsartikel (refereegranskat)abstract
- Wedescribe an efficient first-principles method that can be used tocalculate mixing enthalpies of transition metal nitrides with B1 structureand substitutional disorder at the metal sublattice. The technique isbased on the density functional theory. The independent sublattice modelis suggested for the treatment of disorder-induced local lattice relaxationeffects. It supplements the description of the substitutional disorder withinthe coherent potential approximation. We demonstrate the excellent accuracy ofthe method by comparison with calculations performed by means ofthe projector augumented wave method on supercells constructed as specialquasirandom structures. At the same time, the efficiency of thetechnique allows for total energy calculations on a very finemesh of concentrations which enables a reliable calculation of thesecond concentration derivative of the alloy total energy. This isa first step towards first-principles predictions of concentrations and temperatureintervals where the alloy decomposition proceeds via the spinodal mechanism.We thus calculate electronic structure, lattice parameter, and mixing enthalpiesof the quasibinary alloy c-Ti1−xAlxN. The lattice parameter follows Vegard'slaw at low fractions of AlN but deviates increasingly withincreasing Al content. We show that the asymmetry of themixing enthalpy and its second concentration derivative is associated withsubstantial variations of the electronic structure with alloy composition. Thephase diagram is constructed within the mean-field approximation.
|
|
| 37. |
- Alling, Björn, 1980-, et al.
(författare)
-
Role of stoichiometric and nonstoichiometric defects on the magnetic properties of the half-metallic ferromagnet NiMnSb
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:6, s. 064418-
-
Tidskriftsartikel (refereegranskat)abstract
- The first material to be predicted from first-principles calculations as half-metallic was NiMnSb, and the research on this material has been intense due to its possible applications in spintronics devices. The failure of many experiments to measure spin polarization to more than a fraction of the predicted 100% has partly been blamed on structural defects. In this work a complete first-principles treatise of point defects, including nonstoichiometric antisites, interstitial and vacancy defects, as well as stoichiometric atomic swap defects in NiMnSb, is presented. We find that the formation energies of the defects span a large scale from 0.2 to 14.4 eV. The defects with low formation energies preserve the half-metallic character of the material. We also find that some of the defects increase the magnetic moment and thus can explain the experimentally observed increase of magnetic moments in some samples of NiMnSb. Most interesting in this respect are Mn interstitials which increase the magnetic moment, have a low formation energy, and keep the half-metallic character of the material.
|
|
| 38. |
- Alling, Björn, 1980-
(författare)
-
Theory of the ferromagnetism in Ti1-xCrxN solid solutions
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:5, s. 054408-
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations are used to investigate the magnetic properties of Ti1‑xCrxN solid solutions. We show that the magnetic interactions between Cr spins that favor antiferromagnetism in CrN is changed upon alloying with TiN leading to the appearance of ferromagnetism in the system at approximately x≤0.50 in agreement with experimental reports. Furthermore we suggest that this effect originates in an electron density redistribution from Ti to Cr that decreases the polarization of Crd states with t2g symmetry while it increases the polarization of Crd states with eg symmetry, both changes working in favor of ferromagnetism.
|
|
| 39. |
- Andersen, O., et al.
(författare)
-
Electrical activity of carbon-hydrogen centers in Si
- 2002
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:23, s. 235205-1
-
Tidskriftsartikel (refereegranskat)abstract
- The electrical activity of Cs-H defects in Si has been investigated in a combined modeling and experimental study. High-resolution Laplace capacitance spectroscopy with the uniaxial stress technique has been used to measure the stress-energy tensor and the results are compared with theoretical modeling. At low temperatures, implanted H is trapped as a negative-U center with a donor level in the upper half of the gap. However, at higher temperatures, H migrates closer to the carbon impurity and the donor level falls, crossing the gap. At the same time, an acceptor level is introduced into the upper gap making the defect a positive-U center.
|
|
| 40. |
- Andersson, Andreas, et al.
(författare)
-
Anomalous Nernst effect and heat transport by vortex vacancies in granular superconductors
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:6, s. 060508-1-060508-4
-
Tidskriftsartikel (refereegranskat)abstract
- We study the Nernst effect due to vortex motion in two-dimensional granular superconductors using simulations with Langevin or resistively shunted Josephson-junction dynamics. In particular, we show that the geometric frustration of both regular and irregular granular materials can lead to thermally driven transport of vortices from colder to hotter regions, resulting in a sign reversal of the Nernst signal. We discuss the underlying mechanisms of this anomalous behavior in terms of heat transport by mobile vacancies in an otherwise pinned vortex lattice.
|
|
| 41. |
- Andersson, Andreas, et al.
(författare)
-
Influence of vortices and phase fluctuations on thermoelectric transport properties of superconductors in a magnetic field
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:17, s. 174502-
-
Tidskriftsartikel (refereegranskat)abstract
- We study heat transport and thermoelectric effects in two-dimensional superconductors in a magnetic field. These are modeled as granular Josephson-junction arrays, forming either regular or random lattices. We employ two different models for the dynamics: relaxational model-A dynamics or resistively and capacitively shunted Josephson junction dynamics. We derive expressions for the heat current in these models, which are then used in numerical simulations to calculate the heat conductivity and Nernst coefficient for different temperatures and magnetic fields. At low temperatures and zero magnetic field the heat conductivity in the RCSJ model is calculated analytically from a spin wave approximation, and is seen to have an anomalous logarithmic dependence on the system size, and also to diverge in the completely overdamped limit C -> 0. From our simulations we find at low magnetic fields that the Nernst signal displays a characteristic "tilted hill" profile similar to experiments and a nonmonotonic temperature dependence of the heat conductivity. We also investigate the effects of granularity and randomness, which become important for higher magnetic fields. In this regime geometric frustration strongly influences the results in both regular and random systems and leads to highly nontrivial magnetic field dependencies of the studied transport coefficients.
|
|
| 42. |
- Andersson, Andreas, et al.
(författare)
-
Modeling and simulations of quantum phase slips in ultrathin superconducting wires
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:13
-
Tidskriftsartikel (refereegranskat)abstract
- We study quantum phase slips (QPS) in ultrathin superconducting wires. Starting from an effective one-dimensional microscopic model, which includes electromagnetic fluctuations, we map the problem to a (1+1)-dimensional gas of interacting instantons. We introduce a method to calculate the tunneling amplitude of quantum phase slips directly from Monte Carlo simulations. This allows us to go beyond the dilute instanton gas approximation and study the problem without any limitations of the density of QPS. We find that the tunneling amplitude shows a characteristic scaling behavior near the superconductor-insulator transition. We also calculate the voltage-charge relation of the insulating state, which is the dual of the Josephson current-phase relation in ordinary superconducting weak links. This evolves from a sinusoidal form in the regime of dilute QPS to more exotic shapes for higher QPS densities, where interactions are important.
|
|
| 43. |
- Andersson, Andreas, et al.
(författare)
-
Scaling, finite size effects, and crossovers of the resistivity and current-voltage characteristics in two-dimensional superconductors
- 2013
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:22, s. 224506-
-
Tidskriftsartikel (refereegranskat)abstract
- We revisit the scaling properties of the resistivity and the current-voltage characteristics at and below the Berezinskii-Kosterlitz-Thouless transition, both in zero and nonzero magnetic fields. The scaling properties are derived by integrating the renormalization group flow equations up to a scale where they can be reliably matched to simple analytic expressions. The vortex fugacity turns out to be dangerously irrelevant for these quantities below T-c, thereby altering the scaling behavior. We derive the possible crossover effects as the current, magnetic field, or system size is varied, and find a strong multiplicative logarithmic correction near T-c, all of which is necessary to account for when interpreting experiments and simulation data. Our analysis clarifies a longstanding discrepancy between the finite size dependence found in many simulations and the current-voltage characteristics of experiments. We further show that the logarithmic correction can be avoided by approaching the transition in a magnetic field, thereby simplifying the scaling analysis. We confirm our results by large-scale numerical simulations, and calculate the dynamic critical exponent z, for relaxational Langevin dynamics and for resistively and capacitively shunted Josephson junction dynamics.
|
|
| 44. |
- Andersson, Britt M., et al.
(författare)
-
Thermal conductivity of polycrystalline YBa2Cu4O8
- 1994
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 49:6, s. 4189-4198
-
Tidskriftsartikel (refereegranskat)abstract
- We have measured the thermal conductivity κ and the thermal diffusivity a of a dense bulk ceramic polycrystalline sample of YBa2Cu4O8 (1:2:4) in the temperature range 30–300 K. We find κ≊10 W m-1 K-1 at 100 K, significantly higher than in ceramic YBa2Cu3O7-δ (1:2:3) and approaching the in-plane value for single-crystal 1:2:3, and decreasing to 7.6 W m-1 K-1 at 300 K. The data for this sample can be described by standard theories for phonon thermal conductivity of crystalline materials with boundary, phonon, and electron scattering. The higher κ in 1:2:4 as compared to 1:2:3 is, in this model, due to the smaller point defect scattering in the former. The fitted parameters for the three scattering mechanisms all agree with independent estimates based on simple models; inserting data for electric resistivity, grain size, carrier density, and lattice properties we can predict κ and its T dependence to within about 20%. We also discuss models for the phonon and electron thermal conductivities in some detail, including some second-order effects such as inelastic electron scattering and a T-dependent carrier density.
|
|
| 45. |
- Andersson, Britt M., et al.
(författare)
-
Thermal conductivity of YBa2Cu4O8 dominated by phonon-phonon interactions
- 1993
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 48:5, s. 3575-3578
-
Tidskriftsartikel (refereegranskat)abstract
- The thermal conductivity κ of dense sintered ceramic YBa2Cu4O8 in the range 30–310 K has been measured. At 100 K, κ is 10 W m-1 K-1, approaching the in-plane κ of single crystals of other high-Tc materials. κ decreases rapidly with increasing T to 7.4 W m-1 K-1 at 300 K. Fitting standard models for κ(T) to the data we find that κ is limited mainly by phonon-phonon interactions. Depending on the model used, the best fit is found for effective values of FTHETADebye between 155 and 185 K, characteristic for the acoustic phonon branches, indicating that such phonons carry most of the heat. Finally, we suggest a possible way to test the electron-phonon model for the electrical and thermal conductivities in high-Tc materials.
|
|
| 46. |
|
|
| 47. |
- Andersson, David A., et al.
(författare)
-
Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism
- 2007
-
Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 75, s. 035109-1-035109-6
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.
|
|
| 48. |
- Andersson, David A., et al.
(författare)
-
Monovacancy and divacancy formation and migration in copper : a first-principles theory
- 2004
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:11, s. 115108-
-
Tidskriftsartikel (refereegranskat)abstract
- The formation and migration of monovacancies and divacancies in copper have been studied from first-principles in order to resolve the discrepancies between previously published experimental and theoretical data. The monovacancy and divacancy formation, migration and binding enthalpies as well as the formation volumes have been calculated in the framework of a plane-wave pseudopotential implementation of the density functional theory, with full structural relaxations included. The monovacancy and divacancy formation entropies have been estimated from experimental data by performing a least-squares analysis. We show that the complete set of first-principles data, taking into account the presence of both vacancies and divacancies as well as the temperature dependence of the formation enthalpies and entropies allow one to reproduce the Arrhenius plot of the total vacancy concentration and the diffusion coefficient, both in good agreement with the most accurate experiments.
|
|
| 49. |
- Andersson, David A., et al.
(författare)
-
Theoretical study of CeO(2) doped with tetravalent ions
- 2007
-
Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 76, s. 1741191-17411910
-
Tidskriftsartikel (refereegranskat)abstract
- We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).
|
|
| 50. |
- Andersson, David A., et al.
(författare)
-
Thermodynamics of structural vacancies in titanium monoxide from first principles calculations
- 2005
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:14, s. 144101-
-
Tidskriftsartikel (refereegranskat)abstract
- The structure, stability and electronic properties of the low oxygen oxides of titanium, TiOx with 1/3 <= x <= 3/2, have been studied by means of accurate first-principles calculations. In both stoichiometric and nonstoichiometric TiO there are large fractions of vacant lattice sites. These so-called structural vacancies are essential for understanding the properties and phase stability of titanium oxides. Structures with an ordered arrangement of vacancies were treated with a plane wave pseudo-potential method, while calculations for structures with disordered vacancies were performed within the framework of the Korringa-Kohn-Rostoker Green's function technique. The relaxed structural parameters in general compare well with experimental data, though some discrepancies exist for stoichiometric TiO in the ideal B1 structure, i.e., without any vacancies. The equation of state as well as the elastic properties are also derived. A monoclinic, vacancy-containing, structure of stoichiometric TiO is confirmed to be stable at low temperature and pressure. Experimentally a transition from a stoichiometric cubic structure with disordered vacancies to the ideal B1 structure without any vacancies has been observed at high pressure. It is discussed how this experimental observation relates to the present theoretical results for defect-containing and defect-free TiO.
|
|
