| 1. |
- Hoffmann, Martin, et al.
(författare)
-
Elastic anomalies and long/short-range ordering effects : A first-principles investigation of the AgcPd1-c solid solution
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:9, s. 094106-
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the elastic properties of the binary alloy Ag-Pd. The lattice constant of this system shows significant deviations from the linear behavior anticipated by the semi-empirical Vegard's rule. This effect was formerly studied by assuming total substitutional disorder, and described by the coherent potential approximation (CPA). Theoretical phase diagram investigations have however suggested three ordered phases at low temperatures, and we extend our first-principles investigation to include such scenarios through the adoption of an extended unit cell representation and a recently developed multisublattice generalization of the original CPA. This allows us to explore equilibrium lattice constant and bulk modulus within a unified approach even in the presence of partial long-range order. We obtain significant variations of the bulk modulus in comparison to the totally disordered picture, and again very rich deviations from more intuitive predictions of a simple linear behavior. We follow former suggestions to analyze the different regimes in connection with topological transitions of the Fermi surface, examined through Bloch spectral function calculations.
|
|
| 2. |
- Kuzmin, Mikhail, et al.
(författare)
-
Atomic-Level Understanding of Interfaces in the Synthesis of Crystalline Oxides on Semiconductors : Sr- and Ba/Si(100)(2 x 3) Reconstructions
- 2014
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:4, s. 1894-1902
-
Tidskriftsartikel (refereegranskat)abstract
- The synthesis of novel functional crystalline films on semiconductor substrates calls for atomic-level knowledge and controlling of the initial stages of interface or junction formation. Technologically relevant epitaxial oxide films can be grown on Si(100) surfaces modified by submonolayer alkaline earth adsorbates, e.g., barium (Ba) and strontium (Sr). Nevertheless, the fundamental properties of such surfaces, that is, Ba/Si(100) and Sr/Si(100) reconstructions are still controversial, which hinders a deeper insight into the synthesis of crystalline oxide films on silicon. In this study, scanning tunneling microscopy (STM), low-energy electron diffraction, synchrotron-radiation photoemission, and ab initio calculations have been utilized to examine Sr- and Ba-induced Si(100)(2 x 3) reconstructions that form the first mediating step in the growth of various functional oxide films on Si(100). The presented results elucidate the atomic and electronic structures of the Si(100)(2 x 3)-Sr and -Ba interfaces, giving support to the so-called (2 x 3) dimer vacancy structure. In particular, using STM, we demonstrate an evidence for the Si dimer, one of the main structural elements of metal-induced reconstructions on semiconductor (100) surfaces. It is also shown that in contrast to the dimer vacancy geometry, the other models, proposed for the Sr- and Ba/Si(100)(2 x 3) earlier, cannot be adopted.
|
|
| 3. |
- Levamaki, Henrik, et al.
(författare)
-
Kullback-Leibler and relative Fisher information as descriptors of locality
- 2018
-
Ingår i: International Journal of Quantum Chemistry. - : WILEY. - 0020-7608 .- 1097-461X. ; 118:12
-
Tidskriftsartikel (refereegranskat)abstract
- Kullback-Leibler and relative Fisher information functionals are applied in studying deviation from local density approximation. The reduced density gradient s and the local kinetic energy parameter alpha are key ingredients of these new locality descriptors. The relative Kullback-Leibler information density contains extra knowledge as it is negative where the given probability density is smaller than the reference density. The relative Fisher information incorporates the highest order deviations from the uniform electron gas approximation.
|
|
| 4. |
- Levamäki, Henrik, et al.
(författare)
-
Gradient-level and nonlocal density functional descriptions of Cu-Au intermetallic compounds
- 2018
-
Ingår i: European Physical Journal B. - : SPRINGER. - 1434-6028 .- 1434-6036. ; 91:6
-
Tidskriftsartikel (refereegranskat)abstract
- We use three gradient level and two nonlocal density functional approximations to study the thermodynamic properties of Cu-Au compounds. It is found that a well-designed gradient level approximation (quasi non-uniform approximation, QNA) reproduces the experimental equilibrium volumes and the formation energies of L12 and L10 phases. On the other hand, QNA predicts a non-existent beta(2) phase, which can be remedied only when employing the nonlocal hybrid-level Heyd-Scuseria-Ernzerhof (HSE06) or Perdew-Burke-Ernzerhof (PBE0) approximations. Gradient-level approximations lead to similar electronic structures for the Cu-Au compounds whereas hybrids shift the d-band towards negative energies and account for the complex d-d hybridization more accurately.
|
|
| 5. |
- Li, Wei, et al.
(författare)
-
First-principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys
- 2016
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 108:8
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles alloy theory is used to establish the gamma-surface of Fe-Cr-Ni alloys as function of chemical composition and temperature. The theoretical stacking fault energy (SFE) versus chemistry and temperature trends agree well with experiments. Combining our results with the recent plasticity theory based on the gamma-surface, the stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective stacking fault energy below similar to 18 mJ m(-2). Alloys with SFE above this critical value show both twinning and full slip at room temperature. Interestingly, twinning remains a possible deformation mode in addition to full slip even at elevated temperatures, in line with observations.
|
|
| 6. |
|
|
| 7. |
- Makela, Jaakko, et al.
(författare)
-
Comparison of chemical, electronic, and optical properties of Mg-doped AlGaN
- 2016
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:50, s. 28591-28597
-
Tidskriftsartikel (refereegranskat)abstract
- Hydrogen, carbon, and oxygen are common unintentional impurities of Al(x)Ga(1−x)N crystals. This impurity structure and its interplay with Mg impurities in Al(x)Ga(1−x)N semiconductors are relevant to develop the p-type nitride crystals for various devices (e.g, LEDs, transistors, gas sensors) but are still unclear. Here we have investigated Mg-doped Al0.5Ga0.5N before and after postgrowth annealing with valence-band and core-level photoelectron spectroscopy, photoluminescence, and resistivity measurements. First, it is found that a surface part of the Al0.5Ga0.5N crystal is surprisingly inert with air and stable against air exposure-induced changes. Thus, the relatively surface-sensitive photoelectron spectroscopy measurements reflect in this case also the bulk crystal characteristics. The measurements reveal the presence of deep states up to 1 eV above valence-band maximum before and after the annealing and that oxygen and carbon occupy N lattice sites (i.e., ON and CN). The model where CN-induced acceptor states in the band gap participate in the blue emission (photoluminescence) is supported. Furthermore, the presented Mg 2p core-level spectra demonstrate that part of Mg atoms forms direct bond(s) with oxygen in the bulklike structure of Al0.5Ga0.5N and that the chemical environment of Mg atoms is much richer than was expected previously.
|
|
| 8. |
- Tian, Fuyang, 1980-
(författare)
-
Ab initio atomistic simulation of metals and multicomponent alloys
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ab initio theory provides a powerful tool to understand and predict the behavior of materials. This thesis contains both of these aspects. First we use ab initio alloy theory to investigate a new kind of complex alloy (high-entropy alloy). Second we introduce a novel potential (interlayer potential), which can be extracted from ab inito total energy calculations using the Chen-Möbius inversion method.High-entropy alloys (HEAs) are composed of four or more metallic elements with nearly equimolar composition. In spite of the large number of components, most of the HEAs have a simple solid-solution phase rather than forming complex intermetallic structures. Extensive experiments have reported the unique microstructures and special properties of HEAs. Single-phase HEAs may be divided into three types, i.e. the3d-HEAs adopting the face centered cubic (fcc) phase, the refractory-HEAs with a body centered cubic (bcc) phase, and the HEAs with the duplex fcc-bcc structure. We employ the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) to investigate the electronic structure, the equilibrium volume and the elastic properties of these three-type HEAs.First we compare the CPA with the super cell technique (SC) to assess the performance of the EMTO-CPA method. As typical fcc 3d-HEAs, we consider the CuNiCoFeCrTix systems in the paramagnetic state. Starting from the calculated electronic structure, we give an explanation for the observed magnetic states. Furthermore, we provide a theoretical prediction for the elastic parameters and polycrystalline elastic moduli for CuNiCoFeCrTix (x= 0.0−0.5, 1.0) and NiCoTeCrTi. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys.Refractory-HEAs are composed of Ti, Zr, Hf, V, Nb, Ta, Mo, and W. These HEAs have a simple bcc structure. Taking the TiZrNbMoVx and TiZrVNb HEAs as examples, we provide a detailed investigation of the effect of alloying elements on the elastic parameters and the elastic isotropy. Our results indicate that vanadium enhances the anisotropy and ductility of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence charge concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC ≃ 4.72.With increase of the aluminum content, phase transformations (fcc→(fcc+bcc)→bcc) occur in NiCoFeCrAlx HEAs. Our ab initio results predict that at room temperature the paramagnetic NiCoFeCrAlx HEAs adopt the fcc structure for x ≤ 0.60 and the bcc structure for x ≥ 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.Many modern materials and material systems are layered. The properties related to layers are connected to interactions between atomic layers. We introduce the interlayer potential (ILP), a novel model potential which fully describes the interaction between layers. The ILPs are different from the usual interatomic potentials which present inter- action between atoms. We use the Chen-Möbius inversion method to extract the ILPs from ab initio total energy calculations. The so obtained ILPs can be employed to investigate several physical parameters connected with the particular set of atomic layers, e.g. surface energy, stacking fault energy, elastic parameters, etc.As an application, we adopt the supercell method and the axial interaction model in connection with the ILPs to calculate the stacking fault energy along the fcc ⟨111⟩ direction, including the intrinsic stacking fault energy, extrinsic stacking fault energy and twin stacking fault energy as well as the interactions between the intrinsic stacking faults. We find that the data derived from ILPs are consistent with those obtained in direct ab initio calculations. Along the fcc ⟨111⟩ direction, we study the surface energy and surface relaxation using the ILPs. The phonon dispersions are also described.Our conclusions are as followsthe EMTO-CPAab initioalloy theory can be used to understand and predict the fundamental properties of multicomponent alloys.the interlayer potentials based on the Chen-Möbius inversion method may provide a new way to investigate the properties related to layers in layered materials,the EMTO-CPA alloy theory combined with the Chen-Möbius inversion method offers a powerful technique to study the properties of complex alloys.
|
|
| 9. |
- Tian, Fuyang
(författare)
-
Ab initio Interlayer Potentials For Metals and Alloys
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many modern materials and material systems are layered. The properties related to layers are connected to interactions between atomic layers. In the present thesis, we introduce the interlayer potential (ILP), a novel model potential which fully describes the interaction between layers. The ILPs are different from the usual interatomic potentials which present interaction between atoms. We use the Chen-Möbius inversion method to extract the ILPs from ab initio total energy calculations. The so obtained ILPs can be employed to investigate several physical parameters connected with the particular set of atomic layers, e.g. surface energy, stacking fault energy, elastic parameters, etc.The interactions between the face centered cubic (fcc) (111) planes are described by two different ILPs. Using two close-packed model structures, namely the ABC stacking along the fcc ⟨111⟩ direction and AB stacking along the hcp ⟨0001⟩ direction, we demonstrate how these two ILPs are obtained via the Chen-Möbius method. Density function theory (DFT) is employed to generate the ILPs and also to compute the equilibrium structural properties of elemental metals Al, Ni, Cu, Ag, Au and Pd as well as of Pd-Ag random solid solutions.With the so established ILPs, we adopt the supercell method and the axial interaction model to calculate the stacking fault energy along the fcc ⟨111⟩ direction, including the intrinsic stacking fault energy, extrinsic stacking fault energy and twin stacking fault energy as well as the interactions between the intrinsic stacking faults. We find that the data derived from ILPs are consistent with those obtained in direct ab initio calculations. Along the fcc ⟨111⟩ direction, we study the surface energy and surface relaxation using the ILPs. The phonon dispersions are also described.We conclude that the interlayer potentials based on the Chen-M¨obius inversion technique may provide a new way to investigate the properties related to layers in layered materials.
|
|
| 10. |
|
|
