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| 2. |
- Björketun, Mårten, 1976, et al.
(författare)
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Effect of acceptor dopants on the proton mobility in BaZrO3: A density functional investigation
- 2007
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Ingår i: Phys. Rev. B. ; 76, s. 054307-
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the stability and mobility of protonic defects in acceptor doped BaZrO3 are investigated using density functional theory in conjunction with kinetic modeling. The interaction between the mobile protons and Zr-site substituted trivalent dopants—Ga, Sc, In, Y, and Gd—is assessed by extensive mapping of migration pathways. In all cases, the interaction is attractive, which results in a stabilization, and hence a reduction of the mobility, of the proton. However, its strength and range vary substantially and are found to be strongly correlated to the ionic radius of the dopant. Further, it is shown that the observed correlation is related to a variation in strength of hydrogen bonds formed between protons and next nearest lattice oxygen ions in different environments. Finally, migration barriers and attempt frequencies obtained from first principles are used as input to a jump-diffusion model. The proton self-diffusion coefficient is evaluated for the different substituted phases, and the experimental trend in activation energies is reproduced.
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| 3. |
- Björketun, Mårten, 1976, et al.
(författare)
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Structure and thermodynamic stability of hydrogen interstitials in BaZrO3 perovskite oxide from density functional calculations
- 2007
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Ingår i: Faraday Discuss.. ; 134, s. 247-265
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Tidskriftsartikel (refereegranskat)abstract
- Density functional calculations have been used to study the electronic structure, preferred sites in the lattice, formation energies and vibrational frequencies for hydrogen interstitials in different charge states in the cubic phase of perovskite-structured BaZrO3. By combining ab initio results with thermodynamic modeling, defect formation at finite temperature and pressure has been investigated. We demonstrate how the site selectivity and spatial distribution of dopant atoms in the lattice can be affected by changes in the environmental conditions (atomic chemical potentials, oxygen partial pressure and temperature) used during processing of the material. In addition, we have calculated the thermodynamic parameters of the water uptake reaction for an acceptor-doped BaZrO3 crystal in equilibrium with a humid atmosphere. The interaction energies between a protonic defect and the investigated Ga, Gd, In, Nd, Sc, and Y dopants were found to be attractive, and we show that a simple model of defect association may reproduce an experimentally observed trend in the hydration enthalpy.
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| 4. |
- Karlsson, Maths, 1978, et al.
(författare)
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Using Neutron Spin-Echo To Investigate Proton Dynamics in Proton-Conducting Perovskites
- 2010
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 22:3, s. 740-742
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Tidskriftsartikel (refereegranskat)abstract
- The applicability and potential of neutron spin-echo (NSE) to study proton dynamics in proton-conducting ceramics was studied. The experiment was performed on hydrated BaZr 0,90 Y 0.10 O 2.95 (10Y:BZO), a cubic perovskite with a relatively high proton conductivity. The NSE experiment was performed at the IN15 spectrometer at Institut Laue-Langevin (ILL) in Grenoble, France. The relaxation rate for two Q-values for the temperature 563 K was determined. The first-principles calculations were carried out within the framework of density functional theory (DFT). The calculated diffusion harriers far from Y-dopants are found to he 0.20 and 0.18 eV for the proton transfer and hydroxyl rotation motion, respectively. The binding energy to a Y-dopant is 0.16 eV, and the influence of the Y-dopant on the energetics for the proton is quite extended in space, including both the first and the second coordination shells.
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| 5. |
- Karlsson, Maths, 1978, et al.
(författare)
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Vibrational Properties of Protons in Hydrated BaInxZr1-xO3-x/2
- 2005
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 72, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the local proton dynamics in the proton conducting hydrated perovskite system BaInxZr1-xO3-x/2 (x=0.25-0.75) using infrared spectroscopy and first-principles calculations. We show that oxygen vacancies and dopant atoms in the vicinity of the proton tilt the proton toward a neighboring oxygen creating strongly hydrogen-bonded configurations. This is manifested as a strong redshift of the O-H stretch band in the infrared absorption spectrum. We also find considerable fluctuations of the nearest and next-nearest oxygen-proton distances with time, resulting in additional spectral broadening. By comparing the frequencies of computed O-H stretch modes we can relate specific local configurations to different parts of the broad O-H stretch band. Even though hydrogen-bonded configurations favor proton transfer they hinder the long-range migration by decreasing the reorientational rate. Thus, in order to optimize the proton mobility it is important to avoid extreme configurations caused by either oxygen vacancies or dopant atoms in the perovskite structure.
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| 7. |
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| 8. |
- Sundell, Per G., 1977, et al.
(författare)
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Density-functional calculations of prefactors and activation energies for H diffusion in BaZrO3
- 2007
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Ingår i: Phys. Rev. B. ; 76, s. 094301-
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Tidskriftsartikel (refereegranskat)abstract
- Density-functional calculations are used to investigate hydrogen diffusion in the solid-state proton conductor BaZrO3. Activation energies and prefactors for the rate of proton transfer and reorientation are evaluated for a defect-free region of this simple cubic perovskite-structured oxide. Both semiclassical over-barrier jumps and phonon-assisted tunneling transitions between sites are considered. It is found that the classical barriers for the elementary transfer and reorientation steps are both of the order of 0.2 eV. The quantum-mechanical zero-point motion effects are found to be sizable, to effectively reduce the barrier heights, and to make the prefactors similar for the transfer and reorientation steps. The Flynn-Stoneham model [Phys. Rev. B 1, 3966 (1970)] of phonon-assisted tunneling yields an activation energy of around 0.2 eV and a very small prefactor for proton transfer, whereas the corresponding adiabatic model gives a similar activation energy but a much larger prefactor. It is suggested that the effect of other defects such as dopants has to be included for a proper description of hydrogen diffusion in this material.
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| 9. |
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| 10. |
- Sundell, Per G., 1977
(författare)
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Quantum aspects of hydrogen in metals and oxides from density-functional calculations
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atomic hydrogen dissolved in a solid or adsorbed on a solid surfacedisplays many remarkable features, and is a subject ofgreat technological interest.An increased understanding of hydrogen/solid systems can supportthe development of areas as diverse asproton exchange membranes for fuel cells,heterogeneous catalysis, and growth of semiconductor devices.The behavior of H in solids and on surfaces is also of fundamentalinterest, since the uniquely small mass of the hydrogen nucleusallows for quantum effects such as tunneling at low temperatures. This thesis gives a detailed account of the structure and dynamicsof hydrogen in different systemsusing first-principles calculationsbased on the density-functional theory approach.More specifically, we investigate the basic properties ofH and D in bulk metals, on metal surfaces, and in bulk oxides.On a microscopic scale, these systems are characterized by theirhydrogen site occupancy, vibrational states, and rate of diffusivejumps between sites.Extensions to macroscopic length and time scales provided bymeans of thermodynamic and kinetic modeling alsoallow for a direct comparison with experimental data. In metallic systems, hydrogen atoms generally diffuse very rapidlyand remain mobile down to very low temperatures where no migrationcan be expected to occur classically.We demonstrate how the parameters that characterize this behaviorcan be calculated on a first-principles basis.For H interstitials in bulk Nb and Ta, we show that goodagreement with experiments can be obtained by assumingdiffusive jumps to occur as a sequence ofthermally activated phonon-assisted tunneling transitions attemperatures below approximately 250~K.For H adsorbates on the Cu(001) surface, we show that the hop rateis limited by a non-adiabatic response of the conductionelectrons below 25~K and we explain the experimentallyobserved abrupt transition between nearly temperature independentand thermally activated behavior occurring around 60~K.In hydrated perovskite oxides, proton conductivity typically becomesappreciable only at elevated temperatures.For bulk BaZrO$_3$, we analyze the defect structure under variousenvironmental conditions, and show that the presence of dopant atoms --although necessary to achieve a high concentration of H$^+$ defects-- can lead to a severe reduction of the proton mobility byacting as 'traps'.Taken together, these results provide valuable insightsinto the structure and dynamics of hydrogen in various systemsand into the quantum nature of the interaction of hydrogen withdifferent materials.
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| 12. |
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| 13. |
- Sundell, Per G., 1977, et al.
(författare)
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Thermodynamics of doping and vacancy formation in BaZrO3 perovskite oxide from density functional calculations
- 2006
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Ingår i: Phys. Rev. B. ; 73, s. 104112-
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Tidskriftsartikel (refereegranskat)abstract
- Density functional calculations combined with thermodynamical modeling have been used to investigate defect formation in a perovskite-structured oxide in equilibrium with an oxygen containing atmosphere. We have calculated the electronic structure and formation energies for anion and cation vacancies and for Ga, Gd, In, Nd, Sc, and Y dopants incorporated on different lattice sites in BaZrO3. On energetic grounds, it is found that most of the investigated dopants (with the possible exception of Nd and Gd) preferably substitute for Zr atoms in the lattice. The interaction between dopants was found to be repulsive and correlated with the ionic radius of the dopant, while the dopant-oxygen vacancy interaction was found attractive. We also show that oxygen vacancies are not thermodynamically stable at low temperatures, but will form at the high temperatures and low oxygen partial pressures typically used during synthesis of the material.
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