| 1. |
- Eklöf-Österberg, Carin, 1987, et al.
(författare)
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The role of oxygen vacancies on the vibrational motions of hydride ions in the oxyhydride of barium titanate
- 2020
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 8:13, s. 6360-6371
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxyhydrides, BaTiO3-xHx, represent a novel class of hydride ion conducting materials of interest for several electrochemical applications, but fundamental questions surrounding the defect chemistry and hydride ion transport mechanism remain unclear. Here we report results from powder X-ray diffraction, thermal gravimetric analysis, nuclear magnetic resonance spectroscopy, inelastic neutron scattering (INS), and density functional theory (DFT) simulations on three metal hydride reduced BaTiO3 samples characterized by the simultaneous presence of hydride ions and oxygen vacancies. The INS spectra are characterized by two predominating bands at around 114 (omega(perpendicular to)) and 128 (omega(parallel to)) meV, assigned as fundamental Ti-H vibrational modes perpendicular and parallel to the Ti-H-Ti bond direction, respectively, and four additional, weaker, bands at around 99 (omega(1)), 110 (omega(2)), 137 (omega(3)) and 145 (omega(4)) meV that originate from a range of different local structures associated with different configurations of the hydride ions and oxygen vacancies in the materials. Crucially, the combined analyses of INS and DFT data confirm the presence of both nearest and next-nearest neighbouring oxygen vacancies to the hydride ions. This supports previous findings from quasielastic neutron scattering experiments, that the hydride ion transport is governed by jump diffusion dynamics between neighbouring and next-nearest neighbouring hydride ion-oxygen vacancy local structures. Furthermore, the investigation of the momentum transfer dependence of the INS spectrum is used to derive the mean square displacement of the hydride ions, which is shown to be in excellent agreement with the calculations. Analysis of the mean square displacement confirms that the hydrogen vibrational motions are localized in nature and only very weakly affected by the dynamics of the surrounding perovskite structure. This insight motivates efforts to identify alternative host lattices that allow for a less localization of the hydride ions as a route to higher hydride ion conductivities.
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| 2. |
- Frostenson, Carl Mikael, 1992, et al.
(författare)
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Hard and soft materials: Putting consistent van der Waals density functionals to work
- 2022
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Ingår i: Electronic Structure. - : IOP Publishing. - 2516-1075. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- We present the idea and illustrate potential benefits of having a tool chain of closely related regular, unscreened and screened hybrid exchange-correlation (XC) functionals, all within the consistent formulation of the van derWaals density functional (vdW-DF) method (Hyldgaard et al (2020 J. Phys.: Condens. Matter 32 393001)). Use of this chain of nonempirical XC functionals allows us to map when the inclusion of truly nonlocal exchange and of truly nonlocal correlation is important. Here we begin the mapping by addressing hard and soft material challenges: Magnetic elements, perovskites, and biomolecular problems.We also predict the structure and polarization for a ferroelectric polymer. To facilitate this work and future broader explorations, we present a stress formulation for spin vdW-DF and illustrate the use of a simple stability-modeling scheme. The modeling supplements density functional theory (DFT) (with a specific XC functional) by asserting whether the finding of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies an actual DFT-based prediction of a low-temperature transformation.
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| 3. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
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Band: Vs. polaron: Vibrational motion and chemical expansion of hydride ions as signatures for the electronic character in oxyhydride barium titanate
- 2019
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 7:27, s. 16211-16221
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Tidskriftsartikel (refereegranskat)abstract
- The oxyhydride phase of barium titanate, BaTiO3-xHx, is a mixed hydride ion and electron conductor. The substitution of oxygen with hydrogen to form a hydride ion is accompanied by donation of an electron to the initially empty titanium 3d conduction band. It is not clear, however, whether the electron forms a delocalized state where it is shared among all titanium ions forming a bandstate, or if it localizes on a titanium ion and forms a bound electron polaron. Here, we investigate polaron formation in this material using density-functional theory (DFT) calculations, where the self-interaction error has been corrected by the DFT + U method and the HSE hybrid functional. While calculated formation energies do not provide a conclusive description of the electronic state, a comparison of the results from first-principles phonon calculations with vibrational spectra measured with inelastic neutron scattering (INS) suggests that the electrons form bandstates in bulk BaTiO3-xHx. This is further supported by comparison of the computed chemical expansion of the involved defect species with experimental data of the lattice expansion in the oxyhydride formation. The oxyhydride phase of barium titanate, BaTiO3-xHx, should thus exhibit metallic-like conductivity.
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| 4. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
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BaZrO3 stability under pressure: The role of nonlocal exchange and correlation
- 2020
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 101:22
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Tidskriftsartikel (refereegranskat)abstract
- The ground-stale structure of BaZrO3 is experimentally known to be cubic down to absolute zero. However, there exist several measured properties and experimental characterizations that earlier computational works have failed to accurately describe and explain within this cubic symmetry. Among these properties and observations are the dielectric constant and the parallel mean-squared relative displacement value that tracks the fluctuations in distance for Ba-O atom pairs. Previous density-functional theory (DFT) studies have resolved the issue by assuming that BaZrO(3 )undergoes a phase transition from cubic to tetragonal I4/mcm symmetry, possibly while forming a glasslike state that reflects cubic symmetry on average. In this paper, we show that the set of experimental results can indeed be satisfactorily explained by DFT entirely within the cubic symmetry. We find that past theory limitations arose from the choice of exchange-correlation-functional approximations and that the inclusion of Fock exchange in hybrids significantly improves the DFT performance. We also find that the inclusion of nonlocal correlation effects is beneficial. We conclude by making a prediction for the phase-transition pressure for the transition from cubic to tetragonal symmetry at zero kelvin.
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| 5. |
- Jedvik Granhed, Erik, 1979
(författare)
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On hydrogen point defects in perovskite oxides
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Oxides based on the perovskite structure exhibit a surprisingly large diversity in materials properties and are found in many different applications, several related to clean energy technologies, such as solar cells, batteries and fuel cells. Many properties in materials are the result of lattice imperfections, commonly denoted \emph{defects}, and much effort is devoted to fine tuning materials properties through controlling the defects therein. Therefore, a thorough understanding of defect properties on a microscopic scale is desirable, and first-principles calculations have proven an invaluable tool in complementing experimental observations. In the present thesis density functional theory (DFT) calculations have been employed to describe two types of hydrogen point defects in perovskite oxides with the aim of deepening the understanding as well as to develop tools for modelling and characterising point defects. In paper I a strain tensor formalism for describing the anisotropic volume expansion of a point defect is developed. The formalism is successfully applied to the proton forming a hydroxide ion and the oxygen vacancy in acceptor-doped barium zirconate. It is inferred that both the hydroxide ion and the oxygen vacancy are smaller than the oxygen host ion, but that the difference in size causes an expansion in hydration which could lead to micro-cracking of the material. In paper II the substitutional hydride ion on an oxygen site in barium titanate is investigated. For this oxyhydride material two possible electronic states are permissible leading to different conductive properties; on the one hand the delocalised band-state as predicted by band theory and on the other hand a polaron state, in which an electron localises on one of the titanium ions next to the hydride ion, the description of which requires beyond DFT-methods. The two electronic states are investigated through their influence on the hydrogen vibrations, using both theoretical methods and inelastic neutron scattering measurements, and through their different volume expansion. The conclusion that the electronic state is predominantly band-like is confirmed both through the vibrational characterisation and the strain tensor formalism. The thesis reiterates the usefulness of first-principles calculations in assisting interpretation of experimental data.
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| 6. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
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Size and shape of oxygen vacancies and protons in acceptor-doped barium zirconate
- 2015
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 275, s. 2-8
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Tidskriftsartikel (refereegranskat)abstract
- The defect induced chemical expansion in acceptor-doped barium zirconate is investigated using density-functional theory (DFT) calculations. The two defect species involved in the hydration reaction, the +2 charged oxygen vacancy and the proton interstitial forming a hydroxide ion, are considered both as free defects and in association with the dopants Y, In, Sc and Ga. The defect induced strain tensor lambda is introduced, which provides a natural generalisation of the ordinary chemical expansion to three dimensions and to anisotropic distortions. Both the addition of a vacancy and a proton cause anisotropic distortions and a net contraction of the lattice, indicating that both the vacancy and the hydroxide ion are smaller than the oxygen ion. The contraction is considerably larger for the vacancy and the net effect in hydration, when a vacancy is filled and two protons are added, is an expansion, consistent with the experimental findings. The effect of the dopants on the chemical expansion in hydration is found to be quite small, even if it is assumed that both the vacancy and the proton are fully associated with a dopant atom in the lattice.
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| 7. |
- Jedvik Granhed, Erik, 1979
(författare)
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Vibrational and Structural Characterisation in Two Perovskite Challenges: A Density Functional Theory Study
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The modelling of perovskites using density functional theory (DFT) can sometimes be a challenge with many different states very close in energy. In particular, the tilting of the inscribed octahedron, as well as the formation of electron polarons, leads to states with energy differences in the meV range. To distinguish between these states requires special care. This thesis investigates how the vibrational frequencies and defect-induced strain, or chemical expansion, can be used to distinguish between different states. For the polaron state in oxyhydride BaTiO3, the comparison of calculations of hydrogen-ion vibrational frequencies to neutron scattering experiments is an excellent discriminator. The presence of polarons is deemed highly unlikely in unstrained material, despite the presence of oxygen vacancies. The observation is confirmed by comparisons of the strain tensor, calculated using a here-developed formalism. In BaZrO3 the likelihood of an anti-ferrodistortive phase transition is a direct consequence of the magnitude of the R25-mode frequency. The R25-mode frequency is strongly dependent on the lattice spacing, but it is shown that the main effect of the inclusion of gradient corrections, as well as non-local correlation, is secondary and is mostly a consequence of the adjusted lattice constant. The inclusion of Fock exchange, however, leads to a significant stabilisation of the cubic phase, which is also verified by neutron scattering measurements. This thesis also concludes that the inclusion of Fock exchange, as found in hybrid functionals, is essential for a correct description of vibrational properties in both two studied perovskites.
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| 8. |
- Perrichon, Adrien, 1988, et al.
(författare)
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Local Coordination Environments and Vibrational Dynamics of Protons in Hexagonal and Cubic Sc-Doped BaTiO3 Proton-Conducting Oxides
- 2020
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:16, s. 8643-8651
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Tidskriftsartikel (refereegranskat)abstract
- The proton local coordination environments and vibrational dynamics associated with the two order of magnitude change in proton conductivity in hydrated forms of hexagonal and cubic structured BaTi1-xScxO3Hx (0.16 < x < 0.7) were investigated using optical spectroscopy, neutron spectroscopy, and first-principles calculations. Whereas the cubic structure compositions display a single proton site, we show that protons occupy three distinct sites in compositions exhibiting the hexagonal structure. The principal site is characterized by interoctahedral hydrogen bonds, while two additional low occupancy sites are similar to those in the cubic structure, with classic intraoctahedral geometry. Furthermore, the proton hydrogen bond strength increases with decreasing scandium doping level. We infer from this that the stronger, more energetic hydrogen bonds in the hexagonal structure, resulting from proton sites with lower symmetry (lower multiplicity), are predominantly responsible for the significant reduction in macroscopic conductivity between cubic and hexagonal BaTi1-xScxO3Hx materials, rather than simply the absolute number of protons. Our findings are highly relevant to the field, clarifying the advantages of high-symmetry structures with high-multiplicity proton sites to favorable properties in ceramic proton-conducting oxides.
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| 9. |
- Perrichon, Adrien, 1988, et al.
(författare)
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Unraveling the ground-state structure of BaZrO3 by neutron scattering experiments and first-principle calculations
- 2020
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 32:7, s. 2824-2835
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Tidskriftsartikel (refereegranskat)abstract
- The all-inorganic perovskite barium zirconate, BaZrO3, is a widely used material in a range of different technological applications. However, fundamental questions surrounding the crystal structure of BaZrO3, especially in regard to its ground-state structure, remain. While diffraction techniques indicate a cubic structure all the way down to T = 0 K, several first-principles phonon calculation studies based on density functional theory indicate an imaginary (unstable) phonon mode due to the appearance of an antiferrodistortive transition associated with rigid rotations of ZrO6 octahedra. The first-principles calculations are highly sensitive to the choice of exchange-correlation functional and, using six well-established functional approximations, we show that a correct description about the ground-state structure of BaZrO3 requires the use of hybrid functionals. The ground-state structure of BaZrO3 is found to be cubic, which is corroborated by experimental results obtained from neutron powder diffraction, inelastic neutron scattering, and neutron Compton scattering experiments.
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