| 1. |
- Eklöf-Österberg, Carin, 1987, et al.
(författare)
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Dynamics of Hydride Ions in Metal Hydride-Reduced BaTiO3 Samples Investigated with Quasielastic Neutron Scattering
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:4, s. 2019-2030
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxyhydrides, BaTiO3-xHx, have been recently shown to exhibit hydride-ion (H-) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO3 samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400-700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, <= 250 K, all of the models feature a characteristic jump distance of about 2.8 angstrom, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite lattice and a mean residence time between successive jumps of the order of 0.1 ns. At higher temperatures, >400 K, the jump distance increases to about 4 angstrom, thus of the order of the distance between next-nearest neighboring oxygen atoms or oxygen vacancies, with a mean residence time of the order of picoseconds. A diffusion constant D was computed from the data measured at low and high temperatures, respectively, and takes on values of about 0.4 X 10(-6) cm(-2) s(-1) at the lowest applied temperature of 225 K and between ca. 20 X 10(-6) and 100 X 10(-6) cm(-2) s(-1) at temperatures between 400 and 700 K. Activation energies E-a were derived from the measurements at high temperatures and take on values of about 0.1 eV and show a slight increase with increasing oxygen vacancy concentration.
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| 2. |
- Eklöf-Österberg, Carin, 1987
(författare)
-
Structure-dynamics relationships in perovskite oxyhydrides and alkali silanides
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on investigations of the local structure and dynamics of two classes of hydrogen-containing, energy relevant, materials; perovskite type oxyhydrides BaTiO3-xHx and alkali silanides ASiH3 (A = K and Rb). In the area of oxyhydrides, which are of relevance for the development of electrolytes in fuel cells and batteries, the aim is to elucidate the dynamics and electronic character of the hydrogen species in the materials. This is of great importance for developing new efficient synthesis routes and novel applications for oxyhydrides. The main tools of choice for these investigations are quasielastic and inelastic neutron scattering (QENS and INS, respectively). The results show that the hydride ion exhibits a long-range diffusion with a jump length corresponding to nearest neighbour (NN) jumps at low temperatures (225–250 K) and second nearest neighbour (2NN) jumps at high temperatures (400–700 K). Importantly, the hydride ion diffusivity was shown to be mediated by oxygen vacancies present in the material. Furthermore, the results from INS combined with density functional theory calculations show that the extra electron, originating from the hydride ion, forms a delocalised bandstate, as opposed to a localised polaronic state as suggested elsewhere. In the area of alkali silanides, which are of interest as hydrogen storage materials, the aim was to investigate the origin of the low entropy variation that these materials exhibit during the absorption/desorption process, using QENS. The results point towards complex dynamics, characterised by a quasi-spherical localised jump diffusion with 24 different preferred sites at high temperatures and slower C3 axis rotations as the dynamical motions starts to "freeze in" closer to the phase transition at lower temperatures. Specifically, at high temperatures the SiH3- ions are almost freely rotating, similar to how the ions behaves in a gas, which explains the origin of the low entropy variation and should be something to strive for when developing new hydrogen storage materials.
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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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