| 1. |
- Aleksis, Rihards, et al.
(författare)
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Probing the electronic structure and hydride occupancy in barium titanium oxyhydride through DFT-assisted solid-state NMR
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 24:46, s. 28164-28173
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxhydrides such as BaTiO3−xHy exhibit mixed hydride ion and electron conduction and are an attractive class of materials for developing energy storage devices. However, the underlying mechanism of electric conductivity and its relation to the composition of the material remains unclear. Here we report detailed insights into the hydride local environment, the electronic structure and hydride conduction dynamics of barium titanium oxyhydride. We demonstrate that DFT-assisted solid-state NMR is an excellent tool for differentiating between the different feasible electronic structures in these solids. Our results indicate that upon reduction of BaTiO3 the introduced electrons are delocalized among all Ti atoms forming a bandstate. Furthermore, each vacated anion site is reoccupied by at most a single hydride, or else remains vacant. This single occupied bandstate structure persists at different hydrogen concentrations (y = 0.13–0.31) and a wide range of temperatures (∼100–300 K).
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| 2. |
- Andersson, Ove, et al.
(författare)
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Evidence suggesting kinetic unfreezing of water mobility in two distinct processes in pressure-amorphized clathrate hydrates
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 24:34, s. 20064-20072
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Tidskriftsartikel (refereegranskat)abstract
- Type II clathrate hydrates (CHs) with tetrahydrofuran (THF), cyclobutanone (CB) or 1,3-dioxolane (DXL) guest molecules collapse to an amorphous state near 1 GPa on pressurization below 140 K. On subsequent heating in the 0.2-0.7 GPa range, thermal conductivity and heat capacity results of the homogeneous amorphous solid show two glass transitions, first a thermally weak glass transition, GT1, near 130 K; thereafter a thermally strong glass transition, GT2, which implies a transformation to an ultraviscous liquid on heating. Here we compare the GTs of normal and deuterated samples and samples with different guest molecules. The results show that GT1 and GT2 are unaffected by deuteration of the THF guest and exchange of THF with CB or DXL, whereas the glass transition temperatures (Tgs) shift to higher temperatures on deuteration of water; Tg of GT2 increases by 2.5 K. These results imply that both GTs are associated with the water network. This is corroborated by the fact that GT2 is detected only in the state which is the amorphized CH's counterpart of expanded high density amorphous ice. The results suggest a rare transition sequence of an orientational glass transition followed by a glass to liquid transition, i.e., kinetic unfreezing of H2O reorientational and translational mobility in two distinct processes.
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| 3. |
- B. Brant Carvalho, Paulo H., 1990-, et al.
(författare)
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Neutron scattering study of polyamorphic THF·17(H2O) : toward a generalized picture of amorphous states and structures derived from clathrate hydrates
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 25:21, s. 14981-14991
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Tidskriftsartikel (refereegranskat)abstract
- From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77-140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.
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| 4. |
- B. Brant Carvalho, Paulo H., 1990-
(författare)
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Pressure-Induced Amorphization and Distinct Amorphous States of Clathrate Hydrates
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis summarizes a study on the pressure-induced amorphization (PIA) and structures of amorphous states of clathrate hydrates (CHs).PIA involves the transition of a crystalline material into an amorphous solid in response of mechanical compression at temperatures well below the melting point. The first material observed to undergo PIA was hexagonal ice. More recently it was shown that compounds of water undergo the same phenomenon without decomposition, despite the presence of solutes. CHs, which are crystalline inclusion compounds consisting of water molecules encaging small guest species, undergo PIA at ca. 1–4 GPa below 145 K. The obtained amorphous CH phase can be further densified on isobaric heating at high pressure. This annealing step enables to retain an amorphous material on pressure release. There has been a significant amount of studies into the understanding of the nature of PIA and transformations between amorphous phases of pure ice. The aim of this thesis has been the understanding of the PIA in CHs and its relation to pure ice. New information on the nature of PIA and subsequent amorphous-amorphous transitions in CH systems were gained from structural studies and in situ neutron diffraction played pivotal role due to the sensitivity of neutrons to the light element hydrogen. Here a generalized understanding of the PIA in CHs and a clear image of amorphous CH structures are presented.
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| 5. |
- Barros Brant Carvalho, Paulo Henrique, et al.
(författare)
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Neutron scattering study of polyamorphic THF ∙ (H2O)17 – toward a generalized picture of amorphous states and structures derived from clathrate hydrates
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF ∙ 17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77–140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, very-HDA (VHDA), upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third, recovered amorphous (RA) form. Results from a compilation of neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF ∙ 17H2O solution (~2.5 M). The calculated density of (only in situ observable) HDA and VHDA at 2 GPa and 130 K is 1.287 and 1.328 g/cm3, respectively, whereas that of RA (at 1 atm) is 1.081 g/cm3. Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ~23 H2O. The local water structure in HDA is reminiscent of pure HDA-ice, featuring 5-coordinated H2O. In VHDA, this structure is maintained but the local water structure is densified to resemble pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ~18 H2O and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous, solid solutions of THF and water. The local water structure of water-rich (1:17) amorphous CHs resembles most that of the corresponding amorphous water ices when compared to guest-rich CHs, e.g., Ar ∙ ~6H2O. The proposed significance of different contributions of water local environments presents a simple view to justify neutron structure factor features.
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| 6. |
- Gebresenbut, Girma Hailu, et al.
(författare)
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Superconducting YAu3Si and Antiferromagnetic GdAu3Si with an Interpenetrating Framework Structure Built from 16-Atom Polyhedra
- 2022
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:10, s. 4322-4334
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of reaction mixtures REx(Au0.79Si0.21)100–x (RE = Y and Gd) yielded the compounds REAu3Si which adopt a new structure type, referred to as GdAu3Si structure (tP80, P42/mnm, Z = 16, a = 12.8244(6)/12.7702(2) Å, and c = 9.0883(8)/9.0456(2) Å for GdAu3Si/YAu3Si, respectively). REAu3Si was afforded as millimeter-sized faceted crystal specimens from solution growth employing melts with composition RE18(Au0.79Si0.21)82. In the GdAu3Si structure, the Au and Si atoms are strictly ordered and form a framework built of corner-connected, Si-centered, trigonal prismatic units SiAu6. RE atoms distribute on 3 crystallographically different sites and each attain a 16-atom coordination by 12 Au and 4 Si atoms. These 16-atom polyhedra commonly fill the space of the unit cell. The physical properties of REAu3Si were investigated by heat capacity, electrical resistivity, and magnetometry techniques and are discussed in the light of theoretical predictions. YAu3Si exhibits superconductivity around 1 K, whereas GdAu3Si shows a complex magnetic ordering, likely related to frustrated antiferromagnets exhibiting chiral spin textures. GdAu3Si-type phases with interesting magnetic and transport properties may exist in an extended range of ternary RE–Au–Si systems, similar to the compositionally adjacent cubic 1/1 approximants RE(Au,Si)∼6.
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| 7. |
- Gordeeva, Alisa, 1994-, et al.
(författare)
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Electronic structure characterization of TiO2-II with the α-PbO2 structure by electron-energy-loss-spectroscopy and comparison with anatase, brookite, and rutile
- 2023
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Ingår i: Journal of Solid State Chemistry. - : Academic Press. - 0022-4596 .- 1095-726X. ; 322
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Tidskriftsartikel (refereegranskat)abstract
- TiO2-II is a high pressure form of titania with a density about 2% larger than that of rutile. In contrast to the common polymorphs anatase, brookite and rutile its electronic structure and optical properties are poorly characterized. Here we report on a comparative electron-energy-loss-spectroscopy (EELS) study for which high resolution valence-loss and core-loss EELS data were acquired from nanocrystalline (<75 nm sized) titania particles with an energy resolution of about 0.2 eV. Electronic structure features revealed from titanium L3,2 and oxygen K electron energy loss near-edge structures show a strong similarity of TiO2-II with both rutile and brookite, which is attributed to similarities in the connectivity of octahedral TiO6 units with neighboring ones. From combined valence-loss EELS and UV-VIS diffuse reflectance spectroscopy data the band gap of TiO2-II was determined to be indirect and with a magnitude of ∼3.18 eV, which is very similar to anatase (indirect, ∼3.2 eV), and distinctly different from rutile (direct, ∼3.05 eV) and brookite (direct, ∼3.45 eV).
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| 8. |
- Grinderslev, J. B., et al.
(författare)
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Reorientational Dynamics in Y(BH4)3•xNH3 (x=0, 3, and 7) : The Impact of NH3 on BH4- Dynamics
- 2024
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Ingår i: The Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:11, s. 4431-4439
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Tidskriftsartikel (refereegranskat)abstract
- The reorientational dynamics of Y(BH4)3·xNH3 (x = 0, 3, and 7) was studied using quasielastic neutron scattering (QENS) and neutron spin echo (NSE). The results showed that changing the number of NH3 ligands drastically alters the reorientational mobility of the BH4– anion. From the QENS experiments, it was determined that the BH4– anion performs 2-fold reorientations around the C2 axis in Y(BH4)3, 3-fold reorientations around the C3 axis in Y(BH4)3·3NH3, and either 2-fold reorientations around the C2 axis or 3-fold reorientations around the C3 axis in Y(BH4)3·7NH3. The relaxation time of the BH4– anion at 300 K decreases from 2 × 10–7 s for x = 0 to 1 × 10–12 s for x = 3 and to 7 × 10–13 s for x = 7. In addition to the reorientational dynamics of the BH4– anion, it was shown that the NH3 ligands exhibit 3-fold reorientations around the C3 axis in Y(BH4)3·3NH3 and Y(BH4)3·7NH3 as well as 3-fold quantum mechanical rotational tunneling around the same axis at 5 K. The new insights constitute a significant step toward understanding the relationship between the addition of ligands and the enhanced ionic conductivity observed in systems such as LiBH4·xNH3 and Mg(BH4)2·xCH3NH2.
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| 9. |
- Huang, Yu-Chin, et al.
(författare)
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Structural Analysis of the Gd-Au-Al 1/1 Quasicrystal Approximant Phase across Its Composition-Driven Magnetic Property Changes
- 2023
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 62:36, s. 14668-14677
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Tidskriftsartikel (refereegranskat)abstract
- Gd14AuxAl86–x Tsai-type 1/1 quasicrystal approximants (ACs) exhibit three magnetic orders that can be finely tuned by the valence electron concentration (e/a ratio). This parameter has been considered to be crucial for controlling the long-range magnetic order in quasicrystals (QCs) and ACs. However, the nonlinear trend of the lattice parameter as a function of Au concentration suggests that Gd14AuxAl86–x 1/1 ACs are not following a conventional solid solution behavior. We investigated Gd14AuxAl86–x samples with x values of 52, 53, 56, 61, 66, and 73 by single-crystal X-ray diffraction. Our analysis reveals that increasing Au/Al ordering with increasing x leads to distortions in the icosahedral shell built of the Gd atoms and that trends observed in the interatomic Gd–Gd distances closely correlate with the magnetic property changes across different x values. Our results demonstrate that the e/a ratio alone may be an oversimplified concept for investigating the long-range magnetic order in 1/1 ACs and QCs and that the mixing behavior of the nonmagnetic elements Au and Al plays a significant role in influencing the magnetic behavior of the Gd14AuxAl86–x 1/1 AC system. These findings will contribute to improved understanding towards tailoring magnetic properties in emerging materials.
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| 10. |
- Joshi, Deep C., et al.
(författare)
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2D crystal structure and anisotropic magnetism of GdAu6.75−xAl0.5+x (x ≈ 0.54)
- 2022
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Exploration of the gold-rich part of the ternary Gd–Au–Al system afforded the intermetallic compound GdAu6.75−xAl0.5+x (x ≈ 0.54) which was structurally characterized by single crystal X-ray diffraction (Pnma, a = 18.7847(4) Å, b = 23.8208(5) Å, c = 5.3010(1) Å). GdAu6.75−xAl0.5+x crystallizes in a previously unknown structure type featuring layers of Gd2(Au, Al)29 and Gd2(Au, Al)28 clusters which are arranged as in a close-packing parallel to the ac plane. The Gd substructure corresponds to slightly corrugated 36 nets (dGd–Gd = 5.30–5.41 Å) which are stacked on top of each other along the b direction with alternating short (5.4, 5.6 Å, within layers) and long distances (6.4 Å, between layers). The title compound has been discussed with respect to a quasicrystal approximant (1/1 AC) GdAu5.3Al in the same system. The magnetic properties of GdAu6.75−xAl0.5+x were found to be reminiscent to those of some ternary ACs, with sharp peaks in the temperature dependent magnetization, and metamagnetic-like transitions. The material becomes antiferromagnetic below 25 K; magnetometry results suggest that the antiferromagnetic state is composed of ferromagnetic ac planes, coupled antiferromagnetically along the b direction.
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