| 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. |
- Auer, Henry, et al.
(författare)
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The Hydrogenation of the Zintl Phase NdGa Studied by in situ Neutron Diffraction
- 2019
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Ingår i: Zeitschrift für Anorganische und Allgemeines Chemie. - : Wiley. - 0044-2313 .- 1521-3749. ; 645:3, s. 175-181
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Tidskriftsartikel (refereegranskat)abstract
- The hydrogenation of the Zintl phase NdGa was studied by in situ neutron powder diffraction. We find a compositional range of 0.1 < x < 0.8 in NdGaH1+x. Hydrogen atoms are located in two different positions, in HNd4 tetrahedra, and close to the polyanionic chains. For the latter, the Ga-H distance in NdGaH1.66 is quite long (ca. 200 pm) with a trigonal bipyramidal Nd3Ga2 surrounding of hydrogen atoms. Hydrogen poor NdGaH<1 phases as known for similar systems were not observed. The changing hydrogen content shows no measureable effect on the unit cell volume, but on lattice parameter ratios. Superstructures occur for 0.53 < x < 0.66 and 0.73 < x < 0.8, leading to a doubling or tripling of the lattice parameter a. They are probably caused by partial hydrogen ordering. The threefold superstructure contains a (1)[(Ga-H-Ga-H-Ga)(6-)] moiety with hydrogen bridging two gallium chains.
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| 3. |
- Ek, Gustav, et al.
(författare)
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Hydrogen induced structure and property changes in Eu3Si4
- 2019
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 277, s. 37-45
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Tidskriftsartikel (refereegranskat)abstract
- Hydrides Eu3Si4H2-X were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 degrees C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25-200 degrees C afford a uniform hydride phase with an orthorhombic structure (Immm, a approximate to 4.40 angstrom, b approximate to 3.97 angstrom, c approximate to 19.8 angstrom), whereas at 300 degrees C mixtures of two orthorhombic phases with c approximate to 19.86 and approximate to 19.58 angstrom were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H-2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns pointed to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x
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| 4. |
- 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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| 5. |
- 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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| 6. |
- Guo, Hua, et al.
(författare)
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Trapping of different stages of BaTiO3 reduction with LiH
- 2020
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 10:58, s. 35356-35365
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the hydride reduction of tetragonal BaTiO3 using LiH. The reactions employed molar H : BaTiO3 ratios of 1.2, 3, and 10 and variable temperatures up to 700 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy, thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and 1H magic-angle spinning (MAS) NMR spectroscopy. Effective reduction, as indicated by the formation of dark blue to black colored, cubic-phased, products was observed at temperatures as low as 300 °C. The product obtained at 300 °C corresponded to oxyhydride BaTiO∼2.9H∼0.1, whereas reduction at higher temperatures resulted in simultaneous O defect formation, BaTiO2.9−xH0.1□x, and eventually – at temperatures above 450 °C – to samples void of hydridic H. Concomitantly, the particles of samples reduced at high temperatures (500–600 °C) display substantial surface alteration, which is interpreted as the formation of a TiOx(OH)y shell, and sintering. Diffuse reflectance UV-VIS spectroscopy shows broad absorption in the VIS-NIR region, which is indicative of the presence of n-type free charge carriers. The size of the intrinsic band gap (∼3.2 eV) appears only slightly altered. Mott–Schottky measurements confirm the n-type conductivity and reveal shifts of the conduction band edge in the LiH reduced samples. Thus LiH appears as a versatile reagent to produce various distinct forms of reduced BaTiO3 with tailored electronic properties.
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| 7. |
- Nedumkandathil, Reji, et al.
(författare)
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Hydride Reduction of BaTiO3 ? Oxyhydride Versus O Vacancy Formation
- 2018
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 3:9, s. 11426-11438
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4, and NaAlH4. The reactions employed molar BaTiO3/H ratios of up to 1.8 and temperatures near 600 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA), and 1H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products - indicative of the formation of BaTiO3-xHx - except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4-reduced samples) and 4.033 Å (for MgH2-reduced samples). With increasing H/BaTiO3 ratio, CaH2-, NaAlH4-, and MgH2-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-xHx with x values larger than 0.5. 1H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-xHy?(x-y) with x up to 0.6 and y in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO3-xHx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the ?2 to ?60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO3-xHy?(x-y). Sintering of BaTiO3-xHy?(x-y) in a gaseous H2 atmosphere resulted in more ordered materials, as indicated by considerably sharper 1H resonances.
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| 8. |
- Nedumkandathil, Reji, et al.
(författare)
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Hydride Reduction of BaTiO3 – Oxyhydride vs O-Vacancy Formation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4 and NaAlH4. The reactions employed molar BaTiO3:H ratios of up to 1.8 and temperatures near 600 °C. The air stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA) and solid-state 1H NMR spectroscopy. PXRD showed the formation of cubic products - indicative of the formation of BaTiO3-xHx - except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4 reduces samples) and 4.033 Å (for MgH2 reduced samples). With increasing BaTiO3:H ratio, CaH2, NaAlH4 and MgH2 reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increase of up to 3.5 % for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-xHx with x values larger 0.5. 1H NMR, however, revealed rather low concentrations of H, and, thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-xHy□(x-y) with x up to 0.6, y in a range 0.05 – 0.2 and (x-y) > y, rather than homogeneous solid solutions BaTiO3Hx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of chemical shifts and breadth of the signals signifies the structural disorder in BaTiO3-xHy□(x-y). Sintering of BaTiO3-xHy□(x-y) in a gaseous H2 atmosphere resulted in more ordered materials as indicated by considerably sharper 1H resonances.
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| 9. |
- Nedumkandathil, Reji, et al.
(författare)
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Hydrogen Induced Structure and Property Changes in Eu3Si4
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Hydrides Eu3Si4H2+x were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 °C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25 – 200 ºC afford a uniform hydride phase with an orthorhombic structure (Immm, a ≈ 4.40 Å, b ≈ 3.97 Å, c ≈ 19.8 Å), whereas at 300 ºC mixtures of two orthorhombic phases with c ≈ 19.86 and ≈19.58 Å were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H to be occupied in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns point to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. However, the ordered monoclinic model structures do not provide a good match to the experimental, orthorhombic, PXRD patterns. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x < 2) and are disordered with respect to H in Si2Eu3 interstitials. Hydrides Eu3Si4H2+x decompose at temperatures above 300 °C in a dynamic vacuum into unidentified products. Thus the hydrogenation of Eu3Si4H2+x is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of Eu3Si4H2+x were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 mB which is typical for compounds containing Eu2+ 4f7 ions.
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| 10. |
- Nedumkandathil, Reji, et al.
(författare)
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Hydrogenation induced structure and property changes in GdGa
- 2016
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 239, s. 184-191
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Tidskriftsartikel (refereegranskat)abstract
- Hydrides GdGaH were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50 bar and temperatures from 50 to 500 degrees C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50 bar and temperatures below 500 degrees C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) angstrom, b=12.024(2) angstrom, c=4.1009(6) angstrom) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200 degrees C affords a hydride with x approximate to 1.3, which is stable up to 500 degrees C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200 degrees C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300 degrees C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of similar to 8 mu B which is typical for compounds containing Gd3+ ions.
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| 11. |
- Nedumkandathil, Reji, et al.
(författare)
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Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH3
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:9, s. 5241-5252
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Tidskriftsartikel (refereegranskat)abstract
- The β–α (order–disorder) transition in the silanides ASiH3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (Cp), solid-state 2H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH3– moieties. At temperatures below 200 K ASiH3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH3 and KSiH3, respectively. The transition is accompanied by a large molar volume increase of about 14%. The Cp(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH3– rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH3, the SiH3– anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). 2H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from Cp measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH3– ions are only weakly coordinated in an environment of A+ cations. The orientational flexibility of SiH3– can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.
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| 12. |
- Nedumkandathil, Reji, et al.
(författare)
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The 3R polymorph of CaSi2
- 2015
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 222, s. 18-24
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Tidskriftsartikel (refereegranskat)abstract
- The Zintl phase CaSi2 commonly occurs in the 6R structure where puckered hexagon layers of Si atoms are stacked in an AA'BB'CC' fashion. In this study we show that sintering of CaSi2 in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 degrees C transforms 6R-CaSi2 quantitatively into 3R-CaSi2. In the 3R polymorph (space group R-3m (no. 166), a=3.8284(1), c=15.8966(4), Z=3) puckered hexagon layers are stacked in an ABC fashion. The volume per formula unit is about 3% larger compared to 6R-CaSi2. First principles density functional calculations reveal that 6R and 3R-CaSi2 are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi2 stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi2 should revert to 6R at elevated temperatures, which however is not observed up to 800 degrees C. 3R-CaSi2 may be stabilized by small amounts of incorporated hydrogen and/or defects.
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| 13. |
- Spektor, Kristina, et al.
(författare)
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Crystallization of mesoporous silica SBA-15 in a high pressure hydrothermal environment
- 2017
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Ingår i: High Pressure Research. - : Taylor & Francis. - 0895-7959 .- 1477-2299. ; 37:3, s. 345-359
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Tidskriftsartikel (refereegranskat)abstract
- Mesoporous silica SBA-15 (with similar to 6 nm pore size and similar to 6 nm wall thickness) was exposed to a hydrothermal environment at 2 and 5GPa. The p,T quenched products were investigated by powder X-ray diffraction and transmission electron microscopy. Infrared spectroscopy and thermogravimetric analysis of a sample subjected to 5GPa at room temperature suggests functionalization of both inner and outer pore surface by silanol. Partial transformation to nano-sized (20-50 nm) coesite crystals with nonfaceted morphology was observed during short equilibration times of 2h at 125 degrees C, which is significantly below the melting point of water (similar to 250 degrees C). Untransformed SBA-15 maintained intact pore structure. At 175 degrees C and during 8h, SBA-15 transformed completely into faceted coesite crystals with dimensions 100-300 nm, suggesting Ostwald ripening and thus significant mass transport in the solid water environment. At 2GPa the melting point of water is near 70 degrees C. Partial transformation to nano-sized alpha-quartz was observed at 65 degrees C and during 2h. Untransformed SBA-15 partially pore collapsed. The reduced pore stability of SBA-15 at 2GPa is attributed to the presence of liquid water in the pores due to melting point depression of confined water.
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