| 1. |
- Andersson, Ove, et al.
(författare)
-
Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH3
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:2, s. 024115-
-
Tidskriftsartikel (refereegranskat)abstract
- The potential hydrogen storage compound NH3BH3 has three known structural phases in the temperature and pressure ranges 110–300 K and 0–1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal I4mm phase and the low-temperature orthorhombic Pmn21 phase. Transformations of the I4mm phase into the high-pressure orthorhombic Cmc21 phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc21 phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH3BH3, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn21/Cmc21 phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [ Phys. Rev. B 79 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal I4mm phase is about 0.6 W m−1 K−1 and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.
|
|
| 2. |
- Araujo, Carlos Moyses, et al.
(författare)
-
Pressure-induced structural phase transition in NaBH4
- 2005
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:5, s. 054125-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a combined experimental and theoretical study of the technologically important NaBH4 compound under high pressure. Using Raman spectroscopy at room temperature, we have found that NaBH4 undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of NaBH4, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the alpha-LiAlH4–type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.
|
|
| 3. |
- Dmitriev, Vladimir, et al.
(författare)
-
Pressure-temperature phase diagram of LiBH4: Synchrotron x-ray diffraction experiments and theoretical analysis
- 2008
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 77:17
-
Tidskriftsartikel (refereegranskat)abstract
- An in situ combined high-temperature high-pressure synchrotron radiation diffraction study has been carried out on LiBH4. The phase diagram of LiBH4 is mapped to 10 GPa and 500 K, and four phases are identified. The corresponding structural distortions are analyzed in terms of symmetry-breaking atomic position shifts and anion ordering. Group-theoretical and crystal-chemical considerations reveal a nontrivial layered structure of LiBH4. The layers and their deformations define the structural stability of the observed phases.
|
|
| 4. |
- Filinchuk, Yaroslav, et al.
(författare)
-
Cation Size and Anion Anisotropy in Structural Chemistry of Metal Borohydrides. The Peculiar Pressure Evolution of RbBH4
- 2010
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 49:11, s. 5285-5292
-
Tidskriftsartikel (refereegranskat)abstract
- The pressure evolution of RbBH4 has been characterized by synchrotron powder X-ray diffraction and Raman spectroscopy up to 23 GPa. Diffraction experiments at ambient temperature reveal three phase transitions, at 3.0, 10.4, and 18 GPa (at 2.6, 7.8, and 20 GPa from Raman data), at which the space group symmetry changes in the order Fm-3m(Z=4) → P4/nmm(2) → C222(2) → I-42m(4). Crystal structures and equations of state are reported for all four phases. The three high-pressure structure types are new in the crystal chemistry of borohydrides. RbBH4 polymorphs reveal high coordination numbers (CNs) for cation and anion sites, increasing with pressure from 6 to 8, via an intermediate 4 + 4 coordination. Different arrangements of the tetrahedral BH4 group in the Rb environment define the crystal symmetries of the RbBH4 polymorphs. The structural evolution in the MBH4 series is determined by the cation’s size, as it differs drastically for M = Li (CNs = 4, 6), Na (CN = 6), and Rb. The only structure common to the whole MBH4 family is the cubic one. Its bulk modulus linearly decreases as the ionic radius of M increases, indicating that the compressibility of the material is mainly determined by the repulsive BH4···BH4 interactions.
|
|
| 5. |
- Sundqvist, Bertil, et al.
(författare)
-
Complex hydrides studied by Raman spectroscopy and thermal conductivity measurements under high pressure
- 2006
-
Ingår i: Hydrogen Storage Technologies. - Warrendale : Materials Research Society. ; , s. 7-12
-
Konferensbidrag (refereegranskat)abstract
- The pressure-temperature phase diagrams of alkali metal alanates and borohydrides are of large current interest, and we have recently studied phase transformations under pressure in several of these materials. We here report Raman studies of KBH4 under pressure at room temperature, showing a phase transition near 6 GPa. Although no structural information is yet available, the similarity between KBH4 and NaBH4 suggests the new structure is orthorhombic. We also report studies on LiBH4 showing that the high pressure phase of this material is metastable to zero pressure below 200 K.
|
|
| 6. |
- Sundqvist, Bertil, et al.
(författare)
-
Low temperature phase diagram of NH3BH3
- 2011
-
Ingår i: Materials Research Society Symposium Proceedings. - : Cambridge University Press.
-
Konferensbidrag (refereegranskat)abstract
- The pressure-temperature (p-T) phase diagram of NH3BH3 has been investigated by thermal conductivity measurements up to 1.5 GPa at temperatures between 100 and 300 K, and the phase boundaries between the three known structural phases have been identified. The transformation between the room temperature tetragonal I4mm phase and the low temperature orthorhombic Pmn21 phase (Tc = 218 K at p = 0) shows only a small hysteresis. The transformation into the high pressure orthorhombic Cmc21 phase (at 1.0 GPa near 292 K) has a very strong hysteresis, up to Δp = 0.5 GPa, and below 230 K a fraction of this phase is metastable even at atmospheric pressure.
|
|
| 7. |
- Sundqvist, Bertil, et al.
(författare)
-
Phase transitions in hydrogen storage compounds under pressure
- 2007
-
Ingår i: Journal of Physics Condensed Matter, vol. 19, issue 42. - Bristol : Intitute of Physics. ; , s. 425201-
-
Konferensbidrag (refereegranskat)abstract
- Solid, hydrogen-rich compounds, such as alkali metal hydrides, MAH4, where M is an alkali metal and A is boron or aluminium, may be used for hydrogen storage. We briefly review recent high-pressure work in this field aimed at exploring the phase behaviour, and especially the possibility to find highly dense new structures. In particular we present experimental data on the structure, lattice dynamics, phase diagrams, and thermal properties obtained by us and others by Raman scattering, x-ray diffraction, and thermal conductivity measurements under pressure between 100 and 400 K. From these data and the results of theoretical calculations we map observed structural phases and phase transitions in the pressure–temperature plane for the materials that have so far been investigated under pressure.
|
|
| 8. |
- Talyzin, Alexandr, et al.
(författare)
-
Gentle fragmentation of C60 by strong hydrogenation: a route to synthesizing new materials
- 2004
-
Ingår i: Chemical Physics Letters. - Amsterdam : Elsevier B.V.. - 0009-2614 .- 1873-4448. ; 400:1-3, s. 112-116
-
Tidskriftsartikel (refereegranskat)abstract
- A large number of small hydrogenated fullerenes and unusual polycyclic aromatic hydrocarbons were detected by mass-spectrometry in samples obtained by C60 reaction with hydrogen gas at 673 K. A synthesis of a large number of sequential breakdown products instead of fast formation of amorphous carbon was found due to immediate termination of dangling bonds by hydrogen atoms during the C60 fragmentation process. This opens a possibility to synthesize these unusual materials in bulk amounts. Investigation of breakdown products could also help to select building blocks for synthesis of cage structures by purely chemical methods.
|
|
| 9. |
- Talyzin, Alexandr, et al.
(författare)
-
High pressure phase transition in LiBH4
- 2007
-
Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596. ; 180:2, s. 510-7
-
Tidskriftsartikel (refereegranskat)abstract
- The high-pressure phase transition from ambient pressure alpha-LiBH4 to high-pressure beta-LiBH4 was observed by Raman spectroscopy and X-ray diffraction between 0.8 and 1.1 GPa. The phase boundary between these two phases was mapped over a large range of temperatures using thermal conductivity studies and differential thermal analysis. The structure of the high-pressure phase could not be identified due to small number of experimentally observed reflections, but it was shown that it is different from previously reported theoretical predictions.
|
|
| 10. |
- Talyzin, Alexandr, et al.
(författare)
-
High pressure study of NaAlH4 by Raman spectroscopy up to 17 GPa
- 2006
-
Ingår i: High Pressure Research. - : Informa UK Limited. - 0895-7959 .- 1477-2299. ; 26:3, s. 165-73
-
Tidskriftsartikel (refereegranskat)abstract
- A high-pressure Raman study was carried out on NaA1H(4) up to 17 GPa using the diamond anvil cell method. In the pressure region 2-5 GPa, several of the original modes split. Although this might be a sign of some structural change, the spectral changes do not allow us to claim the existence of a clear phase transition in this pressure range. The spectra revert to their ambient pressure forms on decreasing pressure below < 3.0-1.4 GPa. A phase transition to beta-NaA1H(4) was found at 14-16 GPa. This phase transition is also reversible with an unusually strong hysteresis: the beta-NaA1H(4) can be followed upon decompression down to 3.9 GPa. Analysis of Raman data shows that this phase transition is compatible with a theoretical prediction of a strong volume collapse.
|
|
