| 51. |
- Andersson, Ove, et al.
(författare)
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Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH3
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:2, s. 024115-
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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.
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| 52. |
- Andersson, Ove, et al.
(författare)
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Pressure-induced collapse of ice clathrate and hexagonal ice mixtures formed by freezing
- 2009
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics Publishing LLC. - 0021-9606 .- 1089-7690. ; 131, s. 114503-114513
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Tidskriftsartikel (refereegranskat)abstract
- We report thermal conductivity κ measurements of the pressure-induced collapse of two mixtures of ice and tetrahydrofuran (THF) clathrate hydrate formed by freezing aqueous solutions, THF·23 H2O and THF·20 H2O, one containing twice as much excess water than the other. On pressurizing, κ of the solid mixture first decreases at the onset pressure of 0.8 GPa, as occurs for collapse of pure ice, reaches a local minimum at a pressure of 1.0 GPa, and then increases as occurs for the collapse of the pure clathrate THF·17 H2O. This shows that in the apparently homogeneous mixture, the ice and the clathrate collapse as if the two were in a mechanically mixed state. The manner in which the clathrate aggregate can arrange in the solid indicates that ice occupies the interstitial space in the tightly packed aggregates and H2O molecules belonging to the lattice of one form hydrogen bond with that of the other, a feature that is preserved in their collapsed states. On decompression, the original clathrate is partially recovered in the THF·20 H2O mixture, but the collapsed ice does not transform to the low density amorph. We surmise that on irreversible transformation to the original clathrate, the aggregates expand. Any pressure thus exerted on the small domains of the collapsed ice with a hydrogen bonded interface with the clathrate aggregates could prevent it from transforming to the low density amorph. Measurements of κ are useful in investigating structural collapse of crystals when dilatometry is unable to do so, as κ seems to be more sensitive to pressure-induced changes than the volume.
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| 53. |
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| 54. |
- Andersson, Ove, et al.
(författare)
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Reorientational relaxation in C60 following a pressure induced change in the pentagon/hexagon equilibrium ratio
- 1995
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Ingår i: Physics Letters A. - : Elsevier Science B.V.. - 0375-9601 .- 1873-2429. ; 206:3-4, s. 260-264
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Tidskriftsartikel (refereegranskat)abstract
- The orientational structure of C60 depends on pressure and temperature. Pressurization below the glass transition temperature Tg can freeze in non-equilibrium orientational structures. The relaxation of such structures on heating through Tg has been studied through thermal conductivity measurements and the effects observed are explained in a simple model.
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| 55. |
- Andersson, Ove, et al.
(författare)
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Spontaneous transformation of water's high-density amorph and a two-stage crystallization to ice VI at 1 GPa : a dielectric study
- 2004
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 120:24, s. 11662-11671
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Tidskriftsartikel (refereegranskat)abstract
- Dielectric relaxation spectra of a metastable crystal phase formed on implosive and exothermic transformation of pressure-amorphized hexagonal ice have been measured in situ at 0.97 GPa pressure over a range of temperature. The metastable phase showed no relaxation peak at 130 K and 0.97 GPa. When heated at a fixed pressure of 0.97 GPa, it began to transform at ∼ 145 K exothermally to a phase whose relaxation rate and equilibrium dielectric permittivity increased. A second, but slower exothermic transformation also occurred at ∼ 175 K. After keeping at 213 K, the relaxation rate and equilibrium permittivity reached the known values of these two quantities for ice VI. Thus the metastable phase transformed to ice VI in two stages. It is conjectured that the intermediate phase in this transformation could be ice XII. The rate of transformation is not determined by the reorientational relaxation rate of water molecules in the ices
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| 56. |
- Andersson, Ove, et al.
(författare)
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Sub-Tg features of glasses formed by cooling glycerol under pressure – Additional incompatibility of vibrational with configurational states in the depressurized, high density glass
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145
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Tidskriftsartikel (refereegranskat)abstract
- The vibrational state of a glass is naturally incompatible with its configurational state, which makes the glass structurally unstable. When a glass is kept at constant temperature, both the vibrational and configurational states of a glass change with time until it becomes metastable (equilibrium) liquid and the two states become compatible. The process, known as structural relaxation, occurs at a progressively higher rate during heating, and the properties of a glass change accordingly. We add to this incompatibility by depressurizing a glass that had been formed by cooling a liquid under a high pressure, p, and then investigate the effects of the added incompatibility by studying thermal conductivity, κ, and the heat capacity per unit volume ρCp of the depressurized glass.We use glycerol for the purpose and study first the changes in the features of κ and of ρCp during glass formation oncooling under a set of different p. We then partially depressurize the glass and study the effect of the p-induced instability on the features of and Cp as the glass is isobarically heated to the liquid state.At a given low p, the glass configuration that was formed by cooling at high-p had a higher κ than the glass configuration that was formed by cooling at a low p. The difference is more when the glass is formed at a higher p and/or is depressurized to a lower p. On heating at a low p, its κ decreases before its glass-liquid transition range is reached. The effect is the opposite of the increase in observed on heating a glass at the same p under which it was formed. It is caused by thermally assisted loss of the added incompatibility of configurational and vibrational states of a high-p formed glass kept at low p. If a glass formed under a low-p is pressurized and then heated under high p, it would show the opposite effect, i.e., its κ would first increase to its high p value before its glass-to-liquid transition range.
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| 57. |
- Andersson, Ove, et al.
(författare)
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Thermal conductivity, heat capacity and phase diagram of cyclooctanol in liquid, solid and glassy crystal states under high pressure
- 1990
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Ingår i: Molecular Physics. - : Taylor & Francis Group. - 0026-8976 .- 1362-3028. ; 71:3, s. 523-539
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Tidskriftsartikel (refereegranskat)abstract
- Using the transient hot-wire method, thermal conductivity and heat capacity per unit volume are measured for solid and liquid phases and glassy crystal states of cyclooctanol, and information is provided on the phase diagram under high pressure. A new solid phase (III) is detected and characterized as a normal crystal phase, whereas all other solid phases (I, II, IV, V) are characterized as plastic crystal phases. We find evidence that the plastic crystal phases I, II and IV could each be the source for a distinct glassy crystal state. It is argued for phase II that its possession of both a low thermal conductivity and a low dielectric permittivity could be accounted for by assuming restricted reorientational motion of the molecules. The unusual (although small) decrease of thermal conductivity observed through the glassy to plastic crystal transitions may indicate that phonons can couple to reorientational motion in the plastic crystal phases I, II and IV.
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| 58. |
- Andersson, Ove, et al.
(författare)
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Thermal conductivity of (+)- and (±)-camphor at pressures up to 0·5 GPa and temperatures down to 40 K
- 1992
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Ingår i: Molecular Physics. - : Taylor & Francis. - 0026-8976 .- 1362-3028. ; 76:2, s. 433-444
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Tidskriftsartikel (refereegranskat)abstract
- The transient hot-wire method was used to investigate the thermal conductivity of (+)- and (±)-camphor in the temperature range 40–300 K and for pressures up to 0·5 GPa. Results for the thermal conductivity of crystalline (+)- and (±)-camphor exhibited maximum values at temperatures of 60 and 70 K, respectively, independent of pressure. Above the maxima, the thermal conductivity λ exhibited a less pronounced temperature dependence than the variation λ ∼T-1 which is predicted theoretically for perfect crystals. We showed that the Debye theory can describe the thermal conductivity of molecular crystals like (+)- and (±)-camphor when structural disorder is assumed to provide a significant contribution to the phonon scattering process. We investigated two samples of (±)-camphor of estimated purity 99·3% and 97% and both samples exhibited the same thermal conductivity within experimental inaccuracy.
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| 59. |
- Andersson, Ove, et al.
(författare)
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Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K
- 1996
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54:5, s. 3093-3100
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Tidskriftsartikel (refereegranskat)abstract
- The thermal conductivity λ of C60 shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K GPa-1 and 62 K GPa-1, respectively. With increasing temperature, λ of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of λ is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of dλ/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T≳260 K at atmospheric pressure) λ was almost independent of temperature, a behavior which is far from that of an ordered crystal (λ∝T-1 for T≳Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of λ, (∂ lnλ/∂ lnρ)T, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively.
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| 60. |
- Andersson, Ove, et al.
(författare)
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Thermal conductivity of C60 under high pressure
- 1995
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Ingår i: Science and Technology of Fullerene Materials. - Pittsburgh, PA : Materials Research Society. - 155899260X ; , s. 549-554
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Konferensbidrag (refereegranskat)abstract
- We have measured the thermal conductivity lambda of highly pure polycrystalline C60 in the range 50 to 300 K under pressures up to 1 GPa. The results are discussed in terms of the lattice structure and dynamics. In particular, we discuss the phase diagram as delineated by anomalies observed in lambda and cp at the f.c.c.-to-s.c. transition at 260 K and the glass transition at Tg = 90 K, and also the effect on lambda of the orientational motion in the s.c. phase. The results are found to be compatible with a p/T phase diagram recently suggested by us.
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