Adding a length scale to the polyamorphic ice debate

X-ray scattering and molecular dynamics simulations have been used to correlate the short range oxygen-oxygen structure with the intermediate range ordering (IRO) upon annealing very high density amorphous ice. While it is clear that the IRO that defines the network structure breaks down continuously to a minimum level, where there are weakened correlations extending beyond 7 Angstrom, at this point the local structure (O-O-O angles) is observed to change abruptly, allowing a continuous reemergence of a new IRO network. This is very different from a classic first order transition and helps reconcile previous data.     

Phonon dispersion of ice under pressure

We report measurements of the phonon dispersion of ice Ih under hydrostatic pressure up to 0.5 GPa, at 140 K, using inelastic neutron scattering. They reveal a pronounced softening of various low-energy modes, in particular, those of the transverse acoustic phonon branch in the [100] direction and polarization in the hexagonal plane. We demonstrate with the aid of a lattice dynamical model that these anomalous features in the phonon dispersion are at the origin of the negative thermal expansion (NTE) coefficient in ice below 60 K. Moreover, extrapolation to higher pressures shows that the mode frequencies responsible for the NTE approach zero at approximately 2.5 GPa, which explains the known pressure-induced amorphization (PIA) in ice. These results give the first clear experimental evidence that PIA in ice is due to a lattice instability, i.e., mechanical melting.     

Dielectric relaxation of low-density amorphous ice under pressure

In situ studies at high pressures show that the dielectric relaxation time tau of low-density amorphous (LDA) ice is more than an order of magnitude longer than that of high density amorphous ice. The increase in tau at the transformation to LDA ice with a simultaneous large density decrease shows that, despite an increase in the average intermolecular distance, the structural change leads to restriction for the orientational diffusion of H2O. The origin is most likely the same as in ice I, i.e., due to the ice rules. This result further stresses the crystallinelike nature of LDA ice.     

Phase transition of ZrN under pressure

A first principles constant pressure approach is carried out to probe the high-pressure behaviour of the rocksalt (RS) structured zirconium nitride (ZrN). The existence of first order reconstructive phase transition from the RS crystal to a CsCl-type crystal is, for the first time, established throughout the simulations. Upon decompression, the CsCl type phase converts back to the original RS structure by following the same transformation mechanism, suggesting a reversible phase transformation in ZrN. The RS-to-CsCl phase change is additionally considered through the thermodynamic theorem and projected to take place at around 225?GPa in experiments. The structural parameters and mechanical properties computed are found to be comparable with some of the previous findings. Additionally, we investigate the response of ZrN to uniaxial compression and tension stresses. The uniaxial stresses initially lead to a tetragonal modification of the simulation box having an I4/mmm symmetry and subsequently structural failure that is expected to occurs at about ?10 and 15?GPa in experiments.     

Semiconductor-to-metal transition in GaP under high pressure

The semiconductor-to-metal transition was observed in GaP under very high pressure exceeding 500 kbar. At the transition the electrical resistance decreased almost instantaneously by five orders of magnitude. This study was also carried out on BP and SiC and no drastic change was observed in their electrical resistance.     

Structural phase transition in FeBO under pressure

Using the density functional theory the structural and magnetic properties of iron borate under high pressure have been studied. At about P = 22.7 GPa a first order phase transition to the phase described by the same space group Rc has been found. The phase transition is accompanied by a 9% volume change of the unit cell, a four times decrease of the magnetic moment on Fe, an increase of the charge density at Fe, and a disappearance of the energy gap in the electronic density of states. Received 21 September 2001 and Received in final form 6 January 2002 Published online 6 June 2002     

Superionic transition in ice

The possibility of the proton system in ice to change its configuration is determined by the existence of Bjerrum and ionic defects. In this paper the concentrations of ionic defects are studied as a function of temperature. It is shown that at enough high temperatures the proton system undergoes a first order phase transition into the state with high concentrations of ionic defects. This transition is analogous to the transitions is ordinary superionic conductors. The possibility of experimental observation of this transition is discussed.     

Observation of a pressure-induced first-order polyamorphic transition in a chalcogenide glass at ambient temperature

An apparently first-order polyamorphic transition has been observed with increasing pressure at ambient temperature in a molecular glass of composition Ge(2.5)As(51.25)S(46.25) Raman spectroscopic measurements on pressure-quenched samples and in situ x-ray diffraction measurements indicate that this transition corresponds to a collapse of the ambient-pressure molecular phase to a high-pressure network phase. The high-pressure phase first appears at a pressure of approximately 8-9 GPa and the transformation becomes complete at approximately 14-15 GPa. Calorimetric measurements indicate that the low- and high-pressure phases are thermodynamically distinct and that they coexist in the transition range.     

Phase transition of samarium under high pressure

Abstract

The DAC X-ray power photograph method was employed for studing the phase transition of samarium up to 26.3 GPa. The experimental results show that the dhcp and fcc high pressure phase of Sm appeared at about 4.0 and 12.5 GPa and room temperature respectively. The dhcp phase was kept until 19.6 GPa. A model for Sm-type -? dhcp -? fcc phase transition is provided in this paper.     

Electronic structure of transition metals under pressure

Conclusions In this short review we have tried to demonstrate the advances made in the intensive study of the behavior of the electronic structure of metals under pressure in recent years. The alloys of transition metals, including intermetallic ordered compounds, have for the time being proved to lie outside the scope of this review, mainly because of the lack, in practice, of theoretical calculations in this area. Clearly, progress in this field is also being held up by the lack of detailed experimental investigations devoted to the behavior of alloys under pressure. This is due to the complexities involved in carrying out many traditional experimental procedures under high pressures. We should like to emphasize, however, that at the present time theoretical calculations have taken on a predictive power and this gives reason to hope that progress in calculations will in turn stimulate progress in experimentation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 50–62, December, 1982.     

Structural transition of FeSe under high pressure

The density functional calculations of the energy band structure and density of state for the tetragonal PbO-type phase α-FeSe and hexagonal NiAs-type phase β-FeSe are reported in this paper. The structural phase transition from tetragonal to hexagonal FeSe under high pressure is investigated, it is found that the calculated transition pressure for the α → β phase transformation is 8.5 GPa. Some fluctuations in the transition pressure maybe occurred by different external factors such as temperature and stress condition. There is about 17% volume collapse accompanying the α → β phase transformation.     

Metal-insulator transition of solid halogens under pressure

Abstract

The band structure of solid bromine and chlorine have been calculated at different pressures. Our calculations predict that there is a closure of the gap in the direction perpendicular to the molecular planes at pressures around 150, 330 and 670 kbar for I₂, Br₂ and Cl₂ respectively.     

Structural phase transition of GeAs under pressure

Abstract

We have measured the effect of pressure on structural properties and the optical response of the layered compound GeAs by means of x-ray powder diffraction and optical reflectivity. The low-pressure semiconducting phase is found to be stable up to 13 GPa. At higher pressures the compound crystallizes in the rocksalt structure. Reflectivity measurements show that the structural change is associated with a semiconductor-to metal-transition. Returning to lower pressures from the region of the cubic phase results in the formation of a metastable tetragonal phase.     

Semiconductor-metal phase transition in LaBi under high pressure

Physics of the Solid State - The effect of pressure up to 22 GPa on the electrical resistance and thermopower of lanthanum monobismuthide at room temperature has been studied. A semiconductor-metal...     

Deformation phase transition in vanadium under high pressure

Within the framework of the Landau theory of phase transitions and the density functional method, it is shown that the structural transformation from a body-centered cubic phase to a rhombohedral phase revealed in vanadium at 69 GPa is a first-order deformation phase transition close to a second-order phase transition.