Pressure-induced phase transition

Svane A, Temmerman W and Szotek Z 1999 Theory of pressure-induced phase transitions in cerium chalcogenides Phys. Rev. B 59 7888... 

Just as one may wish to specify the temperature in a molecular dynamics simulation, so may be desired to maintain the system at a constant pressure. This enables the behavior of the system to be explored as a function of the pressure, enabling one to study phenomer such as the onset of pressure-induced phase transitions. Many experimental measuremen are made under conditions of constant temperature and pressure, and so simulations in tl isothermal-isobaric ensemble are most directly relevant to experimental data. Certai structural rearrangements may be achieved more easily in an isobaric simulation than i a simulation at constant volume. Constant pressure conditions may also be importai when the number of particles in the system changes (as in some of the test particle methoc for calculating free energies and chemical potentials see Section 8.9). 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

H. Arashi, Raman spectroscopic study of the pressure-induced phase transition in Ti02, J. Phys. Chem. 

Generally, the following rules apply for pressure-induced phase transitions Pressure-coordination rule by A. Neuhaus with increasing pressure an increase of the coordination number takes place. 

Gas-phase results provide insight into the reaction pathways for isolated HE molecules however, the absence of the condensed-phase environment is believed to affect reaction pathways strongly. Some key questions related to condensed-phase decomposition are as follows (1) How do the temperature and pressure affect the reaction pathways (2) Are there temperature or pressure-induced phase-transitions that play a role in the reaction pathways that may occur (3) What happens to the reaction profiles in a shock-induced detonation These questions can be answered with condensed-phase simulations, but such simulations would require large-scale reactive chemical systems consisting of thousands of atoms. Here we present results of condensed-phase atomistic simulations, which are pushing the envelope toward reaching the required simulation goal. 

A second, less common kind of pressure-induced phase transition (but in one instance at least, of very great importance) is that in which there is no change in primary coordination number and, to a good approximation, no change in nearest-neighbour bond lengths but, nevertheless, a substantial decrease in volume. We consider next, two examples of this latter type of phase transition. 

Variable-temperature X-ray diffraction studies of crystalline substances are useful in the study of phase transitions, thermal expansion and thermal vibrational amplitudes of atoms in solids. Similarly, diffraction studies at high pressures are employed to examine pressure-induced phase transitions. Time-resolved X-ray diffraction studies (Clark Miller, 1990) will be of great value for examining reactions and other transformations. 

Fig. 12. Panel (a) Calculated isotherms for HMX polymorphs, p<10.6 GPa. Circles p-HMX triangles a-HMX squares <5-HMX. Lines connecting data points are only intended as a guide for the eye. Panel (b) blow-up of low-pressure isotherms for /3-HMX (circles) and a-HMX (triangles), showing break in a-HMX result corresponding to pressure-induced phase transition.

Figure 4-29 Pressure dependence of (a) the low-frequency region and (b) high-frequency region Raman peaks of CsVC The vertical dashed lines mark the pressure-induced phase transitions. The symbols (w = weak, m = medium, s = strong) signify the relative intensity of the Raman peaks. (Reproduced with permission from Ref. 50. Copyright 1991 John Wiley Sons, Ltd.)...

By using of the high pressure apparatus, shown at fig. 1 - fig. 3, the pressure-induced phase transitions in some semiconductors were investigated up to 30 GPa [9,10]. The technique of S measurements supposedly will be in detail described in the special paper. Here we only outline the pecular features of it. 

Only few direct pressure syntheses have been reported. LiTiMF6 phases (M = Mn—Ni) were prepared from respective binary fluorides at T = 700-1200°C and 15-70 kbar [27]. The products crystallized in Na2SiF6 and PbSb206 type structures. On the other hand, pressure induced phase transitions are common, e.g. KMnF3 transforms from cubic to tetragonal symmetry. However, in contrast to oxide containing perovkites, the transition temperature rises with increasing pressure [28]. 

Cohen, A. J., and R. G. Gordon (1975). Theory of the lattice energy, equilibrium structure, elastic constants, and pressure-induced phase transitions in alkali-halide crystals. Phys. Rev. B12, 3228 1. 

There are several possible origins for the small intercalate-specific contribution. The first is suggested by the complex Fermi surface of K3 60 (16). In addition to bandwidth variations, which depend directly on the lattice parameter, the volume of the Fermi surface may be subdy different for different intercalates, or may vary with lattice parameter or pressure, or both effects could occur. Another possibility is a pressure-induced phase transition involving molecular orientations. In pure C, fiee molecular rotations freeze out at = 249 K at 1 bar, locking into specific orientations with respect to the crystal axes (17), and... 

Lacks DJ, Gordon RG (1993) Calculations of pressure-induced phase transitions in silica. J Geophys Res 98 22147-22155... 

Peercy PS, Fritz IJ (1974) Pressure-induced phase transition in paratellurite (Te02). Phys Rev Lett 32 466-469... 

Ikeda T, Kobayashi T, Takata M, Takayama T, Sakata M (1998) Charge density distributions of strontium titanate obtained by the maximum entropy method. Solid State Ion 108 151-157 Imai M, Mitamura T, Yaoita K, Tsuji K (1996) Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures. High Pressure Res 15 167-189... 

Another example is provided by pressure-induced phase transitions in which a cusp catastrophe transforms a double-well of the potential energy curve into a single-well and vice versa. In a wide pressure range, between 10 and 100 GPa, stishovite is the stable modification of silica. At about 100 GPa, a phase transition from PA2/mnm. (rutile) to Pmnm(CaCl2) occurs which corresponds to the twinning of the initial tetragonal cell... 

The IR spectrum of Ba3[BN2]2 shows low site symmetry for the BN23 groups.68 Vibrational data for Eu3[BN2]2, however, were interpreted in terms of discrete BN23- units of D jh symmetry.69 IR data were reported for a 1,3,2-oxazaborolidine dimer derived from (V)-a,a-diphenylprolinol.70 The IR and Raman spectra of the new adduct P8012.2BH3 included vPB at 565 cm-1 (IR), 574 cm-1 (Raman), as well as characteristic vBH bands.71 High-pressure Raman spectroscopy was used to follow pressure-induced phase transitions for B12As2.72... 

There have been numerous papers in which IR and Raman spectroscopies were used to follow thermally- and pressure-induced phase transitions, as well... 

High-pressure and —temperature Raman spectroscopy was used to study carbonate ions in aqueous solution in the ranges 1-30 GPa and 25-400°C.353 IR and Raman spectra were used to study the pressure-induced phase transition (2.8 GPa) for KHC03.354... 

High-pressure (to 40 GPa) Raman spectra were used to probe pressure-induced phase transitions for p-Ge3N4.475... 

Correspondingly, low-symmetry atomic or molecular arrangements comprising different types of chemical bonds normally exhibit pronounced anisotropy of the compressibility. Structural reorganizations due to pressure-induced phase transitions are associated with discontinuous volume decreases and normally increasing coordination numbers. These structural changes not only modify the coordination environment in the crystal structure but frequently also the electronic properties of the solid. 

Temperature-induced phase transitions are typically associated with small volume changes so that the change of the internal energy is dominated by the entropy term TAS. For pressure-induced phase transitions with volume changes of the order of 10%, the PAF term dominates and the entropy term can be neglected at room temperature. The corresponding work AW is defined as ... 

For instance, Raman spectroscopy was used to study the effect of pressure and temperature on the phase composition of fluoranil crystals. Figure 2 shows the Raman spectra obtained at a series of increasing pressures, where the changes in band frequency indicate the existence of pressure-induced phase transitions. It was deduced from sharp discontinuities in the Raman spectra that a phase transition took place at a temperature of around 180 K if the pressure was 1 atm, but that this transition shifted to 300 K if the pressure was increased to 0.8 GPa. Other work indicates that this particular phase transition does not entail a change in the crystal space group, but involves displacement within the unit cell. 

At room temperature the pressure-induced phase transition in SmS differs from those in SmSe and SmTe in that it is discontinuous (55). It is not known whether it remains discontinuous at T=0. It was pointed out by Davis (56) that the two modes of behavior could be explained by assuming different mechanisms for the transition, i.e. an f6 - f5d delocalization in SmS and a simple Se(4p)—Sm(55) band-gap closing in the selenide and telluride. This suggestion was supported by APW calculations of the band structure of the Sm monochalcogenides. In the case of SmS the 4/states were found to lie in the band gap, but in SmSe and SmTe they were located below the p-valence bands,... 

---