Pressure-induced structural transformations

Tolbert S H et al 1996 Pressure-induced structural transformation in Si nanocrystais surface and shape effects Phys. Rev. Lett. 76 4384... 

Tolbert SH, Herhold AB, Brus LE, Alivisatos AP (1996) Pressure-induced structural transformations in Si nanocrystals Surface and shape effects. Phys Rev Letters 76 4384-4387 Wickham JN, Herhold AB, Alivisatos AP (2000) Shape change as an indicator of mechanism in the high-pressure structural transformations of CdSe nanocrystals. Phys Rev Letters 84 4515-4515... 

Cohen, R.E. (1991) Bonding and elasticity of stishovite (Si02) at high pressure-linearized augmented plane wave ealeulations, Amer. Mineral., 76, 733-742. Tsuneyuki, S. , Matsui, Y., Aoki, H., and Tsukada.,M. (1989) New pressure induced structural transformations in silica obtained by computer simulation. Nature, 339, 209-211. 

Structural transformation of silica at high pressure is not only geophysically important but also interesting for their variety due to the flexibility of the framework structure. Hemley et al. have found that polymorphs of silica undergoes amorphization under pressure, even at room temperature, just like H2O ice does at low temperature [45]. We have investigated the microscopic mechanism of this pressure-induced structural transformation by an MD simulation. 

Figure 8. Pressure-volume relation for low-quartz ( ), coesite (A), stishovite ( ) and low-eristobalite (o) obtained by MD. Solid lines represent continuous change and broken lines discontinuous change of volume due to pressure-induced structural transformation. Hysteresis is indicated hy arrows. Pressure-volume relation obtained by an experiment is shown in the inset (from Hemley et al. Reference 45).

Although the Cmcm phase of silica has not been found by real experiments, it has been shown that low-cristobalite in fact undergoes pressure-induced structural transformations at room temperature [19, 50], Very recently it was also confirmed that the displacive transformation from low-cristobalite structure to the Cmcm structure really occurs in case of c-GaP04, which probably has larger ionicity than silica. It should also be mentioned that Ga atoms easily turn into sixfold coordination, while P preserves fourfold coordination at high pressure. This tendency is in harmony with the alternate arrangement of the fourfold and sixfold coordinated cations in the Cmcm phase. 

Figure 10. Pressure-induced structural transformation obtained by MD simulations at room temperature, (a) A framework of comer-shared Si04 tetrahedra in a low-pressure phase (low-oristobalite), where silicon atoms are shown with circles. Oxygen atoms related with the structural transformation are numbered, (b) The framework structure deformed under pressure, (c) The framework structure after released pressure.

In summary, we have obtained a microscopic picture of the pressure-induced structural transformation of the polymorphs of silica at room temperature. In most cases the high-pressure phases highly retain structural order since the transformation occurs without diffusion process, although the tendency might be enhanced by the restricted size of the MD cell for the periodic boundary condition. It is conceivable that compression of a macroscopic sample produces small domains of ordered phases with random crystalline directions. The present result also suggest that the application of high pressure at room temperature may provide routes to new polymorphs, which cannot be obtained under equilibrium conditions. 

S. Tsuneyuki, Y. Matsui, H. Aoki, and M. Tsukada, Nature, 339,209 (1989). New Pressure-Induced Structural Transformations in Silica Obtained by Computer Simulation. 

Tsuneyuki S, Matsui Y, Aoki H, Tsukada M (1988b) New pressure-induced structural transformations in sihca obtained by computer simulation. Nature 339 209-211 Tsuneyuki S, Tsukada M, Aoki H, Matsui Y (1988a) First-principles interatomic potential of silica apphed to molecular dynamics. Phys Rev Lett 61 869-872... 

Rozenberg GK, Pasternak MP, Xu WM et al (2003) Pressure-induced structural transformation in the Mott insulator EeL. Phys Rev B 68 064105... 

Besides these thermotropic phase transitions, a variety of pressure-induced phase transformations can be observed," and it has been demonstrated that temperature and pressure have non-congruent effects on the structural and phase behaviour of these systems. 

Pressure-induced phase transformations are known to occur for a wide range of solids. Bulk Si, for example, has the diamond structure at ambient conditions but converts to the 3-tin structure at pressures around lOOkbar. Figure 2.5 shows how it is possible to use the kinds of information we have calculated in this chapter using DFT to predict the existence of pressure-induced phase transformations. It was essentially this idea that was used to make the geologically relevant predictions of the properties of minerals such as MgSi03 that were mentioned as one of the motivating examples in Section 1.2. 

As for krypton hydrate, Desgreniers et aU studied the pressure-induced phase transformations of krypton hydrate at room temperature by using X-ray diffraction measurements. They found that the initial cubic sll of krypton hydrate (KH-I) successively transformed to the cubic si (KH-II), the hexagonal structure (KH-III), and the sO (KH-IV) at 0.3 GPa, 0.6 GPa, and 1.8 GPa, respectively. They also found that the sO phase decomposed at pressures above 3.8 GPa. 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

Wang Z, Saxena SK, Pischedda V et al (2001) In situ X-ray diffraction study of the pressure-induced phase transformation in nanocrystaUine Ce02. Phys Rev B 64 012102 Liu H, Jin C, Zhao Y (2002) Pressure induced structural transitions in nanocrystaUine grained selenium. PhysicaB 315 210-214... 

Zahn D, Leoni S (2004) Mechanism of the pressure induced reconstructive transformation of KCl from the NaCl type to the CsCl type structure. Z Krist 219 345-347... 

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