Crystal structure pressure effects

According to the model, a perturbation at one site is transmitted to all the other sites, but the key point is that the propagation occurs via all the other molecules as a collective process as if all the molecules were connected by a network of springs. It can be seen that the model stresses the concept, already discussed above, that chemical processes at high pressure cannot be simply considered mono- or bimolecular processes. The response function X representing the collective excitations of molecules in the lattice may be viewed as an effective mechanical susceptibility of a reaction cavity subjected to the mechanical perturbation produced by a chemical reaction. It can be related to measurable properties such as elastic constants, phonon frequencies, and Debye-Waller factors and therefore can in principle be obtained from the knowledge of the crystal structure of the system of interest. A perturbation of chemical nature introduced at one site in the crystal (product molecules of a reactive process, ionized or excited host molecules, etc.) acts on all the surrounding molecules with a distribution of forces in the reaction cavity that can be described as a chemical pressure. 

Hazen R. M. (1977). Effects of temperature and pressure on the crystal structure of ferromag-nesian olivine. Amer. Mineral, 62 286-295. 

Hasegawa, R., Kobayashi, M. and Tadokoro, H. Molecular conformation and packing of poly(vinylidene fluoride). Stability of three crystalline forms and the effect of high pressure, Polymer J. 3, 591 (1972) Hasegawa, R., Takahashi, Y., Chatani, Y. and Tadokoro, H. Crystal structures of three crystalline forms of poly(vinylidene fluoride). Polymer J. 3, 600 (1972)... 

As molecular packing calculations involve just simple lattice energy minimizations another set of tests have focused on the finite temperature effects. For this purpose, Sorescu et al. [112] have performed isothermal-isobaric Monte Carlo and molecular dynamics simulations in the temperature range 4.2-325 K, at ambient pressure. It was found that the calculated crystal structures at 300 K were in outstanding agreement with experiment within 2% for lattice dimensions and almost no rotational and translational disorder of the molecules in the unit cell. Moreover, the space group symmetry was maintained throughout the simulations. Finally, the calculated expansion coefficients were determined to be in reasonable accord with experiment. 

Kudoh, Y., Ito, E. Takeda, H. (1987) Effect of pressure on the crystal structure of the perovskite-ype MgSiOj. Phys. Chem. Minerals, 14, 350-4. 

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