STRUCTURE DETERMINATION AT HIGH PRESSURES FROM MOLECULAR DYNAMICS SIMULATIONS. S. K. Sikka, Surinder M. Sharma, R. Chidambaram, High Pressure Physics Division, Bhabha Atomic Research Center Bombay - 400 085

We describe the use of molecular dynamics (MD) simulations to determine the structures of some high pressure phases which give only a few Bragg peaks in their diffraction patterns. In [[alpha]]-quartz, Kingma et al¹ had reported the diffraction pattern of a new phase before amorphization at 21 GPa. Using the coordinates derived from MD simulations, employing the pair potentials of Tsuneyuki et al², we got x-ray patterns in excellent agreement with experimental results - indicative of a tripling of the [[alpha]]-quartz unit cell in its basal plane. A monoclinic cell could then be derived.³

In LiCsSO₄, there has been a controversy about the number and nature of pressure induced phases. We have carried out x-ray diffraction studies on this and have detected two c-c transitions at 4 and 7 GPa and amorphization at ~ 19 GPa⁴. 0ur data rule out the earlier conjecture that the 4 GPa transition is the orthorhombic to monoclinic (a low temperature phase) one. Instead the x-ray pattern here strongly resembles the one calculated from the coordinates of 9.8 GPa phase of LiKSO₄ as computed by an earlier MD simulation⁵. This new structure is related to the ambient one through the softening of the (1/3,1/2,0) mode and has higher oxygen coordination for Li atoms.

Another application to solve the intermediate structure between fcc and hcp structures of Xe in the pressure range 12-75 GPa will also be described.

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2. S. Tsuneyuki, Y. Tsukada and H. Aoki, Phys Rev.Lett. 61, 869 (1988).

3. M. S. Somayazulu, S. M. Sharma and S. K. Sikka, Phys.Rev.Lett. 73, 98 (1994).

4. S. M. Sharma, N. Garg, V. Chitra and S. K. Sikka -to be published.

5. S. L. Chaplot and S. K. Sikka, Phys Rev. B 47, 5710 (1993).