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Crystal structure and bonding in the high-pressure form of fluorite (CaF2)

2001; Elsevier BV; Volume: 62; Issue: 6 Linguagem: Inglês

10.1016/s0022-3697(00)00291-2

1879-2553

E. Kaiser Morris, T.L. Groy, Kurt Leinenweber,

Luminescence Properties of Advanced Materials

Single crystals of CaF2 in the PbCl2 structure were grown at high pressure in a molten calcium hydroxide flux, and the structure was refined by single crystal X-ray diffraction (Pnma, a=6.018(2) Å, b=3.614(1) Å, c=7.023(2) Å). Calcium is in nine-coordination in this structure, with Ca–F distances range from 2.317 to 2.774 Å. The two distinct fluorine atoms are in four and five coordination with calcium. Both the ambient pressure fluorite phase and the high pressure phase of CaF2 have calcium in a close-packed configuration, cubic in fluorite and hexagonal in the PbCl2 phase. This allows a simple comparison of the Ca arrays between the two phases. It is found that the close-packed array in the PbCl2 phase is contracted along a and b by 4.7 and 6.6%, respectively, and elongated along c by 4.8%, relative to the close-packed array in fluorite.