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

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) Angstrom, b = 3.614(1) Angst rom, c = 7.023(2) Angstrom). Calcium is in nine-coordination in this struct ure, with Ca-F distances range from 2.317 to 2.774 Angstrom. The two distin ct fluorine atoms are in four and five coordination with calcium. Both the ambient pressure fluorite phase and the high pressure phase of CaF2 have ca lcium in a close-packed configuration, cubic in fluorite and hexagonal in t he PbCl2 phase. This allows a simple comparison of the Ca arrays between th e 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 alon g c by 4.8%, relative to the close-packed array in fluorite. (C) 2001 Elsev ier Science Ltd. All rights reserved.