Structural change in lead fluorapatite at high pressure

Abstract

The structure of lead fluorapatite (PbFAP; Pb10(PO4)6F2), crystallized from the melt in a platinum capsule at 1,000C and 1 atm, has been investigated by single-crystal X-ray diffraction. Crystal data are a = 9.7638 (6), c = 7.2866 (4) A ˚ , space group P63/m, R = 0.043, Rw = 0.034. We have also studied the com- pressional behaviour of the c-axis channel of PbFAP up to 9 GPa at 25C, using a diamond-anvil cell, synchrotron X-radiation, and Rietveld powder structure refinement. Pressure-volume data for the channel polyhedron of PbFAP fitted to the third-order Birch-Murnaghan equation resulted in KT = 33.2 ± 1.2 GPa when KT 0 is fixed at 4. The c-axis channel of PbFAP is about twice as compress- ible as the unit-cell volume of PbFAP and the channel of calcium apatites. This is attributed to the anomalous nar- rowing of the channel of PbFAP with increase in confining pressure. Flexibility of the apatite channel is a key factor in the scavenging of toxic heavy metals by calcium apatites.