Calcium-lead fluoro-vanadinite apatites. I. Disequilibrium structures

The synthetic vanadinites (PbxCa10-x)(VO4)(6)F-2delta, 1 < x < 9, adopt a P6(3)/m apatite structure with 9.7590 (1) less than or equal to a less than or equal to 10.1179 (1) Angstrom and 7.0434 (3) less than or equal to c less than or equal to 7.4021 (1) Angstrom. The partitioning of calcium and lead over the A(I)(4f) and A(II)(6h) positions is nonstoichiometric with lead preferentially entering the larger A(II) site. High-resolution electron microscopy showed that samples annealed for 10 h at 1073 K are in disequilibrium with calcium- and lead-rich microdomains coexisting at unit-cell scales. For (Pb5Ca5)(VO4)(6)F-2delta, sintering in excess of 2 weeks is required for the metals to order macroscopically. As annealing progresses, c/a, the partitioning coefficient k(Pb)(A(I)/A(II)) and the A(I)O(6) metaprism twist angle (phi) adjust cooperatively to enlarge the apatite channel, and thereby accommodate higher lead content. These results demonstrate that phi is a sensitive measure of disequilibrium and a useful device for monitoring changes in apatite topology as a function of composition.