Calcium-lead fluoro-vanadinite apatites. I. Disequilibrium structures
White, Timothy John
School of Materials Science and Engineering
The synthetic vanadinites (PbxCa10-x)(VO4)6F2δ, 1 < x < 9, adopt a P63/m apatite structure with 9.7590 (1) ≤ a ≤ 10.1179 (1) Å and 7.0434 (3) ≤ c ≤ 7.4021 (1) Å. The partitioning of calcium and lead over the AI(4f) and AII(6h) positions is nonstoichiometric with lead preferentially entering the larger AII 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)6F2δ, sintering in excess of 2 weeks is required for the metals to order macroscopically. As annealing progresses, c/a, the partitioning coefficient kPb(AI/AII) and the AIO6 metaprism twist angle (φ) adjust ooperatively to enlarge the apatite channel, and thereby accommodate higher lead content. These results demonstrate that φ is a sensitive measure of disequilibrium and a useful device for monitoring changes in apatite topology as a function of composition.
Acta crystallographica B
© 2004 International Union of Crystallography. This paper was published in Acta Crystallographica B and is made available as an electronic reprint (preprint) with permission of International Union of Crystallography. The paper can be found at the following DOI: http://dx.doi.org/10.1107/S0108768104001831. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law.