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

• Main Authors: Dong, Zhili, White, Timothy John
• Other Authors: School of Materials Science & Engineering
• Format: Article
• Language: English
• Published: 2011
• Subjects: DRNTU::Engineering::Materials::Ecomaterials
• Online Access: https://hdl.handle.net/10356/79930; http://hdl.handle.net/10220/6900
• Institution: Nanyang Technological University

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.