Calcium-lead fluoro-vanadinite apatites. I. Disequilibrium structures
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 l...
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| Main Authors: | , |
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| Other Authors: | |
| Format: | Article |
| Language: | English |
| Published: |
2011
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| Subjects: | |
| Online Access: | https://hdl.handle.net/10356/79930 http://hdl.handle.net/10220/6900 |
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| Institution: | Nanyang Technological University |
| Language: | English |
| Summary: | 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. |
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