Correlation of Metaprism Twist Angle with Disequilibrium in Calcium-Lead Fluoro-Vanadinite Apatites

Z.L. Dong and T.J.White

Centre for Advance Research of Ecomaterials, Institute of Environmental Science and Engineering, Innovation Centre, Blk 2, Unit 237, Nanyang Technological University, Singapore 637723 (zldong@ntu.edu.sg)

The synthetic vanadinites (Pb_xCa_10-x)(VO₄)₆(F_2-2yO_y), 0<x<9 and 0<y<0.35 adopt the P6₃/m apatite structure with 9.7073(4) £ a £10.1179(1) Å and 7.0153(3) £ c £7.4021(1) Å. For mixed metal compounds (x ¹ 0) the partitioning of calcium and lead over the A^I(4f) and A^II(6h) positions is nonstoichiometric with lead preferentially sequestering to the larger A^II site, as determined through the refinement of powder X-ray diffraction data. High resolution electron microscopy, supported by multislice calculations, revealed that samples annealed for 10 hrs at 800ºC show great disparity in Ca/Pb partitioning, with domains that are essentially calcium or lead pure co-existing at unit cell scales. Only longer sintering in excess of 2 weeks enabled the metals to order macroscopically. As annealing progresses the lattice parameters adjust first through dilation of a (within 1 week) followed by c contraction (between 1 - 2 weeks). The A^IO₆ metaprism twist angle f is a sensitive measure of disequilibrium and can be used to derive ideal cell parameters from triangular anion networks such that

where t = 2.729 + 0.017x is the triangle edge, h_f=0 = 3.576 + 0.017x is the metaprism height at f = 0^o, and x is the stoichiometry in (Pb_xCa_10-x)(VO₄)₆(F_2-2yO_y).