Structural analysis of alkali-metal lead orthovanadates crystallizing with lacunar apatite structure

Orthovanadates NaPb4(VO4)(3) and KPb4(VO4)(3) crystallize with anionic deficient apatite structure. The crystal structure determination of the two compounds indicates that they are isostructural, The following crystal data were obtained: hexagonal, P6(3)/m, Z = 2, a 10.060(2) Angstrom, c = 7.346(1) Angstrom, R = 0.047 (Rw = 0.061) for NaPb4(VO4)(3) and a = 10.111(2) Angstrom, c = 7.448(1) Angstrom, R = 0.076 (Rw = 0.083) for KPb4(VO4)(3). Some of Pb(II) cations are located in sites (6h), The ninefold coordination sites (4f) are equally occupied by the other lead cations and the monovalent ions. The structure can be described as built up from [VO4](3-) tetrahedral and Pb2+ of sixfold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [001], These tunnels are connected by cations of mixed sites (4f) half occupied by Pb(II) and half by Na+ or K+. The existence of this type of lacunar apatite seems to be conditioned by the presence of lone pair cations Pb(II). (C) 1998 Academic Press.