Structural analysis of alkali-metal lead orthovanadates crystallizing withlacunar apatite structure

Orthovanadates NaPb4(VO4)(3) and KPb4(VO4)(3) crystallize with anionic defi cient apatite structure. The crystal structure determination of the two com pounds indicates that they are isostructural, The following crystal data we re 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) Angstr om, 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 h exagonal tunnels running along [001], These tunnels are connected by cation s of mixed sites (4f) half occupied by Pb(II) and half by Na+ or K+. The ex istence of this type of lacunar apatite seems to be conditioned by the pres ence of lone pair cations Pb(II). (C) 1998 Academic Press.