Synthesis and crystal structures of new oxyapatites BiCa4−xLax(VO4)3−x(GeO4)xO, x=1–3

New germanate–vanadates with the general formula BiCa4−xLax(VO4)3−x(GeO4)xO, x=1−3, have been synthesized by the nitrate–citrate method and characterized. Structural refinement based on X-ray powder diffraction data showed that these compounds are isostructural with BiCa4(VO4)3O (space group P63/m (N 176), Z=2, а=9.8327–9.8755(3), and с=7.1203–7.2133(2) ). They may be viewed as continuous series of solid solutions where Ca2+ and V5+ cations in the crystal lattice of vanadate BiCa4(VO4)3O are replaced by La3+ and Ge4+ cations. In the process of substitution, the La3+ cations occupy mainly the 4f lattice sites of the germanate–vanadate oxyapatites, but the filling of the 4f and 6h lattice sites in the germanate BiCaLa3(GeO4)3 is equivalent. IR and Raman spectra were studied. The Raman spectra clearly show a two-mode behavior: the lines of GeO4 and VO4 are separated from each other, and the (V/Ge)O4 tetrahedra exhibit a quasi-independent behavior.