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

New germanate–vanadates with the general formula BiCa 4− x La x (VO 4 ) 3− x (GeO 4 ) x O, 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 BiCa 4 (VO 4 ) 3 O (space group P6 3 /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 Ca 2+ and V 5+ cations in the crystal lattice of vanadate BiCa 4 (VO 4 ) 3 O are replaced by La 3+ and Ge 4+ cations. In the process of substitution, the La 3+ 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 BiCaLa 3 (GeO 4 ) 3 is equivalent. IR and Raman spectra were studied. The Raman spectra clearly show a two-mode behavior: the lines of GeO 4 and VO 4 are separated from each other, and the (V/Ge)O 4 tetrahedra exhibit a quasi-independent behavior.