Synthesis, Rietveld refinements and Raman spectroscopy studies of the solid solution Na1−xKxPb4(VO4)3 (0 ⩽ x ⩽ 1)

In the present study we report the synthesis, crystal structure and Raman spectroscopy studies of Na1−xKxPb4(VO4)3 orthovanadates solid solutions (0 ⩽ x ⩽ 1). Rietveld refinements showed that this solid solution is continuous adopting P63/m (no. 176) space group. Some of Pb(II) cations are located in the (6 h) sites. The ninefold coordination sites (4f) are equally occupied by the other lead cations and the K+ and Na+ monovalent ions. The structure can be described as built up from [VO4]3− tetrahedral and Pb2+ of sixfold coordination cavities (6 h positions), which delimit void hexagonal tunnels running along [0 0 1]. These tunnels are connected by cations of mixed sites (4f) half occupied by Pb(II) and half by Na+/K+ mixed cations. The existence of this type of lacunar apatite seems to be conditioned by the presence of lone pair cations Pb(II). Raman spectra of all the compositions are similar and show some linear shifts in band positions as a function of the composition toward high values due the substitutions of Na+ by K+ with a larger radius. No considerable changes in the temperature dependence of the Raman modes and the corresponding FWHM are observed and thus no temperature induced phase transition is observed in Na0.5K0.5Pb4(VO4)3 up to 650 K.