Rietveld refinements and vibrational spectroscopic studies of Na1−xKxPb4(PO4)3 lacunar apatites (0≤x≤1) Original Research Article

Synthesis of apatites, Na1−xKxPb4(PO4)3 0≤x≤1, with anion vacancy was carried out using solid state reactions. The solid solution of apatite-type structure crystallizes in the hexagonal system, space group P63/m (No 176). Rietveld refinements showed that 75% of Pb2+ cations are located in the (6h) sites; the ninefold coordination sites (4f) are equally occupied by the other 25% lead cations and the K+ and Na+ monovalent ions. The structure can be described as built up from [PO4]3− tetrahedra and Pb2+ of sixfold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [0 0 1]. These tunnels are connected by cations of mixed sites (4f) half occupied by Pb2+ and half by Na+/K+ mixed cations. The assignment of the observed frequencies in the Raman and infrared spectra is discussed on the basis of a unit cell group analysis and by comparison with other apatites. The Raman modes of all the compositions show some linear shifts of the frequencies as a function of the composition toward lower values due the substitutions of Na+ by K+ with a larger radius.