Rietveld refinements and vibrational spectroscopic studies of Na _1-xK_xPb₄(PO₄)₃ lacunar apatites (0≤x≤1)

Synthesis of apatites, Na_1-xK_xPb₄(PO ₄)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 Pb ² 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.