Synthesis, Rietveld refinements and Raman spectroscopic studies of tricationic lacunar apatites Na1−xKxPb4(AsO4)3 (0 ⩽ x ⩽ 1)

Three compounds from the solid solution of lead lacunar ionic apatites Na1−xKxPb4(AsO4)3, with x = 0, 0.5 and 1, were successfully synthesized as single phases by solid state method at 700 °C. The samples were characterized by X-ray diffraction, the site of the metal ions (Na+, K+ and Pb2+) in the solid solutions was analyzed with the Rietveld method. A variation of the a and c lattice parameters in the solid solutions was observed, with an increase of a and c parameters, related to the radius of the corresponding substituted ions. It was found that Pb(II) ions in the solid solutions preferentially occupied the M(1) and M(2) sites in the lacunar anionic apatite structure. The structure contains channels running along the c axis and centred at (0 0 z). The channels are most probably occupied by the lone electron pairs of the Pb2+ cations. Raman spectra of all the compositions are similar and show some linear shifts, especially for the bending modes, in band positions as a function of the composition toward lower 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(AsO4)3 up to 630 K.