Elaboration, Rietveld Refinements and Vibrational Spectroscopic Studies of a new Lacunar Apatite Series: NaPb3-x CaxCd(PO4)3 (0 ≤ × ≤ 1)

Abstract

The solid–solution series NaPb3-xCaxCd(PO4)3 (0 ≤ × ≤ 1) has been synthesised and crystallised in the hexagonal system with the P63/m space group. A refinement study using the Rietveld method shows good agreement between the observed and calculated patterns. The structure can be described as built up from [PO4]3- tetrahedra and Pb2+/Cd2+/Ca2+ of six-fold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [001]. These tunnels are connected by cations of mixed sites (4f) which are half occupied by Pb2+/Cd2+/Ca2+ and half by Na+ cations. The vibrational spectra of all the compositions are similar and show some linear shifts as a function of the composition towards lower values due to the substitutions of Pb2+ by Ca2+ with a smaller radius.