Elaboration,Rietveld Refinements and Vibrational Spectroscopic Studies of a new Lacunar Apatite Series: NaPb 3-x Ca x Cd(PO 4 ) 3 (0 ≤ × ≤ 1)

Abstract

The solid–solution series NaPb_3-xCa_xCd(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.