Elaboration, Vibrational Study and Thermal Behavior of Lacunar Apatites NaPb3−xCdxCa(PO4)3 (0 ≤ x ≤ 1)

Abstract

In this work we are interested in the synthesis of a new solid solution type NaPb_3–xCd_xCa(PO₄)₃; (0 ≤ _x ≤ 1). The lacunar apatite series are synthesized by a solid-state reaction and studied by X-ray diffraction, infrared and Raman scattering spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry. The crystal structure of these compounds was refined using the Rietveld method. These compounds crystallize in the hexagonal system (space group P6₃/_m) with a number of units per crystal lattice Z = 2. The structure is formed by [PO₄]³⁻ tetrahedra and Pb²⁺/Ca²⁺/Cd²⁺ ions, which constitute the basic skeleton of the structure. The Pb²⁺/Ca²⁺/Cd²⁺ ions occupy the 6h sites, whereas the 4f sites are occupied in half by the Pb²⁺/Ca²⁺/Cd²⁺ ions and the other half by the Na⁺ ions. The observed frequencies in the Raman and infrared spectra are explained and discussed based on the factor group analysis and by comparison with similar apatites. The vibrational spectroscopy results are in good agreement with the X-ray diffraction measurements. The internal modes of (PO₄)³⁻ tetrahedra are assigned and corroborate well with the factor group analysis for the symmetry P6₃/m. We also investigated the thermal stability of these apatite materials by differential scanning calorimetry.