Synthesis, crystal structure and optical investigation of the new phosphates: Na₇Mg₁₃<i>Ln</i>(PO₄)₁₂ (<i>Ln</i>=La, Eu)

Two new isostructural rare earth phosphates Na7Mg13Ln(PO4)12 (Ln=La, Eu) have been synthesized and investigated by X-ray diffraction and optical measurements. They crystallize in the orthorhombic system with the Cmc21 space group (Z=4). The crystal structure exhibits a new type of framework built up from LnO8 (Ln=La, Eu), MO6 (M=0.5Mg+0.5Na) and MgOx (x=5, 6) polyhedra and PO4 tetrahedra linked by common corner, edge or face. It can be described in terms of [Mg4MP4O22]∞ layers stacked along the a direction. These layers are interconnected by [Mg4LnP4O36]∞ undulating chains spreading along the b direction. This framework delimits 6 distinct cavities occupied by Na+ cations. The results of the optical study of Na7Mg13La1−xEux(PO4)12 (x=0, 0.02, 0.1, 1) reveal the presence of two different Eu3+ ion environments whereas the X-ray study predicts the existence of only one Eu site. This difference can be explained by the possible presence of the europium element in the sodium sites with small occupancies which cannot be detected by the X-ray structural determination