Synthesis, crystal structure and optical investigation of the new phosphates: Na{sub 7}Mg{sub 13}Ln(PO{sub 4}){sub 12} (Ln=La, Eu)

Two new isostructural rare earth phosphates Na{sub 7}Mg{sub 13}Ln(PO{sub 4}){sub 12} (Ln=La, Eu) have been synthesized and investigated by X-ray diffraction and optical measurements. They crystallize in the orthorhombic system with the Cmc2{sub 1} space group (Z=4). The crystal structure exhibits a new type of framework built up from LnO{sub 8} (Ln=La, Eu), MO{sub 6} (M=0.5Mg+0.5Na) and MgO{sub x} (x=5, 6) polyhedra and PO{sub 4} tetrahedra linked by common corner, edge or face. It can be described in terms of [Mg{sub 4}MP{sub 4}O{sub 22}]{sub {infinity}} layers stacked along the a direction. These layers are interconnected by [Mg{sub 4}LnP{sub 4}O{sub 36}]{sub {infinity}} undulating chains spreading along the b direction. This framework delimits 6 distinct cavities occupied by Na{sup +} cations. The results of the optical study of Na{sub 7}Mg{sub 13}La{sub 1-x}Eu{sub x}(PO{sub 4}){sub 12} (x=0, 0.02, 0.1, 1) reveal the presence of two different Eu{sup 3+} 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. - Graphical abstract: Emission spectra of Na{sub 7}Mg{sub 13}Eu(PO{sub 4}){sub 12} and Na{sub 7}Mg{sub 13}La{sub 0.9}Eu{sub 0.1}(PO{sub 4}){sub 12} compounds for {lambda}{sub exc}=240, 290 and 393.2 nm.