Crystallographic and spectroscopic investigations on nine metal-rare-earth silicates with the apatite structure type

Nine silicates with the apatite structure type (space group P6₃/m) containing both rare-earth elements (REEs: Pr, Nd, Sm, Tb, Ho and Er) and various metals (K, Sr, Ba and Cd) were synthesised by high-temperature flux-growth techniques and characterised by single-crystal X-ray diffraction, scanning electron microscopy, Raman spectroscopy and laser-induced photoluminescence spectroscopy. In all of the compounds, the 6h Wyckoff position is predominantly or solely occupied by REE³⁺ cations, whereas the cations shows a mixed occupancy at the larger, nine-coordinate 4f site with 55-75% of REE³⁺ cations and 45-25% of other metal cations. The O4 ("free" oxygen) site is fully occupied by O^2- anions, except for a Ba-Pr member with full occupancy by F^- anions. The refined formulas are Cd₂Er₈(SiO₄)₆O₂, Cd₂Tb₈(SiO₄)₆O₂, KHo₉(SiO₄)₆O₂, KTb₉(SiO₄)₆O₂, KSm₉(SiO₄)₆O₂, Sr₂Nd₈(SiO₄)₆O₂, Ba₂Nd₈(SiO₄)₆O₂, Ba₂Sm₈(SiO₄)₆O₂ and Ba₄Pr₆(SiO₄)₆F₂. Changes in the metaprism twist angle (φ) and correlations between the unit-cell parameters, average cationic radii (of M⁺/M²⁺-REE³⁺ pairs) and the chemistry of both the synthesised M⁺/M²⁺-REE³⁺ silicate apatites and those reported previously are evaluated. Photoluminescence measurements of undoped samples yielded emission bands in the visible region from green to red; therefore, these compounds are potential candidates for luminescent materials.