Synthesis, crystal structure and optical properties of Na2RE(PO4)(WO4) (RE = Y, Tb–Lu)

Abstract

Phase-pure samples of sodium rare-earth phosphate tungstates Na₂RE(PO₄)(WO₄) (RE = Y, Dy–Lu) and Na₂Y(PO₄)(WO₄):Ln³⁺ (Ln = Eu, Tb) were obtained by reaction of the respective rare-earth oxide with ammonium hydrogen phosphate with sodium tungstate Na₂WO₄·2H₂O as a flux at 1220 K. According to X-ray single-crystal investigations Na₂RE(PO₄)(WO₄) (RE = Y, Tb–Lu) crystallise orthorhombically in space group Ibca (no. 73) (RE = Y, Z = 8, a = 1799.7(4), b = 1210.2(2), c = 683.82(14) pm, wR₂ = 0.040, R₁ = 0.037, 661 reflections, 62 parameters). The crystal structure contains non-condensed phosphate and tungstate tetrahedra with the (3 + 3)-coordinate sodium and eight-coordinate rare-earth ions in the voids. Relevant absorption and infrared spectra as well as the fluorescence spectra of Na₂Y(PO₄)(WO₄):Ln³⁺ (Ln = Eu, Tb) are presented. An FP-LAPW band-structure calculation confirms the assignment of the main absorption as well as the optical band-gap (ε_calc = 5.1 eV; ε_meas = 5.2(1) eV) of Na₂Y(PO₄)(WO₄).