Na2Y3Cl3[TeO3]4: Synthesis, Crystal Structure and Spectroscopic Properties of the Bulk Material and its Luminescent Eu3+-doped Samples

Colorless, platelet-shaped single crystals of Na₂Y₃Cl₃[TeO₃]₄ were synthesized by a reaction of Y₂O₃ and TeO₂ (molar ratio 3:8) in evacuated silica ampoules within five days at 775 °C with an excess of NaCl added as flux and reagent. The new quintenary compound crystallizes in the monoclinic space group C2/c with the lattice parameters a = 2380.69(14), b = 552.76(3), c = 1662.48(9) pm, β =134.045(3)° and Z = 4. The crystal structure presents one of the rare oxotellurates(IV) containing isolated [TeO₃]²⁻ units, which could be verified by Raman spectroscopy. Doping with Eu³⁺ (3 mass-%) at the Y³⁺ sites in eightfold oxygen coordination leads to a strong orange-red luminescence of Na₂Y₃Cl₃[TeO₃]₄:Eu³⁺ under UV excitation with a wavelength of λ =254 nm. Beyond the electronic Eu³⁺ transitions of the 4f⁶ state, the fluorescence excitation spectrum shows an intense broad charge-transfer band resulting from both O²⁻ → Eu³⁺ ligand-to-metal charge-transfer (LMCT) as well as electronic Eu³⁺ d → f and Te⁴⁺ s → p transitions. The emission spectrum features the prominent f → f transitions of Eu³⁺-doped oxidic compounds. The optical band gaps of the undoped and doped samples were determined by diffuse reflectance spectroscopy revealing just minimal doping effects on the crystal structure. The optical band gaps could be assigned in the excitation spectrum in the charge-transfer broad-band region supporting the assumption of an inorganic antenna effect of the Te⁴⁺ cations.