On the crystal structure and optical spectroscopy of rare earth comprising quaternary tungstates Li3Ba2RE3(WO4)8 (RE = La–Nd, Sm–Ho)

Abstract

The quaternary tungstates Li₃Ba₂RE₃(WO₄)₈ (RE = La–Nd, Sm–Ho) were obtained by a ceramic synthesis route and were characterized by powder and single crystal X-ray diffraction. The structures of Li₃Ba₂Pr₃(WO₄)₈ and Li₃Ba₂Tb₃(WO₄)₈ were refined from single crystal diffractometer data: RbLiBi₂(MoO₄)₄ type, space group C2/c, a = 528.57(2), b = 1292.39(6), c = 1934.80(10) pm, β = 91.522(4)°, 2151 F² values, 108 parameters for Li₃Ba₂Pr₃(WO₄)₈ and a = 520.54(2), b = 1272.03(6), c = 1918.85(10) pm, β = 91.948(4)°, 2020 F² values, 108 variables for Li₃Ba₂Tb₃(WO₄)₈. Striking polyhedral building units in these tungstates are WO₄ tetrahedra and LiO₆ octahedra, while the mixed occupied site and the barium atoms have higher coordination numbers, i.e. RE/Li@O₈ and Ba@O₁₀. In addition to the powder quality assessment by means of reflection spectroscopy, the synthesized samples were studied for their suitability as a scintillator material. Therefore, X-ray excited luminescence measurements where performed. Apart from Li₃Ba₂Ce₃(WO₄)₈ and Li₃Ba₂Nd₃(WO₄)₈, all compounds show strong emission under X-ray irradiation. Li₃Ba₂La₃(WO₄)₈ and Li₃Ba₂Gd₃(WO₄)₈ show blue CT luminescence caused by tungstate units, while the other samples show typical and multiple lines due to well known [Xe]4fⁿ → [Xe]4fⁿ transitions.