Microwave-assisted solid-state synthesis of NaRE(MO4)2 phosphors (RE = La, Pr, Eu, Dy; M = Mo, W)

Abstract

A synthetic method was developed to enable the microwave-assisted solid-state preparation of double molybdate and double tungstate scheelite-type phosphors of formula NaRE(MO₄)₂ (RE = La, Pr, Eu, Dy; M = Mo, W). Starting from subgram-scale stoichiometric mixtures of metal carbonates and oxides and with the aid of granular activated charcoal as a microwave susceptor, ternary (NaEu(MO₄)₂), quaternary (NaLa_0.95Eu_0.05(MO₄)₂), and quinary phosphors (NaLa_0.95Pr_0.025Dy_0.025(MO₄)₂) were obtained upon heating in a countertop microwave oven. The synthesis of crystalline and phase-pure materials required heating times ranging from 18 to 27 min, significantly shorter than those typically encountered in solid-state reactions assisted by conventional heating. Depending on chemical composition, the speed-up factor ranged from 30 to 40. More importantly, photoluminescence studies performed on the compositionally complex quinary molybdate NaLa_0.95Pr_0.025Dy_0.025(MoO₄)₂ showed that phosphors synthesized using microwave and conventional heating have nearly identical luminescence responses. The synthetic method described in this contribution is robust, fast, simple, and ideally suited for exploratory synthesis and rapid screening of group VI metalate phosphors, as well as for the preparation of binary precursors to these materials (e.g., Na₂MoO₄ and Na₂WO₄).