Perspective on europium activated fine-grained metal molybdate phosphors for solid state illumination

Abstract

An attempt has been made to synthesize sulfur free Eu³⁺-activated M_1−xMoO₄:Eu_x³⁺ (M = Zn, Mg, Ca, Ni and Co; x = 0.2) phosphors by an oxalate sol–gel process. The process involves gel formation from solutions of metal nitrates in ethanol, ammonium heptamolybdate tetrahydrate in de-ionized water and oxalic acid in ethanol, digestion for 12 h, drying at 100 °C for 24 h, decomposition in air at 600 °C or above to yield fine grained powder exhibiting different morphologies, viz. distorted spherical agglomerates, dumbbell shaped particles, rectangular bars, faceted particles, ellipsoids or parallelepipeds. The optical absorption bands have been attributed to (i) ligand-to-metal charge transfer (LMCT), i.e., oxygen to molybdenum in the MoO₄²⁻group and (ii) f–f electron transitions (viz., ⁷F₀ → ⁵L₆ at 395 nm and ⁷F₀ → ⁵D₂ at 465 nm) of Eu³⁺ ions upon excitation with a wavelength of ∼395 nm, which occupy non-centrosymmetric metal (M) sites. Eu³⁺ ions exhibit a strong electric dipole ⁵D₀ → ⁷F₂ transition emitting red light at ∼612 nm with quantum efficiency in a 1.0 : 1.7 : 3.2 ratio for M_0.8MoO₄:Eu_0.2³⁺ (M = Mg, Zn and Ca) phosphors, respectively. Further, their CIE coordinates are (x = 0.68 and y = 0.32) close to known values (x = 0.665 and y = 0.331) of Y₂O₂S:Eu_0.05³⁺ red phosphor. Hence, M_0.8MoO₄:Eu_0.2³⁺ (M = Mg, Zn and Ca) phosphor can possibly be a potential alternative to Y₂O₂S:Eu_0.05³⁺ for fabrication of near UV InGaN based light emitting diodes. Yet, another phosphor of composition Co_0.8MoO₄:Eu_0.2³⁺ is shown to have promise for white light emission with its CIE coordinates as x = 0.279 and y = 0.245.