The luminescence of NaxEu3+(2−x)/3MoO4 scheelites depends on the number of Eu-clusters occurring in their incommensurately modulated structure

Abstract

Scheelite related compounds with general formula M_n(XO₄)_m are the subject of hefty interest owing to their optical properties, stability and relatively simple preparation. Eu³⁺-containing scheelites are considered as red-emitting phosphors and the main factors affecting their luminescence are thought to be chemical composition and particle size while the influence of their structure is generally ignored. Here we report eight compounds from the Na_xEu(2−x_)/3MoO₄ series prepared by conventional solid-state reaction and present a detailed analysis of their crystal structures. Six of them have modulated structures, a common feature of SRCs, in which dopant Eu³⁺ ions are orderly distributed. Moreover, different amounts of Eu³⁺ dimers are detected in the modulated structures, characterized by weak satellite reflections appearing in the lower angle part of the XRD patterns. These reflections are indexed and incorporated into Rietveld's refinement using superspace (3 + 1)-dimension symmetry. The remarkable feature of the compounds is that the characteristic luminescence parameters, overall (Q^Eu_L) and intrinsic (Q^Eu_Eu) quantum yields, Eu(⁵D₀) lifetimes, and sensitization efficiencies (η_sens), correlate with the number of Eu³⁺ aggregates, but not directly with the composition x of the materials. This provides an efficient tool for understanding and controlling the luminescence properties of scheelite related compounds.