The luminescence of Na_xEu³⁺(2−x_)/₃MoO₄scheelites depends on the number of Eu-clusters occurring in their incommensurately modulated structure

Scheelite related compounds with general formula Mn(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 NaxEu(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 (QEuL) and intrinsic (QEuEu) quantum yields, Eu(5D0) lifetimes, and sensitization efficiencies (ηsens), 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.