Elucidation of the structural features and photoluminescence properties of a hydrothermally-synthesized γ-KEu(MoO4)2 microcrystal phosphor with metastable orthorhombic structure and differences in the luminescence properties by structure transition due to Y3+-dilution

Potassium europium molybdate, KEu(MoO4)(2), is an intriguing material known for its efficient luminescence properties attributed to Eu3+ ions and its polymorphic nature. Despite its significance, research on the metastable gamma-phase has been limited, with no prior reports on its structure, particle morphology, and luminescence characteristics. In this study, both the stable alpha-phase and the metastable gamma-phase of KEu(MoO4)(2) were synthesized using solid-state and hydrothermal reaction methods, and their crystal structures, particle morphologies, and luminescence properties were comprehensively investigated. X-Ray diffraction analysis confirmed the formation of the triclinic alpha-phase and the orthorhombic gamma-phase, with factor analysis results consistent with theoretically optimized structures, facilitating accurate structural determination. Both phases exhibited typical photoluminescence (PL) spectra of Eu3+ ions. However, in the gamma-phase, the D-5(0) -> F-7(2) transition appeared as a non-split peak with minimal Stark effect, attributed to differences in structural symmetry between the phases. A red-shift in the PL excitation edge was observed in the alpha-phase, which was attributed to a narrowed band gap resulting from the broadening of the O-p orbital in the valence band, as indicated by density functional theory (DFT) calculations. Additionally, KEu(MoO4)(2) was doped with Y3+ ions to form K(Eu,Y)(MoO4)(2), revealing a solid solubility limit of 40% in the alpha-phase, while the gamma-phase displayed no solid solution limit. The estimated critical distance of Eu3+ ions suggested that Eu-Eu interactions contributing to concentration quenching are minimal in the stacking direction but significant in the in-plane direction, operating in the second shell.