Optical properties of Eu3+ activated thorium molybdate and thorium tungstate: Structure, local symmetry and photophysical properties

• Local symmetry around Eu3+ ion in thorium molybdate and tungstate.
• Differences in local structure and luminescence properties of europium ion molybdate and tungstate.
• Optimization of mixed molybdate and tungstate for high lumen output.
• Judd–Ofelt and other photophysical parameter of europium ion in two matrices.
• Structure elucidation of Th(WO4)2 and Th(Mo1−xWxO4)2.

Th (MO4)2: Eu3+ (M = Mo, W) phosphor materials have been synthesized via high temperature solid state route and characterized systematically using by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. X-ray diffraction data of Th(WO4)2 and Th(Mo1−xWxO4)2 were refined and structures were derived for the first time using Th(MoO4)2 as an isostructural model compound. Structural features around Eu ion are deduced from photoluminescence study on Eu doped compounds and were co-related with the local structure of Th ion in parent un-doped compounds. An excitation spectrum of Eu3+ shows red shift of CT maxima in case of thorium molybdates. Emission spectral features show that Eu3+ occupies the Th4+ site in the lattice that lacks inversion symmetry in both the molybdate and tungstate. It can be inferred from stark splitting pattern in emission spectrum that local symmetry of Th(MoO4)2: Eu3+ and Th(WO4)2: Eu3+ is less than C3v. Significantly higher value of asymmetry ratio (I) for Th(WO4)2: Eu3+ as compared to Th(MoO4)2: Eu3+ suggests that Eu3+ ions is in a much more asymmetric environment in thorium tungstate. This fact is also supported by lifetime measurement and Judd–ofelt (Ω2) parameter. Judd–Ofelt parameter calculations also show that Th(WO4)2: Eu3+ possess strong covalent character than Th(MoO4)2: Eu3+. It was also observed that maximum luminescence output is not observed in individual molybdate or tungstate but rather in mixed composition of ThMo1.5W0.5 O8 doped with europium ion.

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