Cation-Induced Variation of Micromorphology and Luminescence Properties of Tungstate Phosphors by a Hydrothermal Method

Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO4 by Hydrothermal Conditions

Doping chemistry has become one of the most effective means of tuning materials' properties for diverse applications. In particular for scheelite-type CaWO₄, high-oxidation-state doping is extremely important, since one may expand the scheelite family and further create prospective candidates for novel applications and/or useful spectral signatures for nuclear forensics. However, the chemistry associated with high-valence doping in scheelite-type CaWO₄ is far from understanding. In this work, a series of scheelite-based materials (Ca_1-x-y-zEu_xK_y□_z)WO₄ (□ represents the cation vacancy of the Ca²⁺ site) were synthesized by hydrothermal conditions and solid-state methods and comparatively studied. For the bulk prepared by the solid-state method, occupation of high-oxidation-state Eu³⁺ at the Ca²⁺ sites of CaWO₄ is followed by doping of the low-oxidation-state K⁺ at a nearly equivalent molar amount. The Eu³⁺ local symmetry is thus varied from the original S₄ point group symmetry to C_2v point group symmetry. Surprisingly different from the cases in bulk, for the nanoscale counterparts prepared by hydrothermal conditions, the high-oxidation-state Eu³⁺ was incorporated in CaWO₄ at two distinct sites, and its amount is higher than that of the low-oxidation-state K⁺ even though KOH was used as a mineralizer, creating a certain amount of cation vacancies. Consequently, an apparent split emission of ⁵D₀ → ⁷F₀ was first demonstrated for (Ca_1-x-y-zEu_xK_y□_z)WO₄. The doping chemistry of high oxidation states uncovered in this work not only provides an explanation for the commonly observed spectral changes in rare-earth-ion-modified scheelite structures, but also points out an advanced direction that can guide the design and synthesis of novel functional oxides by solution chemistry routes.

Effects of slight structural distortion on the luminescence performance in (Ca1-x Eux )WO4 luminescent materials

(Ca_1-x Eu_x )WO₄ (x = 0-21 mol%) phosphors were prepared using the classical solid-state reaction method. The influence of Eu³⁺ ion doping on lattice structure was observed using powder X-ray diffraction and Fourier transform infrared spectroscopy. Furthermore, under this influence, the luminescence properties of all samples were analyzed. The results clearly illustrated that the element europium was successfully incorporated into the CaWO₄ lattice with a scheelite structure in the form of a Eu³⁺ ion, which introduced a slight lattice distortion into the CaWO₄ matrix. These lattice distortions had no effect on phase purity, but had regular effects on the intrinsic luminescence of the matrix and the f-f excitation transitions of Eu³⁺ activators. When the Eu³⁺ concentration was increased to 21 mol%, a local luminescence centre of [WO₄ ]^2- groups was detected in the matrix and manifested as the decay curves of [WO₄ ]^2- groups and luminescence changed from single exponential to double exponential fitting. Furthermore, the excitation transitions of Eu³⁺ between different energy levels (such as ⁷ F₀ →⁵ L₆ , ⁷ F₀ →⁵ D₂ ) also produced interesting changes. Based on analysis of photoluminescence spectra and the chromaticity coordinates in this study, it could be verified that the nonreversing energy transfer of [WO₄ ]^2- →Eu³⁺ was efficient and incomplete.

Probing emission and defects in BaWxMo1–xO4 solid solutions: achieving color tunable luminescence by W/Mo ratio and size manipulation

This work highlights the size, structure, and composition manipulation for designing color-tunable phosphors based on doped scheelite micro- and nanocrystals.

Effect of morphology and temperature treatment control on the photocatalytic and photoluminescence properties of SrWO4 crystals

This work reports the combined effect of the morphology and crystallization degree of the strontium tungstate (SrWO₄) scheelite structure on its photocatalytic and photoluminescence properties.

Morphological and structural investigation of SrWO4 microcrystals in relationship with the electrical impedance properties

Synthesis of SrWO₄ crystal with spindles and spheres morphologies at ambient temperature. A structuring in tetragonal scheelite favors mixed conduction properties based on order–disorder transition.