Effect of Synthesis Route on the Radiation-Induced Luminescence and Defect States in Calcium Tungstate

Calcium tungstate ceramic powders were synthesized using two different methods—solid-state and hydrothermal routes. X-ray diffraction studies confirmed the formation of a tetragonal scheelite structure for both samples, as supported by Rietveld refinement. The solid-state samples exhibited higher crystallinity with a crystallite size of ~91 nm, while the hydrothermal powders showed broader peaks corresponding to smaller crystallites (~25 nm). FESEM analysis revealed micro-sized spherical aggregates (~3.5 µm) composed of 40–60 nm nanoparticles in the hydrothermal powders, whereas solid-state samples displayed larger grains of a few micro-meters. Photoluminescence spectra of solid-state samples showed a broad intrinsic emission band from 400–600 nm, attributed to the intrinsic emission of the [WO₄]^2- complex while hydrothermal samples exhibited additional weak defect-related peaks near 550 nm. Under X-ray excitation, these defect states were significantly enhanced, leading to a dual-peak radioluminescence response associated with oxygen vacancies. These findings highlight the influence of synthesis route on the structural and luminescence behavior of CaWO₄ ceramics, emphasizing the role of oxygen vacancies in their radiation-induced luminescence response.