Structural and Optical Properties of Ca_0.9Cu_0.01WO₄ Solid Solution Synthesized by Sonochemistry Method at Room Temperature

In this work, we report the room-temperature synthesis of pure calcium tungstate (CaWO₄) and copper-doped calcium tungstate solid solution (Ca_0.99Cu_0.01WO₄) by using a sonochemistry method. These materials were structurally characterized by X-ray diffraction (XRD) and Raman spectroscopy. The obtained XRD patterns were submitted to a Rietveld refinement showing, in both materials, a tetragonal phase with space group and point group of I4₁/a and C₄⁶, respectively. Microscopy images of both materials, obtained by field emission scanning electron microscopy, showed spherical agglomerated structures composed by spherical nanoparticles, while calcium and tungstate elements were identified by energy-dispersive X-ray spectroscopy for pure calcium tungstate and copper, calcium, and tungstate for Ca_0.99Cu_0.01WO₄ solid solution. The decrease of optical band gap (E_gap) from 4.0 eV (CaWO₄) to 3.45 eV (Ca_0.99Cu_0.01WO₄) confirmed the substitution of calcium atoms for copper atoms in the clusters [CaO₈]. Maximum photoluminescence (PL) emission was shifted from 522 nm in the pure CaWO₄ to 475 nm in the Ca_0.99Cu_0.01WO₄ solid solution. Consequently, there was an increase of PL emissions intensity in the blue and green regions of the visible spectrum, due to electronic transitions between the orbitals O 2p, Cu 3d, and W 5d.