Structural and Optical Properties of Ca 0.9 Cu 0.01 WO 4 Solid Solution Synthesized by Sonochemistry Method at Room Temperature.

In this work, we report the room-temperature synthesis of pure calcium tungstate (CaWO ₄ ) and copper-doped calcium tungstate solid solution (Ca _0.99 Cu _0.01 WO ₄ ) 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 I 4 ₁ /a and C _{4 h} ⁶ , 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 _0.99 Cu _0.01 WO ₄ solid solution. The decrease of optical band gap ( E _gap ) from 4.0 eV (CaWO ₄ ) to 3.45 eV (Ca _0.99 Cu _0.01 WO ₄ ) confirmed the substitution of calcium atoms for copper atoms in the clusters [CaO ₈ ]. Maximum photoluminescence (PL) emission was shifted from 522 nm in the pure CaWO ₄ to 475 nm in the Ca _0.99 Cu _0.01 WO ₄ 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.