Structural, optical and microwave dielectric properties of Sr1−xCaxWO4 ceramics prepared by the solid state reaction route

Strontium calcium tungstates (Sr1−xCax)WO4 crystals (with x=0; 0.1, 0.2, 0.3, 0.4, 0.5, 0.6,0.7, 0.8, 0.9 and 1.0) were prepared by the standard wet milling ceramic preparation method. X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectroscopic techniques indicate that all the crystals present a scheelite-type tetragonal structure without deleterious phases. FT-Raman spectra exhibited six Raman active modes in the range from 100 to 1000 cm−1, while the FT-IR spectra present different infrared active modes for W–O within the wave number range from 500 to 1000 cm−1. Optical properties were investigated by ultraviolet visible (UV–vis) absorption and photoluminescence (PL) measurements. UV–vis absorption measurements evidenced an increase in the values of the optical band gap (from 5.80 to 5.92 eV) with the increase of Ca into the SrWO4 lattice. All compositions show broad blue PL emission at room temperature when excited with 250 nm wavelength. Dielectric constant, temperature coefficient of resonant frequency (τf) and quality factors were measured with the Hakkie–Coleman technique. The permittivity was found to increase with the increase in Ca content, in agreement with the Clausius–Mosotti relation. The value of τf was found to be −44.61 ppm/°C for SrWO4 which increased to −19.69 ppm/°C for CaWO4.