In this paper, aggregated CaWO₄ micro- and nanocrystals were synthesized by the co-precipitation method and processed under microwave-assisted hydrothermal/solvothermal conditions (160 °C for 30 min). According to the X-ray patterns, all crystals exhibited only the scheelite-type tetragonal structure. The data obtained by the Rietveld refinements revealed that the oxygen atoms occupy different positions in the [WO₄] clusters, suggesting the presence of lattice distortions. The crystal shapes as well as its crystallographic orientations were identified by field-emission scanning electron microscopy and high-resolution transmission electron microcopy. Electronic structures of these crystals were evaluated by the first-principles quantum mechanical calculations based on the density functional theory in the B3LYP level. A good correlation was found between the experimental and theoretical Raman and infrared-active modes. A crystal growth mechanism was proposed to explain the morphological evolution. The ultraviolet-visible absorption spectra indicated the existence of intermediary energy levels within the band gap. The highest blue photoluminescence emission, lifetime and quantum yield were observed for the nanocrystals processed in the microwave-assisted solvothermal method.

A model is proposed here to explain how the chemical features of metal oxide varistors can alter their nonohmic physical behavior, based on nonohmic similarities in the electrical properties of ZnO- and ${\mathrm{SnO}}_{\mathrm{2}}$ -based varistors. The proposed model explains the electrical properties of ZnO- and ${\mathrm{SnO}}_{\mathrm{2}}$ -based varistors before and after thermal treatments in oxygen- and nitrogen-rich atmospheres, which cause similar changes in the nonohmic feature of these polycrystalline ceramics with greatly differing chemical compositions and microstructures. The model is based on the key role that oxygen plays in varistor grain boundaries, independently of the type of ceramic system (ZnO-, ${\mathrm{SnO}}_{\mathrm{2}}$ - or even ${\mathrm{SrTiO}}_{\mathrm{3}}$ -based varistors) involved.