Facile growth of centimeter-order, highly crystalline ZnWO₄ single crystals by the flux evaporation technique using molten NaCl

Wolframite-type ZnWO₄ crystals were grown from molten NaCl using the flux method at temperatures between 700 and 1100 °C. Centimeter-order, idiomorphic single crystals of ZnWO₄ were easily obtained from a solute concentration of 40 mol% at 1100 °C, with well-defined facets at {010}, {101}, {-101}, {130}, and {041}. As the heating temperature decreases, the size of the ZnWO₄ crystals also decreases. However, millimeter-order ZnWO₄ single crystals were still grown from a NaCl flux at 700 °C, whereas solid-state reactions gave only indefinite, micron-order particles even at 1100 °C. It is assumed that NaCl and ZnWO₄ form a eutectic composition, resulting in effective crystal growth through a liquid-phase process even below the melting point of NaCl (∼800 °C). In addition, the large-scale evaporation of molten NaCl should facilitate the supersaturation of ZnWO₄ crystals, resulting in centimeter-order growth at 1100 °C. The crystallographic characteristics of the flux-grown ZnWO₄ crystals were also evaluated; elemental analysis and microscopic observations revealed a highly crystalline nature with respect to chemical stoichiometry, impurities, crystal defects, and regularity of the lattice.