Zinc tungstate is an inorganic compound with the chemical formula ZnWO4. It is a white crystalline solid that is practically insoluble in water. It occurs naturally as the zinc-rich mineral sanmartinite and has been investigated as a scintillator, optical material and photocatalyst.
Occurrence Zinc tungstate occurs naturally as sanmartinite, a member of the wolframite group. Natural sanmartinite commonly contains some iron and is generally written (Zn,Fe)WO4.
Sanmartinite was first described in 1948 and is a valid, grandfathered mineral species.
Preparation Zinc tungstate can be prepared by precipitation from aqueous solutions of a soluble zinc salt and sodium tungstate. For example, zinc chloride reacts according to:
ZnCl2 + Na2WO4 → ZnWO4↓ + 2 NaCl
Zinc nitrate can be used in the same way.
Nanocrystalline ZnWO4 has also been prepared by hydrothermal, solvothermal, sonochemical and electrochemical methods.
Structure and properties Zinc tungstate crystallizes in the monoclinic crystal system with the wolframite structure, in space group P2/c (No. 13). The structure consists of distorted ZnO6 and WO6 octahedra arranged in chains.
At pressures above approximately 30 GPa, ZnWO4 undergoes a structural phase transition to a high-pressure phase related to the β-fergusonite structure.
ZnWO4 is a wide-band-gap semiconductor. Reported experimental and calculated band-gap values are generally around 3.5–4 eV, depending on preparation method and measurement technique. Its optical properties underlie its use in scintillation and photocatalysis.
Applications Single crystals of ZnWO4 are used as scintillator materials. They combine high density, good light yield and low intrinsic radioactivity, and have been studied for rare-event and low-background particle-detection experiments.
Zinc tungstate has also been widely investigated as a heterogeneous photocatalyst for degradation of organic contaminants in water. Doping and formation of heterojunctions with other semiconductors can extend its photoresponse and improve charge separation.
Other investigated uses include optical fibres, photoluminescent materials, humidity sensors and electrochemical devices.
Reactions At elevated temperature, zinc tungstate can react with rare-earth tungstates to form mixed zinc–rare-earth tungstates of the general composition ZnRE4W3O16 (RE = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er or Lu).
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