Manganese(II) tungstate is an inorganic compound with the chemical formula MnWO4. It occurs naturally as the mineral hübnerite, the manganese-rich end member of the wolframite mineral series.
Structure and properties Manganese(II) tungstate crystallizes in the monoclinic wolframite structure, with space group P2/c (No. 13).
Its crystal structure consists of distorted MnO6 and WO6 octahedra. The octahedra share edges and form zigzag chains running approximately along the crystallographic c axis. Modern diffraction studies give closely similar cell dimensions and confirm the wolframite-type P2/c structure.
At room temperature MnWO4 is paramagnetic. On cooling it undergoes a series of magnetic phase transitions beginning at a Néel temperature of about 13.5 K. Three principal low-temperature antiferromagnetic phases, conventionally called AF1, AF2 and AF3, occur below approximately 13.5 K. The intermediate AF2 phase has an incommensurate spiral magnetic structure and is also ferroelectric, making MnWO4 a magnetically induced multiferroic material. The multiferroic phase occurs approximately between 7.8 and 12.5 K.
Occurrence Manganese(II) tungstate occurs naturally as hübnerite, MnWO4. Hübnerite forms a solid-solution series with iron-rich ferberite, FeWO4; compositions intermediate between the two are generally referred to as wolframite.
Preparation Manganese tungstate can be prepared by precipitation from aqueous solutions of a manganese(II) salt and a soluble tungstate such as sodium tungstate.
MnCl2 + Na2WO4 → MnWO4↓ + 2 NaCl
Nanocrystalline MnWO4 has also been prepared by solvothermal, hydrothermal and microwave-assisted methods. Coprecipitation from manganese(II) chloride and sodium tungstate gives the monoclinic wolframite phase after suitable treatment.
It can also be prepared by a high-temperature solid-state reaction between manganese and tungsten oxides. One reported reaction uses manganese(III) oxide and tungsten trioxide:
Mn2O3 + 2 WO3 → 2 MnWO4 + O2
Reactions Ammonolysis of amorphous MnWO4 at around 800 °C produces the ternary nitride MnWN2.
Reaction with Pr2WO6 at elevated temperature has been used to prepare the mixed-metal tungstate MnPr2W2O10.
Applications MnWO4 has been investigated as a photocatalyst. Nanostructured MnWO4 can promote the photodegradation of organic pollutants, and manganese tungstate-based materials have also been studied for photocatalytic reduction reactions.
It has also been investigated as the active material in thick-film humidity sensors, in which the electrical impedance of MnWO4-containing films changes with relative humidity.
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