Sodium Tungsten Bronze

Composition and structure

Crystalline structure of NaxWO₃ is affected by value of x. When x < 0.1, NaxWO₃ is WO₃ structure, it uses WO6 octahedral as unit structure. When 0.1 < x < 0.35, NaxWO₃ is tetragonal crystalline. When 0.35 < x < 1, NaxWO₃ has perovskite structure with Na+ vacancy. These two crystalline structure has WO₆ octahedral structure unit. Under different temperature, WO₆ octahedral can flex in different directions to make the temperature change of crystalline structure. It offers the possibility of synthesis for different materials.

Lattice imperfection

NaxWO₃ equals to NaWO₃ without (1-x) Na+. NaWO₃ belongs to complete perovskite structure which is cubic system. There is no space in lattice for they are being taken by Na+、W(v)，O2-. When NaWO₃ loses Na+ and becomes NaxWO₃, lattice appears (1-x) Na+ space, Na+ space made the incomplete of lattice, that’s why NaxWO₃ appears single ion default.

Synthesis

X value of NaxWO₃ is determined by synthesis condition. Under high temperature, Na+ would disperse into lattice or spread out from it, it makes the content of sodium changes in NaxWO₃. Using mixture of sodium tungstate and tungsten oxide and heat them with proper reductant would get the product, the reductant is often W、WO₂、H₂. Also, by electrolysis of sodium tungstate and tungsten oxide can also obtain the product. Chemical equation is as following:
3xNa₂WO₄ + (6-4x) WO₃ +xW=6NaxWO₃
xNa₂WO₄ + (2 -2x) WO₃ +xWO₂ =2NaxWO₃
xNa₂WO₄ + (2 -x) WO₃=2NaxWO₃ +x/2O₂

Property

Close packed structure, chemical inertness and metal luster NaxWO₃ belongs to fake and default ABO₃ perovskite structure. In such structure, O2- and Na+ are in cubic close packing, W and O becoming WO6 octahedron and share the same vertex, Na+ lies in the interspace of WO6 octahedron. So NaxWO₃ can resist all kinds of hydrofluoric acid, insoluble in water, shows ultimate chemical inertness to acid. Its close packed structure makes it have metal luster and inert metal property. As a result sodium tungsten bronze can be used as great anti-corrosion material.

Unstable oxidation state and reduction of W in NaxWO3

Average oxidation number of W in NaxWO₃ is between V-VI, xmol of W is +V oxidation state, but the most stable is +VI, it makes NaxWO₃ have strong reducing property under alkali condition. NaxWO₃ is oxidized when heating in the air, it can be soluble in the strong alkali solution, and can reducing ammonium solution of nitrate.

Color of NaxWO₃ changes with the x value. The reason why crystalline has coloration is there is default in it which is called color center. Different x value of NaxWO₃ would cause difference in absorbing visible light wavelength, thus the crystalline color is different. When x value is bigger, electron transition mainly absorbs short wave blue-purple light, the crystalline would show yellow-orange; as the decrease of x value, electron transition absorbs long wave red-orange light, the crystalline shows blue-purple.

Electrical conductivity

Sodium tungsten bronze has electrical conductivity which is related to its composition. When x>0.25, NaxWO₃ shows metallic electrical conductivity, rate decreases with rising temperature. When x< 0.25, NaxWO₃ shows semiconductor metallic electrical conductivity, rate increases with rising temperature. This special property makes it become newly solid electrolyte, there is already NaxWO₃ used as ion reversible electrode. Synthesis and research on solid electrolyte is an active field of inorganic solid chemistry. Solid electrolyte replaces regular electrolyte solution has become revolution of electrical chemistry.