Sodium tungsten bronze

Prepared in 1823 by the chemist Friedrich Wöhler, sodium tungsten bronze was the first alkali metal bronze to be discovered. Tungsten bronzes owe some of their properties to the relative stability of the tungsten(V) cation that is formed. A similar family of molybdenum bronzes may have been discovered in 1885 by Alfred Stavenhagen and E. Engels, but they are formed in a very narrow range of temperatures and were not reported again until the 1960s. ==Properties==

Sodium tungsten bronze, like other tungsten bronzes, is resistant to chemical reaction under both acidic and basic conditions. Colour is dependent upon the proportion of sodium in the compound, ranging from golden at x ≈ 0.9, through red, orange and deep purple, to blue-black when x ≈ 0.3. The electrical resistivity of the bronze depends on the proportion of sodium in the compound, with specific resistances of 1.66 mΩ being measured for some samples. It has been suggested that electrons, released when the sodium atoms are ionised, are conducted readily through the tungsten t2g and oxygen π orbitals. and UPS spectra: the peak representing the tungsten 5d band becomes more intense as x rises. For values of x below 0.3, the bronze is semiconducting rather than metallic. The first record of superconductivity in a tungsten bronze was in 1964, with a Tc of 0.57 K. ==Structure==

When x = 1, sodium tungsten bronze adopts a cubic phase: the perovskite crystal structure. In this form, the structure consists of corner-sharing WO6 octahedra with sodium ions in the interstitial gaps. For x values between 0.9 and 0.3, the structure remains similar but with an increasing deficiency of sodium ions and a smaller lattice parameter. ==Synthesis==

Wöhler's 1823 synthesis involved reducing sodium tungstate and tungsten trioxide with hydrogen gas at red heat. A more modern approach reduces a melt of the reactants with electricity rather than with hydrogen. Microwave synthesis is also possible, using tungsten powder as the reducing agent. Hydrothermal (both batch and flow) syntheses are also possible. ==Related compounds==

The sodium in this compound can be replaced by other alkali metals to form their tungsten bronzes, and by other metals such as tin and lead. Molybdenum bronzes also exist but are less stable than their tungsten counterparts. ==References==