What Are Tungsten Bronzes I

What are tungsten bronzes?

The tungsten bronzes are a very interesting, but little appreciated, family of materials. They are not related to bronze, an alloy of copper and tin, except coloration. However, the structure of tungsten bronzes are similar to the high-temperature copper oxide superconductors. In fact, the tungsten bronzes were the first oxide superconductors and were the focus of extensive research 10-15 years ago. But by the early 1980s, most of this work had been set aside in favor of other pursuits.

The tungsten bronzes are a group of compounds made up of tungsten trioxide, WO3, and an alkali metal, such as sodium (Na) , potassium (K), rubidium (Rb) , or cesium (Cs). The general chemical form is MxW03, where M=Na, K, Rb, or Cs, and O<x<l. The color of these compounds varies with composition, at x=0.93 the color is a bronzelike golden-yellow, hence the name; at x =0.32 the color is a blue-violet. For this reason tungsten bronzes are use as pigments in dyes allld paints.

The variation in composition also affects the structure of the compound. Imagine a ¢ube with a tungsten atom at each comer, an oxygen atom in the middle of each edge and an atom of an alkali metal in the center of the cube. However, in a tungsten bronze there is not an atom at the center of every cube. When x< 1, only a certain fraction of the cubes will contain an alkali atom. If x is large, close to 1, the structure of the crystal lattice will be cubic. As x decreases, and fewer of the cubes are filled, the structure changes. At about x<0.3, or with less than 30% of the cubes full, the structure becomes hexagonal, with atoms arranged in hexagonal plates.