Molybdenum bronze

In chemistry, molybdenum bronze is a generic name for certain mixed oxides of molybdenum with the generic formula AxMoyOz where A may be hydrogen, an alkali metal cation (such as Li+, Na+, K+), and Tl+. These compounds form deeply coloured plate-like crystals with a metallic sheen, hence their name. These bronzes derive their metallic character partially occupied 4d-bands. The oxidation state in K0.28MoO3 are K+1, O2−, and Mo+5.72. MoO3 is an insulator, with an unfilled 4d band. In chemistry, molybdenum bronze is a generic name for certain mixed oxides of molybdenum with the generic formula AxMoyOz where A may be hydrogen, an alkali metal cation (such as Li+, Na+, K+), and Tl+. These compounds form deeply coloured plate-like crystals with a metallic sheen, hence their name. These bronzes derive their metallic character partially occupied 4d-bands. The oxidation state in K0.28MoO3 are K+1, O2−, and Mo+5.72. MoO3 is an insulator, with an unfilled 4d band. These compounds have been much studied since the 1980s due to their markedly anisotropic electrical properties, reflecting their layered structure. The electrical resistivity can vary considerably depending on the direction, in some cases by 200:1 or more. They are generally non-stoichiometric compounds. Some are metals and some are semiconductors. The first report of a 'molybdenum bronze' was by Alfred Stavenhagen and E. Engels in 1895. They reported that electrolysis of molten Na2MoO4 and MoO3 gave indigo-blue needles with metallic sheen, which they analysed by weight as Na2Mo5O7. The first unambiguous synthesis of alkali molybdenum bronzes was reported only in 1964, by Wold and others. They obtained two potassium bronzes, 'red' K0.26MoO3 and 'blue' K0.28MoO3, by electrolysis of molten K2MoO4+MoO3 at 550 °C and 560 °C, respectively. Sodium bronzes were also obtained by the same method. It was observed that at a slightly higher temperature (about 575 °C and above) only MoO2 is obtained. Another preparation technique involves crystallization from the melt in a temperature gradient. This report also called attention to the marked anisotropic resistivity of the purple lithium bronze Li0.9Mo6O17 and its metal-to-insulator transition at about 24 K. Hydrogen bronzes HxMoO3 were obtained in 1950 by Glemser and Lutz, by ambient-temperature reactions. The hydrogen in these compounds can be replaced by alkali metals by treatment with solutions of the corresponding halides. Reactions are conducted in an autoclave at about 160 °C. Molybdenum bronzes are classified in three major families: