Caesium suboxide

In addition to the above oxides M2O, M2O2, M4O6, MO2 and MO3 in which the alkali metal has the constant oxidation state 4-1, rubidium and caesium also form suboxides in which the formal oxidation state of the metal is considerably lower. Some of these intriguing compounds have been known since the turn of the century but only recently have their structures been elucidated by single crystal X-ray analysis. Partial oxidation of Rb at low temperatures gives RbeO which decomposes above —7.3°C to give copper-coloured metallic crystals of Rb902 ... 

Other examples of compounds of this type are the suboxides of rubidium and caesium, made by fusing the normal oxides, Rb20 and CS2O, with the parent metals. These are lustrous and have good electrical conductivities, diminishing with increase of temperature. There are several for each metal, examples being Rb902, which is copper coloured, and CsyO, which is bronze. 

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