Although the most common perovskite compounds contain oxygen, there are a few perovskite compounds that form without oxygen. Fluoride perovskites such as NaMgF are well known. A large family of metallic perovskite compounds can be represented by RTM (R: rare-earth or other relatively large ion, T: transition metal ion and M: light metalloids). The metalloids occupy the octahedrally coordinated "B" sites in these compounds. RPdB, RRhB and CeRuC are examples. MgCNi is a metallic perovskite compound and has received lot of attention because of its superconducting properties. An even more exotic type of perovskite is represented by the mixed oxide-aurides of Cs and Rb, such as CsAuO, which contain large alkali cations in the traditional "anion" sites, bonded to O and Au anions.