Hydroxyperovskites: An Overlooked Class of Potential Functional Materials

While there is enormous interest in studying oxide perovskites with stoichiometries based upon or derived from ABO3, including oxygen-deficient compositions and organometallics, other closely related topologies have been overlooked. Hydroxyperovskites are such a group. Their structures are perovskite-like octahedral frameworks with vacant cavity A sites, and all oxygen atoms form hydroxyl groups. There are fifteen naturally occurring hydroxyperovskites and numerous synthetic analogues. There are two stoichiometries: BB′(OH)6 and B(OH)3. The former consist of alternating divalent and tetravalent cations (B = Mg, Ca, Mn2+, Fe2+, Co2+, Cu2+, Zn; B′ = Sn, Ge). B(OH)3 structures have only trivalent cations (Al, Fe3+, Ga). The properties and behavior of solid solutions in hydroxyperovskites are largely unexplored. This article summarizes our current knowledge of the crystallography and crystal chemistry of hydroxyperovskites and suggests productive areas of research in relation to their potential as functional materials. It should be evident that much of the findings remains to be discovered.