Discovering New Oxides

Abstract

The formation of many oxides with unpredictable structures can be attributed to a balance between stable and unstable compositions. A surprising number of the members of the Ln4Cu3MoO12 and Ln‘2Ln‘ ‘2Cu3MoO12 (Ln = La−Nd, Sm−Lu) families form, all crystallizing in a hexagonal structure similar to that of YMnO3. Some do not form because either the average lanthanide size is too small (r̄Ln < 105 pm, where rLn ≡ 9-coordinate trivalent lanthanide radii) or the difference between the size of Ln‘ and Ln‘ ‘ is too large (ΔrLn > 14 pm). Furthermore, even if Ln‘4Cu3MoO12 and Ln‘ ‘4Cu3MoO12 form single phases, attempts to synthesize the corresponding solution phase, Ln‘2Ln‘ ‘2Cu3MoO12, may result in a mixture of simpler metal oxides. The explanation illuminates the importance of unstable “umbrella” stoichiometries for the formation of unconventional structures in multication systems.