Discovering New Oxides

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 Ln 4 Cu 3 MoO 12 and Ln‘ 2 Ln‘ ‘ 2 Cu 3 MoO 12 (Ln = La−Nd, Sm−Lu) families form, all crystallizing in a hexagonal structure similar to that of YMnO 3 . Some do not form because either the average lanthanide size is too small ( r̄ Ln < 105 pm, where r Ln ≡ 9-coordinate trivalent lanthanide radii) or the difference between the size of Ln‘ and Ln‘ ‘ is too large (Δ r Ln > 14 pm). Furthermore, even if Ln‘ 4 Cu 3 MoO 12 and Ln‘ ‘ 4 Cu 3 MoO 12 form single phases, attempts to synthesize the corresponding solution phase, Ln‘ 2 Ln‘ ‘ 2 Cu 3 MoO 12, 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.