Liquid-mix disorder in crystalline solids: ScMnO3

The liquid-mix citrate synthesis of ScMnO3, when conducted at temperatures below approximately 700 degrees C, leads to the formation of a (nano)crysta lline oxide with the liquid-mix disorder of the metal atoms preserved. The disordered phase has an excess of oxygen atoms over three per formula, impl ying the presence of both Mn3+ and Mn4+. Synchrotron X-ray powder diffracti on reveals two phases (domains) with nearly identical lattices in the liqui d-mix sample; a bixbyite-type phase (as adopted by Sc2O3 and Mn2O3) and a d efect fluorite-type phase. Examinations of the unit-cell parameters indicat e that most if not all of the Mn4+ and excess oxygen are incorporated into the fluorite-type phase, Annealing at temperatures higher than 700 degrees C induces a rapid transformation of this phase mixture into a single-phase product represented by the stable hexagonal modification of ScMnO3. Both co mponents of the liquid-mix oxide, viz, the metastable bixbyite-type phase a nd the oxidation-stabilized fluorite-type phase, exhibit high strain attrib uted to the size imbalance of the constituent metal atoms. Transformation i nto the stable ordered hexagonal phase is accompanied by an order of magnit ude decrease in the micro-strain and a dramatic increase in the crystallite size, The kinetics of the ordering phase transition have been investigated using conventional X-ray powder diffraction techniques. Disordered (Sc1-xM nx)(2)O3+delta solid solutions, synthesized analogously by the liquid-mix t echnique, transform under exsolution of ScMnO3 into stable solid solutions, The thermodynamically stable solid-solution limits are estimated to be (Sc 0.85Mn0.15)(2)O-3 and (Sc0.10Mn0.90)(2)O-3 at 1000 degrees C for the scandi um and manganese rich ends of the phase diagram, respectively. (C) 1998 Aca demic Press.