ORDER-DISORDER IN A2M3+M5+O6 PEROVSKITES

Using x-ray and neutron diffraction data, the degree of order of the octahedral site cations has been determined for the perovskites Sr2AlNbO6 and Sr2AlTaO6, which have been prepared by several different methods and annealed at temperatures up to 1690-degrees-C. The degree of order generally increases with increasing synthesis temperature. The amount of cation ordering is, therefore, primarily controlled by kinetic processes and not by thermodynamic equilibrium considerations. Increased order obtained with increased heating time confirms this general kinetic limitation on the degree of order. However, annealing Sr2AlNbO6 in the highest temperature region resulted 'in some decrease in order, presumably due to thermodynamic considerations. The cubic edge of both compounds decreases significantly with increasing order. Ordered domains are separated by antiphase boundaries which occur in high concentrations. The cubic cell edge within the ordered domains is significantly smaller than the overall cell edge when the concentration of antiphase boundaries is high. The antiphase boundaries cause significant lattice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A2M3+M5+O6 systems are briefly presented.