ORDER-DISORDER IN A2M3+M5+O6 PEROVSKITES

Using x-ray and neutron diffraction data, the degree of order of the o ctahedral site cations has been determined for the perovskites Sr2AlNb O6 and Sr2AlTaO6, which have been prepared by several different method s and annealed at temperatures up to 1690-degrees-C. The degree of ord er generally increases with increasing synthesis temperature. The amou nt of cation ordering is, therefore, primarily controlled by kinetic p rocesses and not by thermodynamic equilibrium considerations. Increase d order obtained with increased heating time confirms this general kin etic limitation on the degree of order. However, annealing Sr2AlNbO6 i n 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 doma ins are separated by antiphase boundaries which occur in high concentr ations. The cubic cell edge within the ordered domains is significantl y smaller than the overall cell edge when the concentration of antipha se boundaries is high. The antiphase boundaries cause significant latt ice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A2M3+M5+O6 systems are briefly presented.