α-PbO2-related phase appearing in the SnIV-Ta-O system transformed from cation-ordered fluorite-related phase

The cation-ordered alpha-PbO2-related Sn-0.82(IV)(Ta0.94Sn0.06IV)O-4.11 phase, which is a triclinic or monoclinic system with lattice parameters of a = 0.769 nm, b = 0.947 nm, c = 0.769 nm, alpha = 90 degrees, beta = 96.34 degrees and gamma = 90 degrees, was synthesized by annealing of the cation-ordered fluorite-related Sn-0.82(IV)(Ta0.94Sn0.06IV)O-4.11 phase at 973 <= T <= 1073K in O-2; the cation-ordered fluorite-related phase was prepared by the oxidation of the Sn-1.64(II)(Ta1.88Sn0.12IV)O-6.58 pyrochlore. The SnIV and [0.94Ta + 0.06Sn(IV)] were respectively ordered similar to that of SnII and [0.94Ta + 0.06Sn(IV)] as observed in the precursor pyrochlore phase. The superlattice diffraction due to the cation-ordering did not appear in its XRD because of the high density of the anti-phase domain boundaries parallel to {101} direction. The transformation temperature from the fluorite-related form to the alpha-PbO2-related form for the Sn-0.82(IV)(Ta0.94Sn0.06IV)O-4.11 was approximately 250K higher than that observed for the previous Sn-0.81(IV)(Nb0.93Sn0.07IV) O-4.085. The difference in the transformation behavior starting from the precursor pyrochlore phase between the present Sn-Ta-O and the previous Sn-Nb-O systems was discussed on the basis of the characteristics of the Ta- and Nb-ions. c 2014 The Ceramic Society of Japan. All rights reserved.