Pyrochlores. III. X-Ray, neutron, infrared, and dielectric studies of A2Sn2O7 stannates

Abstract

A number of compositions [Formula: see text] and of related mixed phases were prepared and investigated. For the trivalent lanthanons from La to Lu, and for Y, the homogeneous A2Sn2O7 phases were of the cubic pyrochlore type (space group Fd3m, No. 227). Their lattice parameters gave a very good linear relationship when plotted against the Templeton–Dauben radii of A3+. The applicability of other sets of ionic radii was also tested. The lower limit of the r(A3+):r(Sn4+) ratio for cubic pyrochlore stannates is about 1.19 (Ahrens radii), while the upper limit is at least 1.60. Attempts to extend the lower limit by preparing mixed Yb + Sc stannates were only partially successful. Bi2Sn2O7 is not a cubic pyrochlore.The structures of Y2Sn2O7 and Sm2Sn2O7 were refined from X-ray powder data, and that of La2Sn2O7, from neutron powder data. The positional parameter of the majority oxygen atom, x(O2), has been found to increase with the size of A3+. The distortion of the SnO6 octahedra in the pyrochlore structure thus decreases with increasing r(A3+).From a systematic study of the infrared spectra of the stannates it appears that it is the size of the Y3+ ion rather than its chemistry (as distinct from the Ln3+ ions) that determines its place among the 3–4 pyrochlores. No evidence of ferroelectricity was found in Pr, Er, and Lu stannates between room temperature and 4.2 °K.