Pyrochlores. III. X-Ray, neutron, infrared, and dielectric studies of A 2 Sn 2 O 7 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 A 2 Sn 2 O 7 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 A 3+ . The applicability of other sets of ionic radii was also tested. The lower limit of the r(A 3+ ):r(Sn 4+ ) 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. Bi 2 Sn 2 O 7 is not a cubic pyrochlore.The structures of Y 2 Sn 2 O 7 and Sm 2 Sn 2 O 7 were refined from X-ray powder data, and that of La 2 Sn 2 O 7 , from neutron powder data. The positional parameter of the majority oxygen atom, x(O 2 ), has been found to increase with the size of A 3+ . The distortion of the SnO 6 octahedra in the pyrochlore structure thus decreases with increasing r(A 3+ ).From a systematic study of the infrared spectra of the stannates it appears that it is the size of the Y 3+ ion rather than its chemistry (as distinct from the Ln 3+ 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.