Pyrochlores. V. Thermoanalytic, X-ray, neutron, infrared, and dielectric studies of A2Ti2O7 titanates

Abstract

A number of compositions A₂³⁺Ti₂⁴⁺O₇ and of related mixed phases were prepared and investigated. For the trivalent lanthanons from Sm to Lu, and for Y, the homogeneous A₂Ti₂O₇ phases were of the cubic pyrochlore type. Their lattice parameters gave a very good linear relationship when plotted against the Templeton–Dauben radii of A³⁺. The room-temperature stability region of the pyrochlore titanates at ordinary pressures extends from an Ahrens radius ratio, r(A³⁺):r(Ti⁴⁺), of about 1.22 to about 1.50, taking into account homogeneous mixed phases. The compositions Bi₂Ti₂O₇, In₂Ti₂O₇, and Cr₂Ti₂O₇ were not cubic pyrochlores; the results of an X-ray and neutron examination of Sc₂Ti₂O₇ indicate that further work is needed to clarify its structural properties.The structures of Lu, Y, Gd, and Sm titanates were refined from X-ray powder data and that of Y₂(Ti_0.64Zr_0.36)₂O₇, from neutron powder data. The structure of Er₂Ti₂O₇ was re-refined from previously published data without significant improvement. The positional parameter of the majority oxygen atom, x(O₂), in A₂B₂O₇ pyrochlores has been found to increase, and the distortion of the BO₆ octahedra to decrease, with the increasing r(A³⁺):r(B⁴⁺) ratio. This is in agreement with a similar result obtained earlier from Mössbauer spectra for A₂FeSbO₇ pyrochlores.The variation of the frequencies of the principal absorption bands in the infrared spectra of the titanates has been correlated with r(A³⁺). No evidence of ferroelectricity was found in Sm and Er titanates between room temperature and 4.2 °K.The Templeton–Dauben set of empirical crystal radii for trivalent lanthanons has been supplemented by values for Sc, In, Tl, and Bi derived from the C-type sesquioxides and the pyrochlores. The self-consistency and applicability of the expanded set have been tested on several series of isostructural compounds.