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(Ti0.64Zr0.36)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.