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

Abstract

A number of con~positions AZ3+TiZ4+ 0 7 and of related mixed phases were prepared and investigated. For the trivalent lanthanons from Sm to Lu, and for Y, the homogeneous A,Ti207 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 Bi2Ti207, In2Ti207, and Cr,Ti20, were not cubic pyrochlores; the results of an X-ray and neutron examination of Sc2Ti20, indicate that further work is needed to clarify its structural properties.The structures of Lu, Y, Gd, and Snl titanates were refined from X-ray powder data and that of Y,(Ti,,6,Zro.36),07, from neutron powder data. The structure of Er2Ti207 was re-refined from previously published data without significant improvement. The positional parameter of the majority oxygen atom, x(OZ), in A2B207 pyrochlores has been found to increase, and the distortion of the B06 octahedra to decrease, with the increasing r(A3+):y(B4+) ratio. This is in agreement with a similar result obtained earlier from Mossbauer spectra for A2FeSb0, pyrochlores.The variation of the frequencies of the principal absorption bands in the infrared spectra of the titanates has been correlated with ?(A3+). No evidence of ferroelectricity was found in Sm and Er titanates between room temperature and 4.2 OK.The Templeton-Dauben set of empirical crystal radii for trivalent lanthanons has been supplemented by values for Sc, In, TI, and Bi derived from the C-type sesquioxides and the pyrochlores. The selfconsistency and applicability of the expanded set have been tested on several series of isostructural con~pounds.

Compression behavior of Sm2Ti2O7-pyrochlore up to 50 GPa: single-crystal X-ray diffraction and density functional theory calculations

“…The bulk modulus obtained from the stress-strain calculations was 187(1) GPa, in excellent agreement with the value obtained from a fit to the compression data with a third-order BirchMurnaghan equation of state, EOS, which yielded V o = 1056.56(4) Å 3 , B ¼ 185:4ð2Þ GPa and B 0 ¼ 4:28ð1Þ. The ambient pressure volume is well established by several independent measurements (see data in [22] and [23]) as 1071 Å 3 . The DFT value (1056.6 Å 3 ) differs from this value by 1.4 %.…”

“…The scatter in the B(V) relationship is to be expected, as different EOS have been used for the fits. Typically, data have been collected only at very few pressure points, and uncertainties due to the significant [23] and [22] correlation between B and B 0 [26] will then cause some scatter. Clearly, more data with higher accuracy are required to better understand the origin of the scatter in the BðVÞ-relation.…”

Electrical and catalytic properties of some oxides with the fluorite or pyrochlore structure

“…A smaller A ion in pyrochlores A2B2060' causes an increased distortion of the BO 6 octahedra, which leads to an increase in the oxygen position parameter X48f,as defined by Knop [9]. Theoretical calculations, performed by Pannetier on pyrochlores A2B2060' [10], show that this increase in X48 f results in a less negative electrostatic energy of the B206 sublattice, while the energy of the A20' sublattice remains unaffected.…”

Pyrochlores. VI. Preparative chemistry of sodium and silver antimonates and related compounds

“…The minima in the composition fields are bordered by compounds of such small M contents that their formulae are close to Sb,O,; the departure of z from the value of five depends on the amount of Sb(II1) present. (5) Silver antimonates are anomalous in comparison with the sodium and potassium compounds. Apparently, either the cation distributions (or site occupancies, or both) in the Ag antimonates are different from those in the two alkali antimonates, or else the bonding of the Ag atom in the pyrochlore structure differs from that of Na and K. For personal use only.…”

“…However, while silver antimonate has always been found to have a pyrochlore structure, regardless of the method of preparation, NaSbO, has been obtained with the pyrochlore structure only by treating "antimonic acid", in solution or as a 'For Part V, see ref. 5. 2Since detailed structure determinations have not as yet been made for all compounds whose preparations are reported in this paper, the term "pyrochlore" is used to denote any compound that gave an X-ray powder pattern consistent with the space group and dimensions of the classical pyrochlore structure.…”