New Calcium Tantalate Polytypes in the System Ca2Ta2O7 - Sm2Ti2O7

Published

February 1, 2000

Author(s)

I E. Grey, Robert S. Roth

Abstract

New calcium tantalate polytypes, with weberite related structures have been prepared by doping Ca₂Ta₂O₇ with Sm₂Ti₂O₇. Trigonal 6-layer (6T) and monoclinic 5-layer (5M) polytypes containing approximately equal to} 6 mol % and 10 mol % respectively of the Sm₂Ti₂O₇ component were prepared and the structures were determined and refined using X-ray data on flux-grown single crystals. The 6T phase, Ca_1.89Ta_1.86Sm_0.16Ti_0.10O₇, has a = 7.353(1), c = 36.264 (1) , Z = 12, space group P31, and the 5M phase, Ca_1.80Ta_1.80Sm_0.24Ti_0.17O₇, has a = 12.763(1), b = 7.310(1), c = 30.190(1) , b = 94.09 (1), Z=20, space group C2. The 6T structure is related to the 3T weberite structure, having a cubic close packed metal atom array, and a stacking of (001) metal atom layers that alternately have the composition Ta₃Ca and Ca₃Ta. Hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected by isolated TaO₆ octahedra in the Ca₃Ta layers, giving an octahedral framework of composition [Ta₂O₇]^4- in which the 8-coordinated calcium atoms are distributed. The octahedra in the Ca₃Ta layers have only four anions involved in corner sharing. The different weberite polytypes are distinguished by the geometry of the two terminal anions, cis- or trans. In the 6T structure there is an alternation of cis- and trans- octahedra in successive Ca₃Ta layers. The 5M polytype differs from the weberite polytypes in having regular twinning of the metal atom lattice on (001), giving a stacking sequence hccchccccc... The c-stacked blocks alternately have the weberite and pyrochlore structures, with ordering of Sm into the Ca sites in the pyrochlore blocks.

Citation

Journal of Solid State Chemistry

Volume

150

Issue

No. 1

Pub Type

Journals

Keywords

Ca₂Ta₂O₇, crystal structure, weberite polytypes

Citation

Grey, I. and Roth, R. (2000), New Calcium Tantalate Polytypes in the System Ca₂Ta₂O₇ - Sm₂Ti₂O₇, Journal of Solid State Chemistry (Accessed May 18, 2024)