New calcium tantalate polytypes in the system Ca2Ta2O7-Sm2Ti2O7

摘要

New calcium tantalate poly types with weberite-related structures have been prepared by doping Ca2Ta2O7 with Sm2Ti2O7. Trigonal 6-layer (6T) and monoclinic 5-layer (5M) polytypes containing similar to 6 and 10 mol%, respectively of the Sm2Ti2O7 component were prepared and the structures were determined and refined using S-ray data on flux-grown single crystals. The 6T phase, Ca1.89Ta1.86Sm0.16Ti0.10O7, has a = 7.353(1), c = 36.264(1) Angstrom, Z = 12, space group P3(1), and the 5M phase, Ca1.80Ta1.80Sm0.24Ti0.17O7 has a = 12.763(1), b = 7.310(1), c = 30.190(1) Angstrom, beta = 94.09 degrees(1), Z = 20, space group C2. The 6T structure is related to the 3T weberite structure, having a cubic close packed metal atom array as in fluorite and a stacking of (001) metal atom layers (={111}(f)) 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 eight- 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 alternatively have the weberite and pyrochlore structures, with ordering of Sm into the Ca sites in the pyrochlore blocks. (C) 2000 Acadmic Press. [References: 21]