New Calcium Tantalate Polytypes in the System Ca2Ta2O7–Sm2Ti2O7

✍ Scribed by Ian E. Grey; Robert S. Roth

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 581 KB
• Volume: 150
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1999.8573

✦ Synopsis

The 6T structure is related to the 3T weberite structure, having a cubic close packed metal atom array as in 6uorite and a stacking of (001) metal atom layers (,+111, f ) that alternately have the composition Ta 3 Ca and Ca 3 Ta. Hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected by isolated TaO 6 octahedra in the Ca 3 Ta layers, giving an octahedral framework of composition [Ta 2 O 7 ] 4؊ in which the eight-coordinated calcium atoms are distributed. The octahedra in the Ca 3 Ta layers have only four anions involved in corner sharing. The di4erent 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 Ca 3 Ta layers. The 5M polytype di4ers 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.