Characterization of New 5M and 7M Polytypes of Niobia-Doped Ca2Ta2O7
| Autor: | Robert S. Roth, C. Li, Leonid A. Bendersky, J. Chenavas, J. Planes, Ian E. Grey, William G. Mumme |
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| Rok vydání: | 2001 |
| Předmět: |
Materials science
Neutron diffraction chemistry.chemical_element Crystal structure Tungsten Condensed Matter Physics Electronic Optical and Magnetic Materials Tantalate Inorganic Chemistry Metal Crystallography Octahedron chemistry visual_art Materials Chemistry Ceramics and Composites visual_art.visual_art_medium Physical and Theoretical Chemistry Isostructural Monoclinic crystal system |
| Zdroj: | ResearcherID |
| ISSN: | 0022-4596 |
| DOI: | 10.1006/jssc.2001.9308 |
| Popis: | Two new calcium tantalate polytypes have been prepared in the system Ca 2 Ta 2 O 7 –Ca 2 Nb 2 O 7 . Their structures have been determined and refined using single-crystal X-ray diffraction data and powder neutron diffraction data, and their dielectric properties have been measured in the 100 kHz to 10 GHz frequency range. The 7M polytype of pure Ca 2 Ta 2 O 7 has monoclinic symmetry with a =12.726(1), b =7.380(1), c =42.538(1) A, β =95.77(1)°, Z =28, space group C 2. The 7M Ca 2 Ta 1.9 Nb 0.1 O 7 is isostructural, with a =12.7170(6), b =7.3728(3), c =42.464(2) A, β =95.755(5)°. The 5M Ca 2 Ta 1.8 Nb 0.2 O 7 is monoclinic with a =12.7528(4), b =7.3492(2), c =30.2442(8) A, β =94.245(2)°, Z =20, space group C 2. The two polytypes have in common a closest-packing of (001) metal atom layers that alternately have the composition Ca 3 Ta and Ta 3 Ca. The polytypes have mixed hexagonal-cubic stacking sequences of the metal atom layers: 7M= hccccchccccccch … , 5M= hccchccccch … . The cubic-stacked blocks have weberite-type structures in both polytypes, in which hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected via corner linking to isolated TaO 6 octahedra in the Ca 3 Ta layers to give a [Ta 2 O 7 ] 4− framework within which the eight-coordinated calcium atoms are distributed. The polytypes display a broad resonance at about 8 GHz, with relative permittivities of ∼25 and with Q values of about 200. |
| Databáze: | OpenAIRE |
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