High-temperature rutile-derived crystallographic shear structures. I. (020)r CS structures
Abstract
Electron diffraction and single-crystal X-ray diffraction techniques have been applied to a study of the structures of phases in the Fe 2O 3TiO 2 system, for compositions in the range 14-16 wt% Fe 2O 3 and for temperatures above 1450°C. The compounds in this system may be described as (020) rCS phases in which adjacent rutile slabs, infinitely extended along [100] r and [001] r, are displaced by {1}/{2}[011] r across (020) rCS planes. In the CS planes, about two-thirds of the available metal-atom octahedral sites are occupied in a deficient NiAs arrangement. The composition range studied is spanned by a continuous series of (020) r intergrowths formed by the ordered mixing of structures with two different CS plane spacings, namely, 9 × d(020) r and 11 × d(020) r. Conventional structure solution methods could not be applied to these infinitely adaptive structures because of the small intensity data to parameter ratios for the high-order intergrowths. We describe a method of structure solution based on a series of successive approximations. The method was applied to determine the structure of one of the intergrowths using single-crystal X-ray diffraction data. For the temperature range studied, the CS phases are within a few degrees of their melting points, and this is reflected in a departure from complete structural order. The results show both long-range disorder in the intergrowth sequences and an absence of correlation between the metal-atom ordering in one CS plane and the next. This disorder is manifested by diffuse scattering in the diffraction patterns. The (020) rCS structures transform reversibly to the well-known (121) r-(132) r family of ordered rutile CS structures below 1450°C.
- Publication:
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Journal of Solid State Chemistry France
- Pub Date:
- January 1976
- DOI:
- 10.1016/0022-4596(76)90049-9
- Bibcode:
- 1976JSSCh..16..331B