UNSPECIFIED (1998) Investigation of the CsxRb1-xTiOAsO4 series. II. The problems of interpretation of residual electron densities in a polar space group. ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE, 54 (Part 5). pp. 645-651. ISSN 0108-7681

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Abstract

The problems of refinements of structures in a polar space group, such as Pna2(1), are illustrated by the example of crystals in the Cs2xRb2-2xTi2O2As2O8, caesium rubidium titanyl arsenate, series, which are isostructural with the well known non-linear optical crystal KTiOPO4. It is shown in particular that errors in the data collection and/or refinement tend to contribute positive residual electron density in sites which are related by an inversion operation to the real Cs/Rb sites. This phenomenon is a consequence of the presence of two minima in least-squares space, representing here the possible coordinate sets x, y, z and x, y, - z, for a polar structure and is exacerbated in these structures by the presence of strong inversion psuedo-symmetry. It is shown that in the worst case, the positive residual electron density can be eliminated and an improved agreement to the observed data obtained by refinement of a partially micro-twinned disordered structure in which the Cs/Rb occupancy is split between the main sites and the inversion-related sites. It is argued that the true structure is micro-twinned, including partial occupancy of the additional sites.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Journal or Publication Title:	ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE
Publisher:	MUNKSGAARD INT PUBL LTD
ISSN:	0108-7681
Date:	1 October 1998
Volume:	54
Number:	Part 5
Number of Pages:	7
Page Range:	pp. 645-651
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/15249

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