Acta Crystallographica Section B

Structural Science

Volume 56, Part 6 (December 2000)

research papers

Acta Cryst. (2000). B56, 921-935 [ doi:10.1107/S0108768100009514 ]

X-ray structure determination of the monoclinic (121 K) and orthorhombic (85 K) phases of langbeinite-type dithallium dicadmium sulfate

A. Guelylah, G. Madariaga, W. Morgenroth, M. I. Aroyo, T. Breczewski and E. H. Bocanegra

Abstract: The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl₂Cd₂(SO₄)₃ have been determined at 121 and 85 K, respectively, by X-ray diffraction. A precise analysis of these structures shows the existence of some differences compared to langbeinites of type II. The monoclinic structure differs very little from the high-temperature cubic structure and the distortion relating the monoclinic structure to the cubic one is very small. SO₄ tetrahedra seem to rotate under orthorhombic symmetry in the monoclinic phase. A symmetry distortion analysis of the ferroelectric monoclinic distortion discloses the importance of the secondary modes with orthorhombic symmetry, especially for the O atoms of the SO₄ groups.

Keywords: Langbeinite; Low temperature; Twinning.