Acta Crystallographica Section B

Structural Science

Volume 57, Part 3 (June 2001)

research papers

Acta Cryst. (2001). B57, 221-230 [ doi:10.1107/S0108768100019960 ]

X-ray diffraction study of the phase transition of K₂Mn₂(BeF₄)₃: a new type of low-temperature structure for langbeinites

A. Guelylah, G. Madariaga, V. Petricek, T. Breczewski, M. I. Aroyo and E. H. Bocanegra

Abstract: The potassium manganese tetrafluoroberyllate langbeinite compound has been studied in the temperature range 100-300 K. Using DSC measurements, a phase transition has been detected at 213 K. The space group of the low-temperature phase was determined to be P112₁ using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled with respect to the prototypic P2₁3 cubic phase. Lattice parameters were determined by powder diffraction data [a = 10.0690 (8), b = 20.136 (2), c = 10.0329 (4) Å, [gamma] = 90.01 (1)°]. A precise analysis of the BeF₄ tetrahedra in the low-temperature phase shows that two independent tetrahedra rotate in opposite directions along the doubled crystallographic axis. A symmetry mode analysis of the monoclinic distortion is also reported. This is the first report of the existence of such a phase transition in K₂Mn₂(BeF₄)₃ and also of a new type of low-temperature structure for langbeinite compounds.

Keywords: langbeinite; low-temperature structure; twinning.