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

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 P112(1) using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled with respect to the prototypic P2(1)3 cubic phase. Lattice parameters were determined by powder diffraction data [a = 10.0690 (8), b = 20.136 (2), c = 10.0329 (4) A, gamma = 90.01 (1) degrees ]. A precise analysis of the BeF(4) 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 K(2)Mn(2)(BeF(4))(3) and also of a new type of low-temperature structure for langbeinite compounds.

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

The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl(2)Cd(2)(SO(4))(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. SO(4) 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 SO(4) groups.