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-temperat ure phase was determined to be P112(1) using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled wit h respect to the prototypic P2(1)3 cubic phase. Lattice parameters were det ermined by powder diffraction data [a = 10.0690 (8), b = 20.136 (2), c = 10 .0329 (4) Angstrom = 90.01 (1)degrees]. A precise analysis of the BeF4 tetr ahedra in the low-temperature phase shows that two independent tetrahedra r otate in opposite directions along the doubled crystallographic axis. A sym metry 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(Be F4)(3) and also of a new type of low-temperature structure for langbeinite compounds.