title: X-ray diffraction study of the phase transition of K2Mn2(BeF4)3: a new type of low-temperature structure for langbeinites.
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 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.
authors.affiliation: Departamento de Física de la Materia Condensada, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, Bilbao 48080, Spain. wmbgugua@lg.ehu.es
authors.forename: A
authors.initials: A
authors.lastname: Guelylah
authors.forename: G
authors.initials: G
authors.lastname: Madariaga
authors.forename: V
authors.initials: V
authors.lastname: Petricek
authors.forename: T
authors.initials: T
authors.lastname: Breczewski
authors.forename: M I
authors.initials: MI
authors.lastname: Aroyo
authors.forename: E H
authors.initials: EH
authors.lastname: Bocanegra