Crystal structural studies of the alkali and barium transition metal fluorides RbK2Mn2F7, BaNiF4, and a 5 : 3-phase of the system BaLiF3/NaCoF3

At single crystals of the compounds RbK2Mn2F7, BaNiF4, and of a phase 0.618 BaLiF3/0.382 NaCoF3 the X-ray crystal structures were refined. RbK2Mn2F7 i s tetragonal (a = 421.1(1), c = 2188.3(2) pm, I4/mmm, Z = 2) and belongs to the Sr3Ti2O7 type. The average distances are Mn-F: 210.7 pm for the [MnF6] octahedron and A-F: 290.6 resp. 297.1pm for the [AF(9)] resp. [AF(12)] coo rdination of the mixed alkali positions (A = Rb/3+2K/3). BaNiF4 (a = 413.7( 1), b = 1443.1(3), c = 578.1(1) pm, Cmc2(1), Z = 4) is of the orthorhombic BaZnF4 type; Ni-F: 200.3 pm, Ba-F: 274.3 pm for CN6 and CN9, resp.. The pha se of approximate composition 5:3, isolated from a 1:1 batch BaLiF3/NaCoF3, is cubic (a = 801.8(1) pm, Im3, Z = 8 AMF(3)) and forms a strongly disorde red perovskite super-structure, the features of which are discussed.