Polymorphism of K2ZrF6

Heating of K2ZrF6 room temperature monoclinic Form I reveals a complex polymorphism investigated by synchrotron powder temperature-dependent diffraction. The first thermal event at 240 degrees C corresponds to the quasi-simultaneous apparition of four different phases (noted II-V), of which only three could be indexed and two completely characterized structurally. Form II (structure unrefined) is very probably hexagonal (a = 6.4473(1) angstrom, c = 3.8606(1) angstrom at 264 degrees C, Z = 1). Form III is cubic (space group Fm (3) over barm, a = 9.0804(1) angstrom at 264 degrees C, Z = 4), related to K3ZrF7. Form IV remains unindexed, presenting broad diffraction peaks. Form V is present as a weak impurity at this stage, increasing in proportion after the second thermal event at 288 degrees C where a new Form VI is built. Form V is tetragonal (space group P4/nmm, a = 6.3281(1) A, c = 9.3671(2) A at 295 degrees C, Z = 2), and Form VI is orthorhombic (space group Cmcm, a = 6.40840(5) angstrom, b = 20.7697(2) angstrom, c = 19.9888(2) angstrom, at 295 degrees C, Z = 16). At 317 degrees C, Form VI transforms into Form V. At 449 degrees C, a different orthorhombic Form VII is disclosed (space group Pnma, a = 10.3084(1) A, b = 6.0291(1) angstrom, c = 23.8023(2) angstrom, at 495 degrees C, Z = 8), coexisting with the cubic Form III. On cooling, Form VII, of which an approach of the crystal structure is provided, returns to Form V at 411 degrees C, which looks stable down to 139 degrees C giving Form I back. Isolated ZrF6 octahedra are present in Forms III, V, VI, and VII. Form VI contains also dimeric entities Zr2F(12) built from a ZrF7 monocapped trigonal prism sharing a corner with a ZrF6 octahedron. Form VII structure is related to Form V by quadrupling the tetragonal a parameter, and the reversible V. VII transition is very probably topotactic. The coexistence of several phases at each temperature above the first transition at 240 degrees C was observed. Peak multisplitting is observed at many temperatures, which suggests the occurrence of coexisting microphases. The parallel existence of several phases was observed also by F-19 MAS NMR spectroscopy measured from 250 degrees C. 410 degrees C. room temperature.