Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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Ľubomír Smrčok, Armel Le Bail, Miroslav Boca, Aydar Rakhmatullin, 'Polymorphism of K2ZrF6', Cryst. Growth Des. 20 3867−3881 (2020) doi:10.1021/acs.cgd.0c00166

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 °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) Å, c = 3.8606(1) Å at 264 °C, Z = 1). Form III is cubic (space group Fm3̅m, a = 9.0804(1) Å at 264 °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 °C where a new Form VI is built. Form V is tetragonal (space group P4/nmm, a = 6.3281(1) Å, c = 9.3671(2) Å at 295 °C, Z = 2), and Form VI is orthorhombic (space group Cmcm, a = 6.40840(5) Å, b = 20.7697(2) Å, c = 19.9888(2) Å, at 295 °C, Z = 16). At 317 °C, Form VI transforms into Form V. At 449 °C, a different orthorhombic Form VII is disclosed (space group Pnma, a = 10.3084(1) Å, b = 6.0291(1) Å, c = 23.8023(2) Å, at 495 °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 °C, which looks stable down to 139 °C giving Form I back. Isolated ZrF6 octahedra are present in Forms III, V, VI, and VII. Form VI contains also dimeric entities Zr2F12 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 °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 19F MAS NMR spectroscopy measured from 250 °C → 410 °C → room temperature.