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Polymorphism of K2ZrF6

DOI: 10.1021/acs.cgd.0c00166 DOI Help

Authors: Lubomir Smrcok (Institute of Inorganic Chemistry, Slovak Academy of Sciences) , Armel Le Bail (Institut des Molécules et des Matériaux du Mans, CNRS UMR 6283, Université du Maine) , Miroslav Boca (Institute of Inorganic Chemistry, Slovak Academy of Sciences) , Aydar Rakhmatullin (Conditions Extrêmes et Matériaux : Haute température et Irradiation (CEMHTI))
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Crystal Growth & Design

State: Published (Approved)
Published: April 2020
Diamond Proposal Number(s): 12340 , 88927

Abstract: Heating of the 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 to 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 Fm3 ̅m, 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, -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, 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 multi-splitting is observed at many temperatures that suggests the occurrence of coexisting micro-phases. The parallel existence of several phases was observed also by 19F MAS NMR spectroscopy measured from 250 oC → 410 oC → RT.

Journal Keywords: di-potassium zirconium fluoride; polymorphism; synchrotron powder diffraction; 19F MAS NMR

Subject Areas: Chemistry

Instruments: I11-High Resolution Powder Diffraction