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The Jahn-Teller Distortion at High Pressure: The Case of Copper Difluoride

Centre of New Technologies, University of Warsaw, 02-097 Warsaw, Poland
Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszyński University, 01-038 Warsaw, Poland
Crystals 2018, 8(3), 140;
Received: 14 February 2018 / Revised: 15 March 2018 / Accepted: 16 March 2018 / Published: 19 March 2018
(This article belongs to the Special Issue High-Pressure Studies of Crystalline Materials)
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The opposing effects of high pressure (in the GPa range) and the Jahn-Teller distortion led to many intriguing phenomena which are still not well understood. Here we report a combined experimental-theoretical study on the high-pressure behavior of an archetypical Jahn-Teller system, copper difluoride (CuF2). At ambient conditions this compound adopts a distorted rutile structure of P21/c symmetry. Raman scattering measurements performed up to 29 GPa indicate that CuF2 undergoes a phase transition at 9 GPa. We assign the novel high-pressure phase to a distorted fluorite structure of Pbca symmetry, iso-structural with the ambient-pressure structure of AgF2. Density functional theory calculations indicate that the Pbca structure should transform to a non-centrosymmetric Pca21 polymorph above 30 GPa, which, in turn, should be replaced by a cotunnite phase (Pnma symmetry) at 72 GPa. The elongated octahedral coordination of the Cu2+ cation persists up to the Pca21Pnma transition upon which it is replaced by a capped trigonal prism geometry, still bearing signs of a Jahn-Teller distortion. The high-pressure phase transitions of CuF2 resembles those found for difluorides of transition metals of similar radius (MgF2, ZnF2, CoF2), although with a much wider stability range of the fluorite-type structures, and lower dimensionality of the high-pressure polymorphs. Our calculations indicate no region of stability of a nanotubular polymorph observed for the related AgF2 system. View Full-Text
Keywords: copper; fluorides; Jahn-Teller effect; high pressure; polymorphism copper; fluorides; Jahn-Teller effect; high pressure; polymorphism

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Kurzydłowski, D. The Jahn-Teller Distortion at High Pressure: The Case of Copper Difluoride. Crystals 2018, 8, 140.

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