Journal
CRYSTALS
Volume 8, Issue 3, Pages -Publisher
MDPI
DOI: 10.3390/cryst8030140
Keywords
copper; fluorides; Jahn-Teller effect; high pressure; polymorphism
Funding
- Polish National Science Centre (NCN) [UMO-2014/13/D/ST5/02764]
- Interdisciplinary Centre for Mathematical and Computational Modelling (ICM) University of Warsaw [GA67-13]
- Polish National Science Centre (NCN) [UMO-2014/13/D/ST5/02764]
- Interdisciplinary Centre for Mathematical and Computational Modelling (ICM) University of Warsaw [GA67-13]
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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 P2(1/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 Pca2(1) 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 Pca2(1)-Pnma 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.
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