Abstract
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 Pca21–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.
Highlights
In 1937, Jahn and Teller showed that non-linear molecules exhibiting orbital degeneracy will undergo a distortion leading to a lower-energy, and orbitally non-degenerate, structure [1].The so-called Jahn-Teller (JT) effect is strong in systems containing divalent copper (3d9 electron count)
The high-pressure phase transitions of CuF2 resembles those found for difluorides of transition metals of similar radius (MgF2, ZnF2, CoF2 ), with a much wider stability range of the fluorite-type structures, and lower dimensionality of the high-pressure polymorphs
Jahn-Teller distortions we studied the high pressure phase transitions of copper difluoride (CuF2 )
Summary
In 1937, Jahn and Teller showed that non-linear molecules exhibiting orbital degeneracy will undergo a distortion leading to a lower-energy, and orbitally non-degenerate, structure [1]. Due to the fluxional nature of the Jahn-Teller effect many studies have been devoted to tuning this distortion either by chemical substitution [12,13,14,15,16], or by high external pressure [17,18,19,20,21,22] In the latter case pressures above 1 GPa (=10 kbar) are used to induce substantial volume reduction which in turn leads to changes in the electronic and structural properties of the studied system. Jahn-Teller distortions we studied the high pressure phase transitions of copper difluoride (CuF2 ). This compound is one of the simplest binary connections containing the Cu2+ cation. The high-pressure phase transitions of CuF2 are reminiscent of those recently reported for its heavier analogue, AgF2 [37], with the exception that a nanotubular phase found for AgF2 is not observed [36]
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