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

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 (CuF 2). 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 CuF 2 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 AgF 2. 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 Cu 2+ 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 CuF 2 resembles those found for difluorides of transition metals of similar radius (MgF 2 , ZnF 2 , CoF 2), 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 AgF 2 system.

Quest for Compounds at the Verge of Charge Transfer Instabilities: The Case of Silver(II) Chloride †

“…And since they tend to collapse fast under even moderate pressures, it could be that other polymorphic forms, such as the ribbon one, would become competitive at rather low pressures, even preceding the transformation to the layered form. The previously documented pressure-induced transformations of CuF 2 [47] and AgF 2 [11] as well as a large body of data for transition metal difluorides and dichlorides (see also [48] and references therein) seem to suggest this scenario as a viable one.…”

“…The common tangent method [17,46] allows for a rather crude estimate for the formation pressure of AgCl 2 of 35 GPa (at T → 0 K), and likely even higher pressures at elevated temperature. The more precise estimate requires calculations in the function of pressure to be performed, also including the ribbon polymorph, which should exhibit substantial compressibility, and several viable high-pressure polymorphs [11,47,48]. Moreover, while drawing the computed volume-based conclusions one should always remember that despite great performance of HSE06 functional for describing crystal and electronic structure of solids, the reproduction of van der Waals interactions is still imperfect.…”

Dramatic enhancement of spin–spin coupling and quenching of magnetic dimensionality in compressed silver difluoride

“…2a). Persistence of the broad band gap is typical of undoped d 9 systems, 21,[29][30][31][32] and it points out to the robustness of the localized AFM interactions. These interactions which lead to insulating behaviour tend to act opposite to the band broadening which is usually seen at elevated pressure and consequently the band gap closing becomes much more difficult than for the diamagnetic semiconductors.…”

High-Pressure Phase Transitions of Zinc Difluoride up to 55 GPa

“…Numerous experimental and theoretical investigations were conducted in order to establish the phase sequence for these compounds. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] The motivation for these studies was the fact that the behaviour of difluorides at these conditions can serve as an analogue of the high-pressure transformations of dioxides, in particular SiO2. 7,8 Moreover high-pressure studies on MF2 systems revealed a surprisingly wide range of structures adopted by these compounds.…”