Abstract: The interplay between the Jahn-Teller (JT) effect and octahedron tilting in transition-metal perovskites is investigated as a function of pressure. Our focus is on its effects on the exchange and electron-phonon interactions, both having a strong influence on materials properties. We demonstrate that the JT distortion in Cu2+ and Mn3+ is reduced upon compression and is eventually suppressed at pressures above 20 GPa. X-ray diffraction and x-ray absorption measurements in A2CuCl4 layer perovskites (A: Rb, CnH2n+1NH3; n=1?3) show that, although pressure slightly reduces the long Cu-Cl distance in comparison to the Cu-Cu distance in the layer, the JT distortion is stable in the 0?20 GPa range. The difference between lattice (?C0=0.14 GPa?1) and local CuCl6 (?0=0.016 GPa?1) compressibilities, together with the high stability of the JT distortion, lead to CuCl6 tilts upon compression. The evolution of the elongated CuCl6 octahedron in A2CuCl4, as well as MnF6 in CsMnF4 and MnO6 in LaMnO3 and DyMnO3, toward a nearly regular octahedron takes place above 20 GPa, in agreement with experimental results and a model analysis based on the JT energy derived from optical absorption spectroscopy: EJT=0.25?0.45 eV/Cu2+, EJT=0.45 eV/Mn3+ (CsMnF4), and EJT=0.25 eV/Mn3+ (LaMnO3). The proposed model clarifies controversial results about pressure-induced JT quenching in Cu2+ and Mn3+ systems, providing an efficient complementary means to predict pressure behavior in perovskites containing JT transition-metal ions.

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