Abstract: This work investigates why Jahn-Teller (JT) transition-metal ions like Cu2+ (d9) or Mn3+ (d4) with octahedral coordination exhibit low-symmetry distortions of the CuF6 4- (or MnF6 3-) octahedron, when Cu2+ replaces Zn2+ either in the cubic perovskite KZnF3 or in the layer perovskite K2ZnF4. The aim is to establish correlations between the splitting ?e of the parent Oh eg(x2 - y2, 3z2 - r2) d orbitals and the low-symmetry local distortion ?, allowing us to unravel the local structure of Cu2+ introduced as an impurity into such perovskites. However, the question of whether tetragonal a1g - b1g splitting ?e of Cu2+ is proportional to ? or ?e contains additional contributions from the rest of the lattice (crystal anisotropy) is controversial. Here we show that ?e depends linearly on ?. High-pressure experiments and compound series involving JT ions give support for the proposed scenario and provide correlations relating ? and ?e in Cu2+ and Mn3+ systems, from which the electron-vibration coupling for the E?e JT effect is obtained experimentally.

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