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Register a new userStatic and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)
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G. Klupp
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We report the temperature dependent mid- and near-infrared spectra of K4C60, Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C604- which brings the fulleride anion from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K4C60 and Rb4C60. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high temperature. In K4C60 and Rb4C60 we suggest a transition from a static to a dynamic Jahn-Teller state without changing the average structure. We studied the librations of these two fullerides by temperature dependent inelastic neutron scattering and conclude that both pseudorotation and jump reorientation are present in the dynamic Jahn-Teller state.
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The Jahn-Teller (JT) distortion that can remove electronic degeneracies in partially occupied states and results in systematic atomic displacements is a common underlying feature to many of the intriguing phenomena observed in 3d perovskites, encompassing magnetism, superconductivity, orbital ordering and colossal magnetoresistance. Although the seminal Jahn and Teller theorem has been postulated almost a century ago, the origins of this effect in perovskite materials are still debated, including propositions such as super exchange, spin-phonon coupling, sterically induced lattice distortions, and strong dynamical correlation effects. Here we analyze the driving forces behind the Jahn-Teller motions and associated electronic fingerprints in a full range of ABX3 compounds. We identify (i) compounds that are prone to an electronically-driven instabilities (i.e. a pure JT effect) such as KCrF3, KCuF3 or LaVO3 and proceed to relax the structures, finding quantitatively the JTD in excellent agreement with experiment; (ii) compounds such as LaMnO3 or LaTiO3 that do not show electronically driven JTD despite orbital degeneracies, because their strongly hybridized B, d-X, p states supply but too weak JT forces to overcome the needed atomic distortions; (iii) although LaVO3 exhibits similar B, d-X, p hybridizations as LaTiO3, the former compound exhibits a robust electronic instability while LaTiO3 has zero stabilization energy, the reason being that LaVO3 has two electrons t2g2 relative to LaTiO3 with just one t2g1. (iv) We explain the trends in orbital ordering whereby electrons occupy orbitals that point to orthogonal directions between all nearest-neighbor 3d atoms. We thereby provide a unified vision to explain octahedra deformations in perovskites that, at odds with common wisdom, does not require the celebrated Mott-Hubbard mechanism.
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