The physics underlying the remarkable properties of the manganite LaMnO₃ and its doped alloys is rich and complex. The actual path followed by a given system towards ferromagnetism and/or metallization, through chemical substitution, thermal treatment or pressure involves modifications of a delicate balance between delocalization and carriers trapping. Even in the parent stoichiometric compound LaMnO₃ the actual relationship between the conduction state and local distortion - related to the Jahn-Teller character of the Mn³⁺ ion- is not fully understood. The present study aims to describe the local process accompanying the pressure-induced abrupt changes in the resistivity in the Jahn-Teller distorted LaMnO₃.
We observed directly the modifications of the average local distortions around the manganese atoms induced by application of high pressures, in the range 0-15 GPa, from the modifications of X- ray absorption spectra in the near edge range. The MnO₆ octahedra keep unchanged by application of hydroscopic pressures up to 8 GPa, whereas above this value a continuous long Mn-O distance reduction is observed. However, the Jahn-Teller bond splitting persists over the whole pressure range. A total quenching of this splitting is expected to take place only for pressures larger than 30 GPa, i.e. at pressure where the metal to insulator transition also takes place. An hysteresis in the XANES features is observed down to ambient pressure, suggesting the coexistence of MnO₆ distorted and undistorted units.

Ramos et al., Phys. Rev. B 75 (2007) 052103.

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