Jahn-Teller-induced crossover of the paramagnetic response in the singly valent e(g) system LaMn7O12

We investigate the high-temperature magnetic and transport properties of LaMn7O12, which displays a similar perovskitelike structure and the same single-valent Mn3+ properties of LaMnO3 but a much simpler Jahn-Teller (JT) distortion at T_JT=650 K. We find that the magnetic response of LaMn7O12 is similar to that of LaMnO3 below T_JT, but strikingly different in the undistorted phase above T_JT, where the Curie-Weiss susceptibility is strongly suppressed. Electrical resistivity and thermopower measurements unveil a concomitant crossover from non-adiabatic to adiabatic small polaron regime. This suggests that the above suppression is due to low-spin electron-hole dimers formed by the e_g charge transfer between Mn sites and stabilized by the slow JT dynamics above T_JT.