Dielectric anomaly at the orbital order-disorder transition in LaMnO3+δ

We report a novel dielectric anomaly around the Jahn-Teller orbital order-disorder transition temperature TJT in LaMnO3+δ. The transition has been characterized by resistivity (ρ) versus temperature (T), calorimetry, and temperature dependent x-ray diffraction studies. Measurements of complex dielectric permittivity ε* (ε'-iε'') over a low-frequency range (1 Hz-10 MHz) across TJT reveal a distinct anomaly. This observation, and the reported relatively high static dielectric constant at T = 0 (ε0∼18-20), possibly indicate that the orbital order gives rise to intrinsic polarization that undergoes transition at TJT. The frequency dispersion of the dielectric response at any given temperature, however, reveals that the dielectric response consists of Maxwell-Wagner component, due to interfaces, within such a low frequency range. The TJT and the nature of the anomaly in ε'(ω,T),ε''(ω,T) at TJT, of course, vary-from a sharp upward feature to a smeared plateau and then a downward trend-depending on the Mn4+ concentration of the sample. The observation of an intrinsic dielectric response due to long-range orbital order in LaMnO3-where no ferroelectric order is possible due to the absence of off-centre distortion in MnO6 octahedra-may throw a new light onto these classes of materials vis-a-vis multiferroic materials