Dielectric anomaly at the orbital order–disorder transition in LaMnO_3+δ

Abstract

We report a novel dielectric anomaly around the Jahn-Teller orbital\norder-disorder transition temperature T_JT in LaMnO_(3+delta). The transition\nhas been characterized by resistivity (rho)versus temperature (T), calorimetry,\nand temperature-dependent X-ray diffraction studies. Measurements of complex\ndielectric permittivity epsilon* (= epsilon'-i.epsilon'') over a low-frequency\nrange (1 Hz - 10 MHz)across T_JT reveal a distinct anomaly. This observation,\nand the reported relatively high static dielectric constant at T = 0 (epsilon0\n\\~18-20), possibly indicate that the orbital order gives rise to intrinsic\npolarization that undergoes transition at T_JT. The frequency dispersion of the\ndielectric response at any given temperature, however, reveals that the\ndielectric response consists of Maxwell-Wagner component, due to interfaces,\nwithin such a low frequency range. The T_JT and the nature of the anomaly in\nepsilon'(omega,T), epsilon''(omega,T) at T_JT, of course, vary - from a sharp\nupward feature to a smeared plateau and then a downward trend - depending on\nthe Mn^4+ concentration of the sample. The observation of an intrinsic\ndielectric response due to long-range orbital order in LaMnO_3 - where no\nferroelectric order is possible due to the absence of off-centre distortion in\nMnO_6 octahedra - may throw a new light onto these classes of materials\nvis-a-vis multiferroic materials.\n