Dielectric properties of the charge-ordered mixed oxide CaMn7O12

We present in this paper a detailed investigation of the dielectric properties of the mixed oxide CaMn7O12 that shows a charge-ordering transition at T-CO = 440 K. Surprisingly, this compound presents a high dielectric constant at room temperature. Data taken at several frequencies and temperatures point to relaxor dielectric behaviour, that can be attributed to the electronic inhomogeneities present in the material. Extrinsic Maxwell-Wagner effects make a significant contribution to this dielectric response, as revealed by the studies made on samples of different particle size and using different types of contact. The intrinsic dielectric constant of this material is epsilon'(r(intrinsic)) similar to 30 at room temperature, a value that is relatively high for this type of compound and that we relate to the presence of the electronic process of charge ordering in this material.