Dielectric and polarization experiments in high loss dielectrics: A word of caution

The recent quest for improved functional materials like high permittivity dielectrics and/or multiferroics has triggered an intense wave of research. Many materials have been checked for their dielectric permittivity or their polarization state. In this report, we call for caution when samples are simultaneously displaying an insulating behavior and a defect-related conductivity. Many oxides containing mixed valent cations or oxygen vacancies fall into this category. In such cases, most of the standard experiments may result in an effective high dielectric permittivity, which may not be related to ferroelectric polarization. Here we list a few examples of possible discrepancies between measured parameters and their expected microscopic origin.