Dielectric Properties of Single Crystals of MnO and of Mixed Crystals of MnO/CoO and MnO/NiO

Abstract

AbstractDielectric constant (K′), loss (K″), and conductivity (σ) of single crystals of MnO and solid solutions of MnO/CoO and MnO/NiO have been measured as a function of frequency in the range 102 to 107 Hz and at temperatures between 30 and 450 °C. The value for K′ of MnO at 30 °C is 18.1 and practically frequency‐independent. In the solid solutions of MnO/CoO and MnO/NiO (50:50 composition) K′ decreases with frequency to a frequency‐independent value of 16.4 and 14.4, respectively. The K″‐values of the solid solutions are much larger at lower frequencies (compared to MnO). The larger values of K′ and K″ for the solid solutions at low frequencies are due to the space charge polarization. In all crystals, the dielectric constant increases with temperature but decreases with frequency. The dependence of the high‐temperature dielectric constant on frequency is attributed to the creation and destruction of dipoles at such temperatures. log ΔK′ vs. 1/T plots (where ΔK′ is the difference between dielectric constant values at any temperature T and room temperature) are straight lines for these compounds giving activation energy values of 0.57, 0.41, and 0.35 eV for MnO, MnO/CoO, and MnO/NiO, respectively. The plots of log σ vs. 1/T also give straight lines for all the crystals studied, and one obtains activation energies of 0.6, 0.52, and 0.52 eV, respectively, for the above mentioned compounds. The proximity of these activation energy values for the two processes suggest that the same charge carriers are responsible for changes in K′ and K″. The lower activation energy for the solid solutions is attributed to lattice distortion. In solid solutions, the room temperature conductivity exhibits a maximum — and the activation energy for conduction a minimum — near the composition Mn0.9X0.1O (where X stands for Co or Ni) which is explained as being due to scattering of charge carriers at larger concentrations than 10% for X.