High-pressure dielectric behavior of polycrystalline CaMoO4: The role of grain boundaries

The dielectric behavior of polycrystalline CaMoO4 was investigated at pressures up to 36.0 GPa using in situ impedance measurements. Grain boundaries played a dominant role in the electrical transport process. Grain boundary microstructures rearranged with the application of pressures, and the relaxation activation energy increased with increasing pressure in the tetragonal phase but decreased in the monoclinic phase. The variation of the bulk resistance with pressure was attributed to defects generated during the compression. The increasing grain boundary resistance with pressure in the tetragonal phase was caused by the increased number of dangling bonds. In the tetragonal phase, localization around O atoms weakened with increasing pressure, which promoted the polarization of MoO electric diploes and led to an increase of the relative dielectric constant. In addition, the dielectric loss tangent of CaMoO4 was significantly reduced in the low frequency range after a pressure cycle. This work demonstrates that regulation of the polycrystalline dielectric performance by modifying the grain boundary distribution under compression can be used as an effective method to improve the bulk properties of ABO4-type dielectrics.