Pressure-directed enhanced ionic conductivity in MgMoO4 mixed-phases

Abstract

The mixed conductors have attracted much attention for potential applications in fuel cells, sensors, supercapacitors. Here, the electrical transport behavior of MgMoO₄ were systematically investigated at pressures up to 30.0 GPa using impedance spectroscopy measurements and theoretical calculations. The discontinuous changes in electrical parameters reflect the pressure-induced phase transition from β-MgMoO₄ to γ-MgMoO₄. The transport mechanism was determined to be mixed ionic-electronic conduction, and the contributions of carriers were distinguished. In the pressure range where the low- and high-pressure phase coexist, ionic transport plays a dominant role. The optimal solution for ionic conductivity of MgMoO₄ occurs in the mixed phases, and the ionic conductivity has been increased by two orders of magnitude in comparison to that observed at ambient conditions. The resistances were analyzed with band gap, transference number and the relative ionic diffusion coefficient. The pressure constrains the atomic amplitude and limits ion-phonon scattering, resulting in a significant enhancement of the ionic diffusion coefficient and facilitates the ionic conduction of γ-MgMoO₄. Meanwhile, it is found that the grain boundary response was weakened under compression, and the static dielectric constant was improved by pressure. These results offer valuable insights for optimizing and applying ABO₄-based mixed conductors.