Pressure-Induced Enhancement of Ionic Conductivity and Interstitial Migration Capability in Li ₃ PO ₄

The ionic transport properties of Li₃PO₄ under high pressure up to 20.0 GPa were systematically studied with alternating-current impedance spectra measurements and first-principles calculations. Li₃PO₄ underwent a change in the transport mechanism from mixed ionic-electronic conduction to predominantly ionic conduction at approximately 7.1 GPa without a structural phase transition. It then reverted to mixed ionic-electronic conduction at approximately 11.8 GPa due to a pressure-induced structural phase transition from the <i>Pmn</i>2₁ phase to the <i>Pnma</i> phase. The conductivity of Li₃PO₄ increased by 2 orders of magnitude after structural phase transition, primarily due to the enhanced Li⁺ ion migration rate. Vacancy migration barriers increased with pressure in all crystallographic directions, but interstitial barriers decreased in Li₃PO₄. Li⁺ ions exhibited a stronger interstitial migration capability under high pressure. The reduced charge density within the interstitial position provided supporting evidence for an absence of the electronic conduction mechanism over the pressure range of 7.1 to 11.8 GPa in Li₃PO₄. These findings could be utilized to propose innovative strategies for enhancing the ionic conductivity of solid electrolytes.