The effect of anion substitution and nonstoichiometry on the fast ion conductor Cu ₆PSe ₅i

Cu ₆PSe ₅I is a fast ion conductor exhibiting an order-of-magnitude change in electrical conductivity at its 265 K superionic-covalent phase transition. Replacing 5% of the selenide anions in this compound with sulfide results in a substantial enhancement of conductivity as well as a decrease in the phase transition temperature. Replacing 5% of the iodide anions with bromide produces a similar enhancement of conductivity, but has no effect upon the phase transition temperature. Introducing 2% halide vacancies into the Cu ₆PSe ₅I lattice via nonstoichiometric sample preparation has little effect upon the superionic conductivity, but significantly enhances conductivity and reduces activation energy in the covalent phase. Finally, the introduction of 1% vacancies into the chalcogenide sites via nonstoichiometric sample preparation shifts the superionic-covalent phase transition to higher temperatures.