Hydroxide-ion incorporation and conduction mechanisms in tin pyrophosphate – a first-principles study

Abstract

The incorporation and conduction mechanisms of hydroxide ions in tin pyrophosphate (SnP₂O₇) have been investigated theoretically on the basis of first-principles calculations. Hydroxide ions are not simply incorporated into interstitial sites in the crystal, and the incorporation is accompanied by dissociation of a P₂O₇ unit (pyrophosphate ion) into two PO₄ units. From the calculated formation energies of various native defects, it was found that the interstitial proton and hydroxide ion are the dominant defect species and their concentrations can be controlled by doping heterovalent cations. The long-range migration of the hydroxide ion was observed in the first-principles molecular dynamics simulations, but its frequency during the simulation was extremely low. In fact, the calculated potential barrier for the conduction pathway was extremely high (1.92 eV), and the estimated bulk conductivity in SnP₂O₇ was much lower than the experimentally reported conductivities. These results indicate that the experimentally observed hydroxide-ion conductivity of SnP₂O₇ cannot be simply explained by motion of hydroxide ions through the crystal lattice.