The atomic-scale picture of the proton conduction mechanism in tin pyrophosphate, SnP2O7, has theoretically been investigated using first-principles calculations, to clarify the intrinsic proton conductivity in the bulk region. Protons in the crystal lattice reside around oxide ions and migrate by rotation around single oxide ions and hopping between adjacent oxide ions by a mechanism similar to that in other proton-conducting oxides. The calculated proton conductivity has weak anisotropy reflecting the monoclinic structure (unique axis: b), particularly in the ca-plane. The main origin of the anisotropic conductivity is the relatively fast long-range migration pathways along the unique b-axis and in the [101] direction (potential barrier: 0.57 eV) versus that along the [101̅] direction (potential barrier: 0.64 eV). The apparent activation energy of the estimated proton conductivity is as high as ∼1.1 eV with the proton trapping effect by dopants (association energy: 0.59 eV), leading to the low proton conductivity in the bulk region. This suggests that the reported fast proton conductivity in the literature may be due to other unexpected proton migration routes, such as surfaces, grain boundaries, and secondary phases with residual phosphoric acid.

The incorporation and conduction mechanisms of hydroxide ions in tin pyrophosphate (SnP 2 O 7 ) 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 P 2 O 7 unit (pyrophosphate ion) into two PO 4 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 SnP 2 O 7 was much lower than the experimentally reported conductivities. These results indicate that the experimentally observed hydroxide-ion conductivity of SnP 2 O 7 cannot be simply explained by motion of hydroxide ions through the crystal lattice.