Hydrogen Bond Dynamics in Proton-Conducting Lanthanum Arsenate

The proton incorporation and migration mechanisms in lanthanum ortho-arsenate, LaAsO4, have theoretically been investigated using first-principles calculations in conjunction with the nudged elastic band (NEB) method and the kinetic Monte Carlo (KMC) method. Nine proton sites and twenty migration paths connecting the proton sites were found in the crystal, which result in the anisotropic proton diffusivity and conductivity of LaAsO4. First-principles phonon calculations have also been performed to discuss the relation between the host lattice flexibility and relaxation around a migrating proton. The involved host lattice relaxation during the proton migration mainly consists of rotations and translations of AsO4 units with little distortion of the tetrahedra. This reflects the relatively low eigenfrequencies of vibrational modes attributed to the AsO4 rotations and translations in comparison with the AsO4 bending and stretching modes. Such dynamics of the host lattice and protons should be common in proton-conducting oxides with rigid oxygen tetrahedral units (XO4) isolated in the crystals.