This procedure has been used extensively in previous studies to successfully predict GB structures. All geometry optimizations, AIMD and electronic structure calculations were carried out using the implementation of DFT within CP2K. Geometry optimization is performed until the force on ions is less than 0.01 eV Å-1 . When optimizing GB geometries, lattice vectors parallel to the GB are held fixed and the lattice vector normal to the GB is allowed to relax.