First-principles study of structural stability and electronic properties of tetragonal and orthorhombic as well as monoclinic K_0.5Na_0.5NbO₃

The energetic stability, structural and electronic properties of tetragonal, orthorhombic, as well as monoclinic K_0.5Na_0.5NbO₃ are systematically studied using first-principles supercell model and virtual crystal approximation based on density functional theory with local density approximation and generalized gradient approximation. Our calculated results show that the total energy differences among the three K_0.5Na_0.5NbO₃ phases are small, which are well consistent with the easy phase equilibrium at the phase boundary in the experiments. Furthermore, we also find that the total energy of the monoclinic phase is lower than that of the other two phases, which suggests that the monoclinic structure is energetically more stable than the others. Moreover, the calculated structural parameters are in good agreement with experimental values. In addition, the electronic structure results show that the bonding interaction in the monoclinic structure is stronger than that in the other structures, also indicating that the monoclinic structure is the most stable one.