Structure, Phase Transition, and Electronic Properties of K₁-_xNa_xNbO₃ Solid Solutions from First-Principles Theory

With extensive first-principles calculations, we investigated the geometric structure, phase transition, and electronic properties of orthorhombic, monoclinic, and tetragonal K_1-xNa_xNbO₃ (KNN) as functions of the Na content. We found that KNN undergoes an orthorhombic-to-monoclinic-to-orthorhombic phase transition when the Na content is gradually increased. We also found that the polarization vector of the monoclinic phase can be rotated more easily than those of the orthorhombic and tetragonal phases, giving rise to an enhanced piezoelectric response of the monoclinic KNN. Furthermore, our calculations provide an interpretation for the experimentally observed unusual broad peak of the KNN piezoelectric parameters.