Γ phonons and microscopic structure of orthorhombicKNbO3from first-principles calculations

From a series of total energy calculations by the full-potential linear muffin-tin orbital method, the total energy hypersurface as a function of atomic displacements from equilibrium positions has been fitted for different Γ phonon modes in orthorhombic ${\mathrm{KNbO}}_3$. Frequencies and eigenvectors of all transverse optical Γ phonons have been calculated in the harmonic approximation, and in the quantum oscillator scheme, for ${\mathit{A}}_2$ and ${\mathit{B}}_2$ modes. The microscopic structure of the orthorhombic phase has been analyzed in a series of supercell calculations for different patterns of Nb displacements, providing indications in favor of the chain structure with oppositely directed neighboring chains.