First principles analysis of vibrational modes in KNbO₃

The vibrational modes in KNbO3 are analyzed using first principles total-energy calculations based on local density functional theory. Normal mode frequencies and eigenvectors are determined for the cubic perovskite structure with wave vectors at the center (Γ) and corner (R) of the Brillouin zone using the frozen-phonon technique. The frequencies are in good agreement with available experimental results with the possible exception of the lowest-frequency zone-center (ferroelectric) mode. Our calculations give a ferroelectric mode which becomes unstable with increasing volume. However, at the equilibrium volume it is stable with a frequency of ∼100 cm−1.