First-Principles Determination of Chain-Structure Instability in KNbO3

A complete mapping in the Brillouin zone of the structural instability associated with the ferroelectric phase transitions of ${\mathrm{KNbO}}_3$ has been obtained by first-principles calculations using a linear response approach. The wave-vector dependence of the instability reveals pronounced two-dimensional character, which corresponds to chains oriented along $〈100〉$ directions of displaced Nb atoms. The results are discussed in relation to models of the ferroelectric phase transitions.