Structural instabilities inKTaO3andKNbO3described by the nonlinear oxygen polarizability model

The validity of the nonlinear polarizability model to describe the different structural behavior as a function of temperature observed in ${\mathrm{KTaO}}_3$ and ${\mathrm{KNbO}}_3$ is analyzed. To this purpose, the total energy for different atomic distortions is evaluated and the results are compared with recent total-energy studies using ab initio methods. It is shown that the large O 2p–Nb 4d hybridization observed in first-principles calculations and a model description through a nonlinear oxygen polarizability are actually not contra- dictory explanations for the origin of ferroelectricity in perovskites.