Off-center instability of Nb⁵⁺ in KNbO₃ under ambient pressure

Highlights: • Off-center displacements of Nb⁵⁺ are explained by the pseudo Jahn–Teller effect. • Ambient pressure weakens the pseudo Jahn–Teller effect. • Enlarging the HOMO–LUMO energy gap reduces potential barriers. • Stiffness of [NbO₆] determines upper bound for the HOMO–LUMO gap. - Abstract: Off-center displacement of Nb in KNbO₃ is explained in terms of the pseudo Jahn–Teller effect in octahedral clusters [NbO₆]. Strength of this symmetry-breaking effect is determined by electron-vibrational coupling and by the HOMO–LUMO energy gap. Under applied ambient pressure, the energy gap increases weakening the pseudo Jahn–Teller instability, reducing off-center displacement of Nb in trigonal wells, and lowering potential barriers between wells. Sensitivity of the off-center displacement of Nb to applied pressure indicates closeness of the HOMO–LUMO gap in KNbO₃ to its critical value when the off-center displacement of Nb approaches zero. This limit, however, is not reached due to lattice anharmonicity of KNbO₃ with respect to volumetric compression. The suggested theory fits well with the local structure obtained in XAFS.