First-Principles Computations for KTN

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Solid solutions of KTaO₃ and KNbO₃ (KTN) exhibit unusual dielectric properties at ferroelectric phase transitions, which are usually associated with a Nb instability that is often explained by the large Nb dynamical charge in oxides. Using non-self-consistent VASP computations, we showed earlier that the Nb dynamical charge in layerd KTN strongly depends on the local environment of Nb. In the present study, we have performed self-consistent first-principles computations for fixed composition KTa7/8Nb1/8O3 in different configurations shown. We computed the equilibrium ionic positions, Born charges, force constants, vibration frequencies and modes. We also discuss the question whether Nb has a multi-well potential, and the field dependence of susceptibility.