Large-scale quantum chemical modeling of the phase transitions in KTN solid solutions

摘要

The large-scale modeling of the atomic and electronic structure of KNb_xTa_(1-x)O_3 (KTN) perovskite solid solutions is performed using the Intermediate Neglect of the Differential Overlap (INDO) method based on the Hartree-Fock formalism. It is found that periodic Nb impurities in KTaO_3 reveal coherent off-center displacements already at the smallest calculated concentration, x = 0.125. The calculated magnitude of < 111 > Nb off-center displacement is 0.27 a.u., which is close to the XAFS observation at 70 K and x=0.09. In contrast, Ta impurities in KNbO_3 remain on-center, due to higher ionicity of Ta, as compared to Nb. Using the calculated energy gain caused by the off-center displacements of Nb atoms, the non-empirical Ginzburg-Landau-Devonshire functional with concentration-dependent coefficients is constructed. Analysis of INDO results for several Nb concentrations in KTN allows calculate the lowest critical Nb concentration, x_(cr)=0.025, corresponding to the quantum displacive limit for the paraelectric-ferroelectric transition at 0 K. This value is only slightly higher than the experimental one.