Electron Charge Density Study of (Na_1-xK_x)NbO₃ in Cubic Structure

The electron charge density study by synchrotron-radiation powder diffraction has been performed for the cubic structure of (Na_1-xK_x)NbO₃ (NKN: 0≤x≤1) at 1000 K to investigate the atomic substitution effect on chemical bonding. Our precise analysis using the maximum entropy method (MEM)/Rietveld method demonstrates that the covalent bonding is formed on the Nb–O bond whereas the Na/K atoms are ionic in the entire composition range. The thermal motion of the Na ion in NaNbO₃ is fairly larger than that of the K ion in KNbO₃. The charge density distributions around the O atoms in NaNbO₃ are extended in the directions perpendicular to the Nb–O bond, which can be related to the rotational mode of the Nb–O₆ octahedron driving the antiferroic phase transition. No such anisotropy is clearly observed in KNbO₃ around the O atoms. These behaviors are closely related to the change in the tolerance of the perovskite structure caused by the atomic substitution.