Local structure and lattice covalency of complex perovskites BaM_0.2Ta_0.8O_3−xN_x (M = Li, Na, Mg) studied by X-ray diffraction and X-ray absorption spectroscopy

Local structural aspects of complex perovskites BaM_0.2Ta_0.8O_3−xN_x (M = Li, Na, Mg) were studied by the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies at the Ta L₃-edge, and also by the diffraction line broadening analysis of high-resolution synchrotron X-ray diffraction (XRD) data. The line broadening analysis based on the Williamson-Hall method indicated that the lattice periodicity of BaM_0.2Ta_0.8O_3−xN_x are disturbed remarkably, with a dependence on the size mismatch between M^{+ / 2+} and Ta⁵⁺. The Ta L₃-edge EXAFS revealed that part of the Ta−O/N bonds are contracted by the existence of the large alkali cation on the shared octahedral site. Meanwhile, the XANES implied the distortion of Ta(O,N)₆ from ideal octahedral geometry, where the curve fitting of the spectra provided additional information on the bonding character of Ta.