Structural characteristics (Ba0.94Gd0.06)(Ti0.97Mg0.03)O3 in cubic structure determined by high-energy synchrotron-radiation powder diffraction

Structural study by high-energy synchrotron-radiation powder diffraction has been performed for (Ba0.94Gd0.06)(Ti0.97Mg0.03)O3 (BGTM) at 473 K to investigate the substitution effects of Gd and Mg on the cubic structure of BaTiO3. Our precise electron-density analysis using the maximum entropy method (MEM)/Rietveld method demonstrates that the bonding electron density between the Ti and O atoms in BGTM decreases with the substitution of Gd and Mg elements, which provides a significant contrast to the fact that the Ti-O bond length is shortened in BGTM. No obvious change is observed on the Ba-O ionic bonding. It is considered that the lowering of the cubic-tetragonal phase transition temperature in BGTM is attributed to the decrease in covalency on the Ti-O bond in BGTM. The features of thermal motions of the constituent atoms are discussed on the basis of the mean square thermal displacement parameters by taking account of the chemical bonding nature and tolerance factor of the BaTiO3 system. (author)