Charge density distributions of strontium titanate obtained by the maximum entropy method

Strontium titanate (SrTiO₃) shows a displacive phase transition at 110 K. In order to study electron level structural changes throughout the phase transition, the charge density distributions of SrTiO₃ were obtained by applying the Maximum Entropy Method (MEM) to X-ray diffraction data at Room Temperature (R.T.) and 70 K. From the MEM charge densities at R.T., it was found that Ti-O shows a rather strong covalent bond and that no covalency of Sr atoms was detected, which means that the Sr atom exists as an ion. From these results, chemical bonds of SrTiO₃ could be described as the Sr ion floating electro-statically between three dimensional Ti-O networks. The symmetry of the lattice changes through the 110 K phase transition, which causes the super-lattice reflection at the R point. The displacement of oxygen atoms throughout the phase transition can be easily seen by taking the difference in charge density distribution between these two temperatures. The MEM charge densities at 70 K reveal that the structural changes throughout the phase transition are rather small, that is, the Ti-O strong covalent bond is maintained. The nature of the chemical bond of Sr atoms is basically unchanged.