ELECTRON DENSITY DISTRIBUTION OF SUPERCONDUCTOR Sr₂RuO₄ BY MAXIMUM ENTROPY METHOD. Y. Sekii, H. Ishibashi and K. Nakahigashi, College of Integrated Arts and Sciences, Osaka Prefecture University, Sakai, Osaka 593, Japan

The electron density distribution in Sr₂RuO₄, which is a K₂NiF₄ structure type superconductor, was determined by the combination of powder X-ray diffraction data and maximum entropy method. The high-Tc superconductors commonly have a layered perovskite structure containing CuO₂ planes, in which the electronic structure is characterized by large hybridization between Cu d and O p states. On the other hand, Sr₂RuO₄ is a recently discovered superconductor which contains RuO₂ planes instead of CuO₂ planes. Theoretical calculation showed that there also exists the large hybridization between Ru d and O p states in Sr₂RuO₄. (T. Oguchi, Phys. Rev. B, 1995, 51, 1385-1388) However, no direct observation of hybridized Ru d and O p states has been done. Therefore, it is important to examine the electron density distribution of Sr₂RuO₄ and to compare the theoretical results.

The Sr₂RuO₄ powder used in the present work was prepared by heating the mixed powder of ruthenium and strontium carbonate in air at 1200deg.C for 24 hours. Powder pattern was measured at room temperature by [[theta]]-2[[theta]] step scanning using the CuK[[alpha]] radiation. Whole powder pattern fitting was applied in order to obtain integrated intensities for each reflection. The integrated intensities were converted into the structure factors by the ordinary least-squares refinement. Consequently, 60 structure factors were obtained. Based on these data, the electron density distribution was calculated by the maximum entropy method. The resultant density map shows the strong covalent bondings between Ru and octahedrally coordinated O atoms. The RuO₆ octahedron is small tetragonally distorted, so Ru-O bondings are divided into two types, i.e., the equatorial bond (Ru-O1, ~1.93Å) and apical one (Ru-O2, ~2.06Å). Nevertheless the atomic distance of Ru-O2 is longer than that of Ru-O1, the electron density at middle point of Ru-O2 was much higher than that of Ru-O1.