Application of Small-Crystal Diffraction Technique Using Synchrotron Radiation to Structural Studies of Ceramics

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Both experimental and theoretical approaches to the cahrge density distribution study become more important in the field of materials science. However, several kinds of effects limit the accuracy of the observed kinematical structure factors from imperfect crystals. They are extinction, multiple scattering, absorption, counting statistics, thermal diffuse scattering, etc. The extinction and absorption can be reduced significantrly by using a smaller crystal and/or a shorter wavelength. Since both solutions tend to decrease the diffracted intensities, the use of synchrotron radiations is strongly recommended for accurate structure analyzes.In this study, we employed an improved single crystal diffraction technique to overcome the problems mentioned above. The technique has features of (1) using a small crystal of about 10mm in size, (2) using a vertically polarized synchrotron radiations at the beam line 14A equipped with a horizontal-type four-circle diffractometer suitable for the charge density study.Promising results have come out. For example, the three-dimensional diffraction data from a K_2NiF_4-type CaYAlO_4 single crystal of about 10mm in size have been found to be scarcely affected by the secondary extinction with a minimum value for the secondary extinction transmission factor Ys of 0.988 for the 200 strongest reflection. The value should be compared to 0.5855 calculated for a spherical specimen with diameter of 112mm mounted on the Ag rotating-anode four-circle diffractometer. The analysis of the extinction-free data on CaYAlO_4 provided detailed information about the composition, atomic positions and the electron density distribution, which have never been obtained by the conventional methods.Various structural studies were performed successfully on ceramic materials, such as BaTiO_3, lead-containing relaxr materials, CuInSe_2, MnS_2, LaTa_3O_9, LaNb_3O9 rare-earth sesquioxides, etc.