STRUCTURE ANALYSIS OF CaYAlO₄ UNDER EXTINCTION-FREE CONDITION USING SR. N. Ishizawa, K.Morita, Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta Yokohama 226 and N. Kodama, Tosoh Cooperation, Hayakawa, Ayase 252, Japan

Structure of the K₂NiF₄-type CaYAlO₄ single crystal has been determined at the beam line 14A of the Photon Factory, KEK, by using a small specimen of 12x12x6 um³ to reduce the extinction effect. The wavelength of 0.75 Å, which is 508 eV below the Y K edge, was selected to avoid the contamination due to the fluorescence of yttrium in the compound and to reduce the absorption effect. The least-squares refinement gave a final R value of 0.011 for 426 independent reflections in the range 2[[theta]][[second]]138deg.. The minimum value of the secondary extinction transmission factor was 0.996 for the 200 strongest reflection when the extinction correction was included in the least-squares refinement, suggesting that the secondary extinction was practically negligible. Presence of excess Ca and deficient Y atoms from the nominal composition and associated oxygen vacancies at the apical O(2) site of the AlO₆ octahedron were detected. The Ca atoms are supposed to be displaced along the c axis to accommodate the oxygen vacancy at O(2) site. Apparent large anisotropy of the thermal parameters of oxygen atoms seems to be due to the variation of possible orientations of oxygen octahedra caused by the distribution of neighboring Ca and Y atoms and oxygen vacancies. This result explains the inhomogeneously broadened line width of the R line fluorescence spectrum created by the variation of the crystal field of Cr³⁺ in Cr-doped CaYAlO₄. The SR results are compared with the Ag-K[[alpha]] rotating anode experiment. Crystal data are; tetragonal I4/mmm, a=3.6437(1) Å, c=11.8682(3) Å, Z=2, m=53.6 cm^-1