4-DIMENSIONAL CRYSTALLOGRAPHIC ANALYSIS OF THE INCOMMENSURATE MODULATION IN A BI2SR2CACU2O8 SINGLE-CRYSTAL

The incommensurately modulated structure in single crystals of nominal composition Bi2Sr2CaCu2O8 has been solved with the aid of four-dimensional crystal-lography. 700 unique X-ray reflections were collected, including 265 fundamentals, 350 first-order and 85 second-order satellites. The overall weighted R factor (wR) is 0.073, while the partial wR values for fundamentals and first- and second-order satellites are 0.066, 0.086 and 0.133 respectively. Up to second-order harmonics were included in the modulation function along with the temperature factors, B(ij). In addition, Ca is partially replaced by Sr and Bi, and Sr sites have some vacancies; Bi has a large first- and second-order modulation of the B factor, which suggests a static disorder in the Bi sites; and the CuO2 planes suffer a measurable modulation of their average structure. These findings are in fair agreement with earlier results by Petricek, Gao, Lee & Coppens [Phys. Rev. B (1990), 42, 387-392]. However, our modulation amplitudes are in significant disagreement. A small c-axis component to the modulation wavevector was observed along with substantial disorder in the interplanar phasing of the modulation waves. Diffuse scattering at 'unallowed' Bragg positions was also measured.