Consideration of the Sample Eccentricity in the Measurement of Unit Cell Parameters of Small Single Crystals in Bond's Scheme Implemented in Modern Diffractometers

We discuss the methodological questions of measuring unit cell parameters of small crystals in Bond's scheme on a commercial diffractometer equipped with a three-circle goniometer and a 2D detector. The effect of the sample eccentricity on the positions of X-ray diffraction reflections during rotation about phi and omega axes is examined. The approach is developed to eliminate the effect of the sample eccentricity related to rotation about the phi axis by positioning one of the crystallographic directions along the omega axis. The unit cell parameters are measured for the reference Si and Ge single crystals used in this work with relative error Delta a/a no worse than 6.5 center dot 10(-5).