Abstract
The criteria developed in the error analysis of 2θ powder data for cubic and uniaxial phases [Frevel (1978). J. Appl. Cryst. 11, 184–189] are extended to biaxial single-crystal data. A methodology is described for applying the focusing matrix method to exhaustive sets of 2θ data for pinacoid and prism reflections measured on a four-circle diffractometer. A spheroidal crystal of hyperpure Si is used to calibrate a Syntex P{\bar 1} automated diffractometer and to establish an effective wavelength for graphite-monochromated Mo Kβ 1 β 3 radiation. A high-quality single-crystal of cis-[bis(7,9-dimethylhypoxanthine) (ethylenediamine)platinum(II)] hexafluorophosphate, [Pt(C2H8N2)(C 7H8N4O)2].(PF6)2, serves as a test case for comparing the normal least-squared analysis with the focusing matrix method. It is concluded that an absolute accuracy greater than one part in 2000 for cell constants of biaxial crystals is difficult to achieve from automated four-circle diffractometer data (2θ ≤ 40°) for non-spheroidal crystals with linear dimensions ca 0.2 mm.
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