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A new goniometer for X-ray powder diffractometry and its setting are described. The diffraction angles are easily and quickly measured with high accuracy (10−3°θ), even if experiments require the use of some large or heavy device such as a cryostat or furnace. An original mechanical design allows a direct measurement of the angular position, 20, of the receiving slit, by means of an incremental (10−3°) photoelectric encoder. Three independent stepping motors, for the θ and 2θ scans and the sample translation, allow one to define with a very good reproducibility the `zero' goniometer position. Diagrams of Pb3O4 and Pb(NO3)2 have been recorded in continuous scanning (0.05° θ/min): differences between θ of the two diffraction lines recorded on each side of the goniometer do not exceed ±2 × 10−3°, and reproducibility of measurements is better than +1 × 10−3°. Sensitivity of angular measurements is very good; it is proved by studying, in the range 9–40° (θ), the thermal expansion of Pb(NO3)2 between 273 and 291 K. Moreover, use of a large radius (500 mm) and of a set of undivergent slits improves angular resolution, principally for θ<20°, which is very important for molecular-compound studies: an example is given showing diffraction line splitting due to a monoclinic triclinic phase transition.
