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The structure of p-nitropyridine N-oxide has been redetermined at both 30 and 300K. Data were collected in a step-scan mode and intensities obtained through profile-analysis. The b axis, which is normal to the molecular planes, contracts considerably on cooling, and thermal parameters decrease by a factor of about six, in agreement with predictions based on a simple harmonic model. The orientation of the thermal ellipsoids is essentially the same at both temperatures. Comparison of bond lengths at the two temperatures clearly demonstrates the necessity of a bond-length correction for shortening due to thermal motion. Bond lengths agree well after correction according to a rigid-body model. Difference densities were calculated with positional parameters and hydrogen thermal parameters from an independent low-temperature neutron study. Densities calculated with data cut-offs of sin 0/2= 0.65, 0.75, and 1/~-1 indicate an appreciable increase in noise level on inclusion of high-order reflections. The height of the lone-pair density increases with data cut-off while the densities in the bond regions are much less affected, thus confirming the hypothesis that the lone-pair electrons contribute to high-order scattering. Similarly, difference maps based on a high order X-ray refinement (sin 0/2 > 0.75 A-1) underestimated the density in the lone-pair regions.

A beryllium-walled cryostat is described which allows collection of nearly a full sphere of accurate three-dimensional X-ray diffractometer data at liquid-nitrogen (LN2) and liquid-helium (LHe) temperatures. Cryogen consumption is 0.3-0.4 liters of LHe per hour at 30~K and 0.1-0.2 liters of LN2 per hour at 78 °K. The specimen is mounted on a copper block, the temperature of which is controlled to within 0.1 °. Measurements with a powder sample of TbVO4 show that at 33~JK the specimen temperature is within 2 ° of the temperature of the copper block. The cryostat has been used for the collection of several low-temperature data sets. Some special modifications in data-collection and dataprocessing software are described.

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