A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step‐scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least‐squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.

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The crystal structure of p-terphenyl has been refined using two sets of X-ray data and two projections of neutron data. The lengths of the carbon--carbon bonds, which have been determined to an accuracy of + 0-005/~, differ from those in benzene in a manner which is difficult to explain simply in terms of bond environment. The molecule departs slightly from planarity, apparently as a result of packing forces. The neutron analysis has shown that there are in-plane steric deformations of the ortho hydrogen atoms which are nevertheless 0.3/~ closer together than the sum of their van der Waals radii. The central ring oscillates more strongly about the long axis of the molecule than do the two end rings. The thermal librations are generally consistent with potential energy functions calculated for rotation about the symmetry axes of the molecule. The final X-ray difference synthesis shows features resulting from the aspherical distribution of the bonding electrons, but there is evidence of strong interaction between the bonding electron distribution and the X-ray thermal parameters. Because of this effect the difference synthesis cannot be interpreted solely in terms of the electron redistribution due to bonding, and the thermal parameters may be considerably in error.

A summary is given of the development of an improved method to handle powder diffraction data. The resulting method, now called the Rietveld method, uses powder diffraction stepscanned intensities instead of integrated powder intensities. This enables the full use of the information content of a powder diagram. The method has revitalized the use of powder diffraction in structure determination. An unexpected and later development is the use of the method in quantitative phase analysis. This is now an essential tool in many industrial processes.