Mo K..cap alpha.. Bragg diffraction data collected from single crystals of the title phases were used in least-squares refinements of structural models that (for chinot =0) included a priori distribution parameters for iron on three of the four metal-atom sites of the known M/sub 23/C/sub 6/ tau-carbide structure. Conventional data for the crystal with chi=0.7/sub 4/ were supplemented by synchrotron-radiation (SR) diffraction data collected at energies near CrK and FeK absorption edges. Results show that distribution parameters whose differences from chemical-average values have statistical and physical significance can be derived from extensive, precise conventional diffraction data. They demonstrate that iron atoms preferentially substitute for chromium at sites not bonded to carbon atoms; when 30-35% of the sites that are bonded to carbon are occupied by iron, the tau solid solution becomes thermodynamically unstable relative to other phases. Calculations with the SR data sets collected near the CrK-edge energy suggest a (sin theta)/lambda dependence of the real part of the dispersion correction to the atomic scattering factor for chromium as well as significant departures from the theoretical free-atom calculations for these corrections at (sin theta)/lambda=0.
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A 1987 study studied this question.