X-Ray Measurement of Short-Range Order in Cu3Au

Abstract

The new measurement of short‐range order in a single crystal of Cu₃Au differs from the original Cowley measurement in four important ways: Allowance was made for a size‐effect modulation, allowance was made for a temperature factor in the diffuse intensity, the measurements were made in the vicinity of 100 to be closer to the origin of reciprocal space, and the intensities were obtained in absolute units by comparison with polystyrene rather than using α(0)=1.0. The ratio of the 100 and 300 peaks indicates that in disordered Cu₃Au the value of 2M(Cu) is about twice 2M(Au) similar to the ratio found in the ordered form. Values for α(i) out to tenth neighbors were obtained for T=405°C, T=450°C, and for a sample quenched from 800°C. There was the same abnormally small value of α(3) found by Cowley. The new values are approximately 35% larger than the Cowley values. Presumably the increase is due mostly to the different method of standardizing. The measured values of α(i) out to i=10 are reproduced very well by the Cowley theory of short‐range order, with no adjustable parameter but the ratio V₂/V₁. The present coefficients give V₂/V₁=+0.10, which would seem more reasonable than the original negative value.