Neutron diffraction study of partial radial densities in γ-CuCl, including an appendix on the effect of instrumental resolution on radial density analysis

Neutron diffraction has been used to study the structural disorder in γ-CuCl at 25 and 366°C. Powder patterns of ⁶³Cu^NCl, ^NCu^NCl and ⁶⁵Cu^NCl (N= natural abundance) were treated by integrated intensity (II) and radial density (RD) analyses. Using II, a satisfactory fit to these 366°C data was made with the anharmonic model described by Sakata, Hoshino & Harada [Acta Cryst. (1974). A30, 655-661]. This fit resulted in an anharmonic parameter β_Cu = 1.2 ± 0.6 × 10^-12 erg Å^-3, which is in excellent agreement with the value of β_Cu = 1.15 ± 0.66 × 10^-12 erg Å^-3 obtained from a single-crystal study at room temperature by Sakata et al. (1974). To eliminate the indistinguishability of the anharmonic and statistically disordered models inherent in II, an energy analysis was made of the diffuse scattering. It was found that at 25 °C, 50 to 70% of the diffuse scattering is elastic. The non-monotonic κ dependence of this elastic diffuse scattering implies a model of correlated static displacements of Cu atoms and is inconsistent with the anharmonic model. Radial density analyses of the three spectra were fitted to a model based on harmonic vibration to obtain partial radial densities _{QCuCu, QClCl} and _QCuCl,. Although the_QClCl, was typical of that expected for a solid structure, _QCuCu,, at 366°C was found to be more 'liquid-like'. Quantitative conclusions from this radial density analysis are limited by the strong dependence of the results on instrumental resolution. An estimate of the effect of instrumental resolution on the measured radial density function is contained in Appendix B.