Diffuse scattering and low-energy phonons in superionic conductor Cu$_{1.8S}$Se

The neutron diffuse and inelastic scattering were studied in the superionic $\alpha $-phase of copper selenide. In neutron diffraction experiments on Cu$_{1.85}$Se single crystal the diffuse scattering features were observed along [111] direction in vicinity of (400) and (422) reflections. In inelastic neutron scattering measurements performed with time-of-flight spectrometer the elastic and inelastic scattering processes were separated and a strong inelastic scattering was observed also along [111] nearby (400) and (022). This shows that diffuse scattering found in conventional diffraction experiment is mainly inelastic and most probably comes from the low-energy phonons. Such phonons with optic-like behaviour of transverse acoustic modes at $q/q_{m} \quad >$ 0.2-0.4 were found earlier in $\alpha $-Cu$_{1.85}$Se [1]. [1] S.A.~Danilkin, A.N.~Skomorokhov, A.~Hoser, H.~Fuess, V.~Rajevac, N.N.~Bickulova, Crystal structure and lattice dynamics of superionic copper selenide Cu$_{2-\delta }$Se, J. Alloys and Compounds, 2003, v. 361, p. 57-61.