Phonon Density of States from a Crystal-analyzer Inverse-geometry Spectrometer: A Study on Ordered Solid Hydrogen Sulfide and Hydrogen Chloride

Abstract

We have studied two “ice-like” systems, H₂S and HCl, which, despite their molecular simplicity, exhibit a rich polymorphism in solid phase. We performed incoherent neutron scattering measurements employing a new crystal-analyzer inverse-geometry neutron spectrometer, TOSCA-I. After a detailed data reduction procedure, we devised a technique aiming to estimate and remove the multi-phonon part of the spectra. Finally, the H-projected phonon densities of states, Z_H(ω), were extracted and the respective H mean squared displacements, 〈u²〉_H, determined. These Z_H(ω) show good statistical accuracy and resolution and represent an important benchmark for future lattice-dynamics simulations and phonon dispersion curve measurements. Since some thermal softening of the phonon modes was visible in both H₂S and HCl, we tried to quantify this anharmonic effect through the zero-point proton mean squared displacement, 〈u²_inter〉_H,0, and the zero-point proton mean kinetic energy, 〈E_k,inter〉_H,0. Finally, approximate Grüneisen parameters for 〈u²_inter〉_H,0 and 〈E_k,inter〉_H,0 were calculated exploiting D₂S neutron diffraction data.