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

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, ZH(omega), were extracted and the respective H mean squared displacements, <u2>H, determined. These ZH(omega) 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>H0 and the zero-point proton mean kinetic energy, <Ek>H0. Finally, approximate Grueneisen parameters for <u2>H0 and <Ek>H0 were calculated exploiting D2S neutron diffraction data.