Incoherent inelastic neutron scattering at small angles: studying phase transitions to orientationally disordered HCl

Phase transitions in simple H-bonded systems to structures characterized by a high degree of orientational disorder, like HCl-I and II, have not yet been studied in detail through neutron spectroscopy, given the complicated interplay between elastic, quasi-elastic and phonon scattering. Some attempts have been accomplished using coherent neutron scattering on deuterated single-crystals, providing interesting but incomplete results. On the contrary, in the present study we will show how incoherent inelastic neutron scattering at small angles (namely in the kinematic regime attained by BRISP) can offer important and unique information on these phase transitions, being able to disentangle the various scattering contributions through the spanning of the accessible momentum transfer range. Simulated examples obtained via molecular dynamics concerning HCl-I will be reported and commented.