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 orientationaldisorder, like HCl-I and II, have not yet been studied in detail through neutron spectroscopy, given thecomplicated interplay between elastic, quasi-elastic and phonon scattering. Some attempts have beenaccomplished using coherent neutron scattering on deuterated single-crystals, providing interesting butincomplete results. On the contrary, in the present study we will show how incoherent inelastic neutronscattering at small angles (namely in the kinematic regime attained by BRISP) can offer important andunique information on these phase transitions, being able to disentangle the various scatteringcontributions through the spanning of the accessible momentum transfer range. Simulated examplesobtained via molecular dynamics concerning HCl-I will be reported and commented.