Crystal Structure and Phase Transition of Hydrogen Chloride

Detailed structural studies of HCl single crystal were performed by X-ray and neutron diffraction experiments. A face-centered cubic lattice with a twelve-fold disordered orientation of HCl molecules was found only above 120 K. Below 120 K, the molecules form hydrogen bonded zigzag chains and a synthetic twin structure of an orthorhombic lattice with the (101) twin boundary is realized. No appreciable change of the crystal structure was seen in the diffraction experiments at the transition point 98 K. The mechanism of the phase transition can be explained by considering a fluctuation of chains and molecular flippings.