Disorder-order structural transition of single crystal hydrogen chloride under high pressure-temperature.

Hydrogen chloride (HCl) is the simplest hydrogen-bonded molecule and has attracted a great deal of attention owing to its interesting structural changes triggered by pressure or temperature. The structural properties of solid HCl have been investigated by Brillouin scattering in the pressure range of 0-20 GPa under high temperature, combined with external heating in a diamond anvil cell. Three elastic constants and two moduli of the single crystal sample were observed at high pressure-temperature and each of them grows monotonously with pressure along a separate isotherm. The pressure dependence of elastic anisotropy proves that the disorder-order transition pressures are 4.5 GPa, 5.4 GPa and 8.8 GPa for the 300 K, 390 K and 470 K isotherms, respectively. The current work discovered the disorder-order structural transition in HCl and extended its phase diagram to the high pressure-temperature range, also providing a new insight into other simple hydrogen-bonded molecular compounds.