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 0-20 GPa under high temperature, combined with externally heated method in a diamond anvil cell. Three elastic constants and two moduli in single crystal sample were presented at high pressure-temperature and each of them grows monotonously with pressure along 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 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 high-pressure and high-temperature range, also providing new insight into other simple hydrogen-bonded molecular compounds.