By using in situ Brillouin spectroscopy in a diamond-anvil cell, we have measured the direction-dependent acoustic velocities and the refractive index in the orientationally disordered and hydrogen-bonded solid phase I of hydrogen chloride ͑HCl͒ at pressures up to 4.5 GPa and 300 K, leading to the first determinations of adiabatic elastic constants (C 11 , C 12 , and C 44 ), bulk modulus (B S ), and the elastic anisotropy ͑A͒ to 4.5 GPa. These results are compared with those of solids of the same type H 2 S and NH 3 . We investigate the effect of the strength of hydrogen bonds on the A value which is sensitive to the rotation-translation coupling of molecular motions under compression. ͓S0163-1829͑99͒01318-1͔
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