High-pressure elastic properties of liquid and solid ammonia

Acoustic velocity, adiabatic elastic constants, adiabatic bulk modulus, and the refractive index of liquid and solid NH3 were determined under pressures up to 3.5 GPa at room temperature, by Brillouin spectroscopy with in situ identification for the orientation of single crystal NH3 in a diamond-anvil cell. Values of three elastic constants and bulk modulus for fee crystalline NH3 (phase III) are, e.g., C-11=23.2, C-12=18.8, and C-44=8.9 GPa, and B-s=20.2 Cpa at a pressure of 3.0 GPa, respectively. The elastic anisotropy obtained from the elastic constants shows a large value (4.06 at 3.0 GPa), indicating that rotation-translation coupling of the molecular motions is effective in the orientationally disordered phase m. Comparisons of the pressure dependence of NH, elastic anisotropy with those for other molecular solids suggest that the hydrogen bond influences on the molecular rotation under compression.