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 und er pressures up to 3.5 GPa at room temperature, by Brillouin spectrosc opy with in situ identification for the orientation of single crystal NH3 in a diamond-anvil cell. Values of three elastic constants and bul k 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 constan ts shows a large value (4.06 at 3.0 GPa), indicating that rotation-tra nslation coupling of the molecular motions is effective in the orienta tionally 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 comp ression.