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 ${\mathrm{NH}}_{3}$ 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 ${\mathrm{NH}}_{3}$ in a diamond-anvil cell. Values of three elastic constants and bulk modulus for fcc crystalline ${\mathrm{NH}}_{3}$ (phase III) are, e.g., ${C}_{11}=23.2$, ${C}_{12}=18.8$, and ${C}_{44}=8.9$ GPa, and ${B}_{S}=20.2$ GPa 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 III. Comparisons of the pressure dependence of ${\mathrm{NH}}_{3}$ elastic anisotropy with those for other molecular solids suggest that the hydrogen bond influences on the molecular rotation under compression.