Abstract
Adiabatic elastic constants and adiabatic bulk modulus have been determined for the orientationally disordered phase III (plastic phase) of NH3 at pressures up to 3. 5 GPa at room temperature, by Brillouin measurements with in situ identification for the orientation of single crystal NH3 in a diamond anvil cell. Typical values of three elastic constants for f. c. c, crystalline NH3 are C11=21. 1, C12=17. 1, and C44=8. 1 GPa at pressure of 2. 56 GPa. The elastic anisotropy (A=4. 06 at 2. 56 GPa) is almost constant against pressure, and these properties are compared with those of other plastic solids.
