Approach to Melting in Ammonia as a Critical Transition

The molar volumes of ammonia solid I and solid II were measured from 0.5 to 14.0 kbar and 185 to 320 K. Over regions that extend 3 kbar and 20 K into the solid phases, variations in compressibility and thermal expansion can be described by power laws with exponents similar to those usually associated with critical transitions. It is suggested that breaking of hydrogen bonds may account for the extreme softening of solid ammonia.