Spontaneous liquifaction of isomerizable molecular crystals.

A lattice vacancy raises the energy of the neighboring (flexible) molecule in a crystal, which may be enough to isomerize it to a tautomer that does not fit the lattice site, thus creating a liquidlike local region embedding the vacancy. Similar regions may appear elsewhere in the lattice and the regions may ultimately merge. Thus a crystal may spontaneously liquefy over a period of hours to years at a temperature below its normal melting point. Simultaneous heat capacity and heat absorption measurements of several such molecular crystals show that they spontaneously liquefy at a temperature far below their reputed melting point, according to a non-exponential rate kinetics and a temperature dependent rate constant, and do not crystallize on cooling. (c) 2007 American Institute of Physics.