First order enantiotropic solid state phase transitions -
the unfinished Mnyukh revolution
Prof. Frank H. Herbstein, Departmnet of Chemistry, Technion
Mnyukh's direct observation of phase transitions in eight organic
crystals by optical microscopy has demonstrated conclusively that
these proceed by a nucleation-and-growth mechanism. Nucleation is
not homogenous (as it is presumed to be in most theoretical treatments) but
localized at defects in the parent phase; growth can be either epitaxial and
oriented when parent and daughter phases have closely similar structures,
or randomly orientated if there are substantial structural differences
between the phases. The Mnyukh description also fits the so-called lambda
transition in NH4Cl at 242 K, whose investigation in 1922 by F. Simon first
gave rise to doubts (expressed most clearly by A. R. Ubbelohde some fifty
years ago) about the applicability of classical thermodynamics to some phase
transitions because of the occurrence of hysteresis. The Mnyukh description
fits, but not entirely, the observations of Asher Schmidt et al.
(J. Am. Chem. Soc. 121, 11291-11299,1999) on the phase transition in
2-(2,4-dinitrobenzyl)-3-methyl-pyridine at 318 K and those of Mitkevich et al.
(Acta Cryst, B55, 799-806,1999) on the transition at ‰185 K in
4,4'-dichlorobenzophenone. Does the Ubbelohde approach still have a role
to play?