AN INTERPRETATION FOR THE THERMODYNAMIC FEATURES OF ICE IH[--]ICE XI TRANSFORMATION

The unusual manner of entropy change at the ice Ih<->ice XI transforma tion has been interpreted in terms of an equilibrium involving the str ain energy at the ice Ih/ice XI interface, the Gibbs energy and the gr ain-boundary energy, and a formalism given. The persistence of;Ih belo w its first order phase transformation temperature of 72 K and the per sistence of ice XI, the proton-ordered phase, at temperatures up to 11 0 K are attributed to the predominance of the strain energy which oppo ses the transformation favored by the Gibbs energy. Calculations using the measured entropy yield a strain energy of 3.5 J/mol at 72 K, whic h decreases on heating toward 120 K. Consequences of the theory are: ( i) Thermodynamic quantities measured below 72 K would change on therma lly cycling the sample through the transformation region, which has be en observed; (ii) A procedure that reduces the strain at the ice nh-ic e XI boundary will increase the extent of transformation, i.e., mechan ically deforming or grinding a partially transformed sample, or subjec ting it to high energy irradiation to create more point defects that c an bind with the ice Ih/ice XI interface, would transform ice Ih and i ce XI more in either direction. (C) 1998 American Institute of Physics . [S0021-9606(98)00242-6].