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Thermodynamic Stability of Ice II and Its Hydrogen-Disordered Counterpart: Role of Zero-Point Energy

Last update: March 8, 2016

Our paper has been published in The Journal of Physical Chemistry B.

 

Nakamura, T., Matsumoto, M., Yagasaki, T. & Tanaka, H. Thermodynamic Stability of Ice II and Its Hydrogen-Disordered Counterpart: Role of Zero-Point Energy. J. Phys. Chem. B, 2016, 120 (8), pp 1843–1848 DOI:10.1021/acs.jpcb.5b09544

 

There are more than 10 crystal polymorphs for ice.  They are roughly divided into hydrogen-ordered and -disoirdered phases.  In the hydrogen-ordered phase, directions of all the hydrogen bonds are decided, while they are randomized in the disordered counterpart.  (This randomness is evaluated experimentally as the Pauling's residual entropy.)

 

Almost all hydrogen-disordered ice phase has its hydrogen-ordered counterpart.  Ice Ih, which is the normal ice, is an exapmle of the hydrogen-disordered phase and it changes to its hydrogen-ordered counterpart, ice XI (eleven).  Ices Ih and XI are considered to be the siblings.

 

Ice II is the only exception without siblings.  This high-pressure phase of ice appears as a hydrogen-ordered phase above 2000 atm, and it does not change to the hydrogen-disordered counterpart by heating.  It rather melts or changes to another ice with different structure, ice III.

 

Why is ice II alone?  We investigated the stability of a hypothetical hydrogen-disordered counterpart of ice II, named ice IId, by theoretical calculations.  We found that the hydrogen bonds in ice II accepts large stresses and ice II is on an exquisite balance of the hydrogen bond network.  Disorder in the structure gets off the balance and unstabilized the structure badly.

 

Theoretical study is thus useful to consider why some structure does not appear in reality.