Macdowell Luis G, Sanz Eduardo, Vega Carlos, Abascal José Luis F
Dpto. de Química Física, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain.
J Chem Phys. 2004 Nov 22;121(20):10145-58. doi: 10.1063/1.1808693.
A close analytical estimate for the combinatorial entropy of partially ordered ice phases is presented. The expression obtained is very general, as it can be used for any ice phase obeying the Bernal-Fowler rules. The only input required is a number of crystallographic parameters, and the experimentally observed proton site occupancies. For fully disordered phases such as hexagonal ice, it recovers the result deduced by Pauling, while for fully ordered ice it is found to vanish. Although the space groups determined for ice I, VI, and VII require random proton site occupancies, it is found that such random allocation of protons does not necessarily imply random orientational disorder. The theoretical estimate for the combinatorial entropy is employed together with free energy calculations in order to obtain the phase diagram of ice from 0 to 10 GPa. Overall qualitative agreement with experiment is found for the TIP4P model of water. An accurate estimate of the combinatorial entropy is found to play an important role in determining the stability of partially ordered ice phases, such as ice III and ice V.
给出了部分有序冰相组合熵的一种精确分析估计。所得到的表达式非常通用，因为它可用于任何遵循伯纳尔 - 福勒规则的冰相。唯一需要的输入是一些晶体学参数以及实验观测到的质子位点占有率。对于诸如六方冰之类的完全无序相，它恢复了鲍林推导的结果，而对于完全有序的冰，发现其为零。尽管确定冰I、VI和VII的空间群需要随机的质子位点占有率，但发现质子的这种随机分配并不一定意味着随机取向无序。组合熵的理论估计与自由能计算一起用于获得0至10吉帕压力下冰的相图。对于水的TIP4P模型，发现与实验总体上定性一致。发现组合熵的精确估计在确定部分有序冰相（如冰III和冰V）的稳定性方面起着重要作用。
