Deuteration-induced structural phase transitions in some hydrogen-bonded crystals

A review is given, mainly based on heat capacity measurements, of the deuteration-induced structural phase transitions recently found in some rather diverse types of hydrogen-bonded crystals: i.e. M(3)H(XO(4))(2)-type crystals with strong hydrogen bonds, 9-hydroxyphenalenone derivatives with strong intramolecular hydrogen bonds, and (NH4)(2)MX(6)-type crystals with weak asymmetric hydrogen bonds. For M(3)H(XO(4))(2)-type crystals and 9-hydroxyphenalenone derivatives, the hydrogen-bond distance seems to be one of the determinant parameters, while for (NH4)(2)MX(6)-type crystals it is as yet unclear. However, some common characteristics of the heat capacity behavior are noted: (1) the magnitude of the transition entropy is compatible with order-disorder-type phase transitions, and (2) unusually the values of the heat capacity of the undeuterated compound are larger than those of the deuterated compound at the lowest temperature range.