Isotope induced phase transition in ammonium salts and hydrogen-bonded crystals

Phase transition is accompanied with changes of internal energy and entropy. Lattice vibration is related to entropy and this entropy is an important factor to decide transition temperature in displacement-type phase transition. Various intermolecular interactions are another factor to determine free energy related to phase transition. The origin of intermolecular interactions is the charge of nuclei and electrons. Some ammonium salts, acidic sulfates and acidic selenates were found to show significant deuterium-induced phase transition. This deuterium isotope effect is examined on (NH4)2MCl6 type crystals (M = Pt, Pd, Te, Se). Transition entropy, heat capacity, and the nature of the deuterium-induced phase transition are discussed. anomalous heat capacity of Rb3H(D)SO4 is examined. Other compounds showing deuterium-induced phase transition are briefly mentioned. (T.H.)