The thermodynamics of discontinuous orientational phase transitions in quantum crystals

Generalized Clausius-Clapeyron relations are derived for discontinuous orientational phase transitions in the quantum-mechanical regime. Some qualitative features of the isotope effect on the transition temperature are explained. The quantum correction to the density dependence of the h.c.p.-f.c.c. transition temperature in solid hydrogen, the stability of the partially ordered phase of solid CH4 and the anomalous isotope effect in ammonium bromide are treated in thermodynamic terms.