The effects of isotopes on the λ-transition and structural phase transition in ammonium chloride

Abstract

A thermal analysis employing differential scanning calorimetry (DSC) was undertaken to study the effects of isotopes 14N– 15N, H–D, and Cl– 35Cl on the λ-transition and structural CsCl⇔NaCl type phase transition of ammonium chloride using 14NH 4Cl, 15NH 4Cl, 14ND 4Cl, 15ND 4Cl, 14NH 4 35Cl, 15NH 4 35Cl, and 14ND 4 35Cl. The endothermic and exothermic peak temperatures of the λ-transition were 7°C higher when D was substituted for H in 14NH 4Cl, 15NH 4Cl, and 14NH 4 35Cl. In the structural phase transition, the lighter isotope-substituted ammonium chloride exhibited a higher transition temperature, except that there was a change in the endothermic peak temperature when 14N– 15N was substituted in 14NH 4Cl, 14ND 4Cl, and 14NH 4 35Cl and in the exothermic peak temperature when Cl– 35Cl was substituted in 14NH 4Cl and 14ND 4Cl. The isotope effect also tended to expand the hysteresis as a result of the difference between the endothermic and exothermic peak temperatures except for the H–D and Cl– 35Cl exchanged samples in 15NH 4Cl. No clear relationship was found between the isotope effect of the λ- and structural CsCl⇔NaCl type phase transitions.