Thermochimica Acta, Vol.336, No.1-2, 127-131, 1999
The effects of isotopes on the lambda-transition and structural phase transition in ammonium chloride
A thermal analysis employing differential scanning calorimetry (DSC) was undertaken to study the effects of isotopes N-14-N-15, H-D, and Cl-Cl-35 on the lambda-transition and structural CsCl double left right arrow NaCl type phase transition of ammonium chloride using (NH4Cl)-N-14, (NH4Cl)-N-15, (ND4Cl)-N-15, (ND4Cl)-N-14, (NH4Cl)-N-14-Cl-35, (NH4Cl)-N-14-Cl-35, and (ND4Cl)-N-14-Cl-35. The endothermic and exothermic peak temperatures of the h-transition were 7 degrees C higher when D was substituted for H in (NH4Cl)-N-14, (NH4Cl)-N-15, and (NH4Cl)-N-14-Cl-35. 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 N-14-N-15 was substituted in (NH4Cl)-N-14, (ND4Cl)-N-14, and (NH4Cl)-N-14-Cl-35 and in the exothermic peak temperature when Cl-Cl-35 was substituted in (NH4Cl)-N-14 and (ND4Cl)-N-14. 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-Cl-35 exchanged samples in (NH4Cl)-N-15. No clear relationship was found between the isotope effect of the lambda- and structural CsCl double left right arrow NaCl type phase transitions.