NEUTRON-DIFFRACTION AND CALORIMETRIC STUDIES OF METHYLAMMONIUM CHLORIDE

The heat capacity of fully deuterated methylammonium chloride (CD3ND3Cl) was measured in the temperature range 14-300 K with an adiabatic calorimeter. Two first-order phase transitions, beta to gamma and gamma to alpha, were observed at 211.5 K and 254.0 K, respectively. Both transitions, especially the former, exhibited unusually large super-heating effect. The transition entropies were 9.703 JK-1 mol-1 and 11.50 JK-1 mol-1, respectively. These thermodynamic properties associated with the transitions were similar in magnitude to those of the protonated analog (CH3NH3Cl), showing minor isotope effect on the transition. Neutron powder diffraction experiment for the room temperature alpha phase of CD3ND3Cl was performed with a time-of-flight type diffractometer using a spallation pulsed neutron source. The observed d-spacing range was 0.5-3.2 angstrom. Structure refinement with the Rietveld method revealed the existence of the four-fold orientational disorder of the methylammonium ion around its C-N axis. This result indicated that the reorientational motion of the methylammonium ion is hindered mainly by the N-D ... Cl hydrogen bonds in the alpha phase. The transition mechanisms were discussed from both thermodynamic and structural points of view.