NEUTRON-DIFFRACTION AND CALORIMETRIC STUDIES OF METHYLAMMONIUM IODIDE

A powerful combination of calorimetric and high-resolution neutron powder-diffraction techniques has been used to study the phase transitions in deuterated methylammonium iodide. The neutron powder-diffraction measurements have confirmed the fourfold disorder of the deuterium atoms about the C3 axis of the methylammonium ion in the tetragonal (P4/nmm) alpha' phase. The structure of the metastable delta-phase was determined using a novel method of chemically constrained profile refinement. It was found to be orthorhombic (Pbma, Z = 4, a = 7.1743, b = 7.0967, c = 8.8323 angstrom) in which the deuterium atoms were completely ordered in an anti-parallel manner along the b axis. Heat capacities of the alpha' and delta-phases of CD3ND3I were measured in the temperature range 13-303 K. A continuous lambda-type anomaly was found at the delta-alpha' phase transition which occurred at 164.0 K with an associated transition entropy of 8.8 J K-1 mol-1. The results are very similar to those of CH3NH3I measured previously, indicating that deuteration of the methylammonium ion does not affect the mechanism of the delta-alpha' phase transition of CD3ND3I.