Phase Transitions and Ionic Motions in Solid Trimethylethylammonium Iodide Studied by ¹H and ¹²⁷I NMR, Electrical Conductivity, X‐ray Diffraction, and Thermal Analysis

Abstract

Abstract 1 H and 127 I NMR, differential thermal analysis (DTA), differential scanning calorimetry (DSC), electrical conductivity, and powder and single crystal X‐ray diffractions were studied for trimethylethylammonium iodide. Three solid phases, named I, II, and III, were obtained above 410 K, between 336 and 410 K, and below 336 K, respectively. Powder X‐ray patterns showed that Phase I and II have a NaCl‐type cubic structure ( a = 10.00(1) Å, Z = 4) and a tetragonal structure ( a = 8.430(5), c = 6.216(4) Å, Z = 2), respectively. The structure of Phase III was determined to be orthorhombic (Pnam (#62), a = 11.294(4), b = 10.729(2), c = 7.158(6) Å, Z = 4) by single crystal X‐ray diffraction. In Phase III, the C 3 reorientation of CH 3 groups and the C 3 reorientation of the whole (CH 3 ) 3 N group in the cation were observed by 1 H NMR. In Phase II, the cation is expected to perform the quasi‐isotropic reorientation. In Phase I, the cation was found to carry out the isotropic reorientation, while the spherical anion the translational self‐diffusion, as evidenced by 1 H and 127 I NMR, and electrical conductivity. Phase II and I are, therefore, dynamically disordered phases; especially, Phase I is highly disordered. From the dynamical behaviour of the cation and anion, and thermal properties, Phase I can be classified into an ionic plastic phase with a NaCl‐type structure.