14N NMR study of the domain structure of tetramethylammonium cadmium chloride (TMCC)

Abstract

AbstractQuadrupolar perturbed 14N NMR investigations are performed on single crystals of tetramethylammonium cadmium chloride, (CH3)4NCdCl3 (TMCC), in the temperature range of 297 to 80 K. At the phase transitions at 118 and 104 K the quadrupole coupling tensor (QCT) shows abrupt changes in accordance with their first order type. The largest QCT component is parallel to the hexagonal c‐axis in the high temperature phase and rotates by 90° at the transition into the monoclinic phase II at 118 K. In complete agreement with a recent 35Cl NMR study, the 14N spectra in phase II show clearly a ferro‐elastic twin domain structure. The saturated rotational angle between the epitaxially grown domains of Exp ≈ 4 is in a good agreement with the theoretical value of S = 3.55 calculated from the ferro‐elastic strain tensor. The ordering behaviour below the phase transition at 118 K is related with the rotation of the largest component VZZ of the QCT in a plane perpendicular to the c‐axis and with changes of its principal axes values. The component VZZ is parallel to the N bond direction. A shift of the N atoms within the plane in relation with the phase transition was also found in recent low temperature X‐ray diffraction measurements. In the whole temperature range investigated, the 14N NMR spectra are exclusively determined by the lattice symmetry but not by distortions of the local tetrahedral symmetry at the N sites of the tetramethylammonium groups.