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

摘要

Quadrupolar perturbed N-14 NMR investigations are performed on single crystals of tetramethylammonium cadmium chloride, (CH3)(4)NCdCl3 (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 90degrees at the transition into the monoclinic phase II at 118 K. In complete agreement with a recent Cl-35 NMR study, the N-14 spectra in phase II show clearly a ferro-elastic twin domain structure. The saturated rotational angle between the epitaxially grown domains of Theta(Exp) approximate to 4 is in a good agreement with the theoretical value of Theta(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 V-ZZ of the QCT in a plane perpendicular to the c-axis and with changes of its principal axes values. The component V-ZZ is parallel to the C-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 N-14 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. [References: 32]