Cl-35 NMR studies of the domain structure of tetramethylammonium cadmium chloride (TMCC) at lower temperatures

Quadrupolar perturbed Cl-35 NMR and Cl-35 NQR investigations are performed on single crystals of tetramethylammonium cadmium chloride, (CH3)(4)NCdCl3 (TMCC), in order to study the formation of ferroelastic domains in the ferr oelastic phase II below 118 K in which the hexagonal symmetry of the high t emperature phase I is lost. The experimental results cannot simply be expla ined in terms of the well known monoclinic domain structure but additionall y a twin domain structure is found to exist in phase II: six orientational domains are verified from the NMR rotational pattern at 113 K in contrast t o the expected number of three for a classical 6/mF2/m transition. The rota tional angle Theta between the epitaxially grown twins is calculated from t he strain tensors of the three orientation states, applying an algebraic ap proach. The theoretical value Theta = 3.5 degrees is in very good correspon dence with the experimental result from the Cl-35 NMR studies (similar to4 degrees). Similar results were also derived in recent x-ray studies in phas e II. In the low temperature phase III below 104 K the number of non-equiva lent Cl positions is consistent with both the space group of phase III and the sixfold enlarged size of the unit cell in phase III with respect to tha t in phase I as observed in XRD studies.