Magnetic resonance investigations of phase transitions and modulation wavemotions in incommensurate solids

We measured Rb-87 nuclear magnetic resonance (NMR) and Cl-35 nuclear quadru pole resonance (NQR) Hahn spin-echo magnetization decays in the incommensur ate (I) phase of Rb2ZnCl4 and, in each case. obtained a Hahn echo decay tha t Mas shorter than the Carr-Purcell-Meiboom-Gill decay and one which decaye d with a lime constant proportional to the: cube of the echo lime. From the se measurements we obtained from both the Rb-87 NMR and Cl-35 NQR measureme nts values for the diffusion coefficients that are comparable in magnitude. a fact that strongly supports the existence of slow modulation wave diffus ionlike motions in the I phase, since such motions should affect both Rb an d CI ions similarly. In addition, we used Rb-87 two-dimensional exchange-di fference NMR to study atomic motions in the incommensurate (1) and paraelec tric (P) phases to elucidate the nature of the I-P transition. We measured as a function of the mixing time the frequency shifts of the cross peaks fr om the main diagonal and observed a gradual increase towards an asymptotic value in the I phase but a sudden jump to the final value in the P phase. W e interpreted the motions observed in the P phase as normal modes arising f rom simultaneous reorientations of ZnCl4 tetrahedra and corresponding Rb io ns displacements between two sites. These normal modes freeze out in the 1 phase and change to the diffusionlike motion of the modulation wave. We als o performed Cl-15 NQR lineshape and T-1 measurements in K2ZnCl4 and obtaine d conclusive evidence for the presence of a narrow Iq (singly modulated) I phase between 146 and 149 K.