Nature of the incommensurate-paraelectric transition: a two dimensional exchange-difference NMR study

87Rb two-dimensional exchange-difference NMR was used to study collective motions in the incommensurate (I) and paraelectric (P) phases of Rb2ZnCl4, which elucidate the nature of the I-P transition. We measured the cross-peak frequency displacement vs mixing time 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. The P phase results are identified as normal modes arising from simultaneous displacement of the Rb ions and rotations of the ZnCl4 between two sites, and these modes freeze out in the I phase to become the modulation wave.