87Rb spin diffusion in ferroelectric RbH2PO4 studied by two dimensional exchange NMR
We separate the contributions of spectral spin diffusion and chemical exchange in the 2D exchange NMR spectra of 87Rb in the pseudo-spin glass Rb1-x(ND4)xD2PO4 by studying the 87Rb spin diffusion in the isostructural compound RbH2PO4 at 85K, where the system is frozen in the ferroelectric phase state. The fact that the spin-diffusion time (TSD) of a particular point in the 2D spectrum depends essentially on its distance from the diagonal, allowed, even for the case of an unresolved 2D spectrum, to determine TSD as a function of the frequency separation Delta over two orders of magnitude. In accordance with existing theories, T-1SD(Delta) was found to be of Gaussian shape. However, we found huge discrepancies between the calculated and the experimentally determined second moments. This failure of the theory is not understood at present. Copyright 1999 Academic Press.