Chapter 5 - NMR Studies of Disorder in Condensed Matter Systems

Abstract

Abstract This review summarizes nuclear magnetic resonance (NMR) studies in condensed matter systems exploring the effects of disorder. Highly ordered systems have a well-defined order parameter, which is zero at high-temperature phase and finite at low temperatures. In contrast to magnetic spin glasses, dipolar glasses show that the cusp in the dielectric susceptibility is rounded and the freezing takes place over an unusually large-temperature interval far above the glass transition temperature TG. Many a times, it is difficult to define a glass transition temperature for a given system. The microscopic origin of this behaviour is quite interesting and is a subject of intensive research. Origin of disorder is different in different systems. In dipolar glasses, the origin of disorder is traced to the shuttling of H-bonds between the possible positions. In parent compounds, at higher temperatures, there is equal probability of H-bonds being found between the two oxygens of the two adjacent PO4 groups, while below a particular temperature Tf (defined as the ferroelectric (FE) transition temperature), they freeze to one of the possible positions. In a dipolar glass system (FE + AFE mixed system), the system goes to a frustrated state with the disappearance of the phase transition resulting in an orientational disorder. As a result of this, the rate of H-bond motion in these systems, now will have a wide range of distribution starting from 10− 3 to 10− 12 s. Similarly in FE relaxors, one can find substitutional disorder due to the changes in the charges of the different cations resulting in random-fields (RFs) and random bonds. The main sources of disorder in conducting polymers (CPs) are sp2 defects in the chain, chain ends, chain entanglements, voids, morphological and doping defects. CPs are partially crystalline and partially amorphous, the volume fraction of the crystalline regions and the size of the crystalline coherence length play a dominant role in the charge transport. In general, the disorder induced localization plays a dominant role in the transport properties of CPs. Our main aim in this review is to study the effect of disorder in dipolar glasses, FE relaxors and CPs. NMR is a powerful local probe for providing information about the distribution and dynamics of local RFs, and characteristic of such systems. While the quadrupole-perturbed NMR line shape analysis gives details about the distribution of local RFs, spin–lattice relaxation studies can give information on the dynamics in the frustrated state of these systems. Many modifications and new methods have been adopted to use NMR to explore the effects of disorder on measurable quantities. Other than hydrogen, many NMR friendly nuclei such as deuteron, 87Rb, 45Sc, 93Nb, 207Pb, 205Tl, etc. are used to study these systems. Further, two-dimensional deuterium NMR exchange spectroscopy is also applied to study these materials.