Dipolar glasses

Classical relaxors [22,23] are perovskite soUd solutions like PbMgi/3Nb2/303 (PMN), which exhibit both site and charge disorder resulting in random fields in addition to random bonds. In contrast to dipolar glasses where the elementary dipole moments exist on the atomic scale, the relaxor state is characterized by the presence of polar clusters of nanometric size. The dynamical properties of relaxor ferroelectrics are determined by the presence of these polar nanoclusters [24]. PMN remains cubic to the lowest temperatures measured. One expects that the disorder -type dynamics found in the cubic phase of BaTiOs, characterized by two timescales, is somehow translated into the... 

The present results demonstrate that the basic difference between relaxors and dipolar glasses is their response to applied electric fields polar nanoclusters, corresponding to the frozen anisotropic component in the NMR spectra, can be oriented in a strong enough applied electric field and a ferroelectric phase can be induced. This is not the case in dipolar glasses, where the response is due to single dipoles which cannot be ordered by applied electric fields. 

Close parallels can be drawn between the dielectric response of a relaxor and that of a random assembly of electric dipoles - a dipolar glass . The matter is clearly one of very considerable complexity depending as it must upon the sizes, charges and polarizabilities of the ions, and thermal history. 

Key Words Dipolar glasses, Ferroelectric relaxors, Conducting polymers, NMR line shape, Disorder, Local polarization related to the line shape, Symmetric/asymmetric quadrupole-perturbed NMR, H-bonded systems, Spin-lattice relaxation, Edwards-Anderson order parameter, Dimensionality of conduction, Proton, Deuteron tunnelling. 

Disordered systems can be broadly classified into spin glasses, dipolar glasses/pseudo-spin glasses, canonical glasses, conducting polymers (CPs),... 

A pulsed NMR spectrometer, with a variable frequency and variable temperature facility is best suited for the study of these disordered systems.26 27 For dipolar glasses and relaxor systems, a spin echo Fourier transformation NMR spectra of the system have been measured in a wide bore superconducting magnet ( typically at 9.2 T). 

In this section, we will be concentrating mainly on the use of NMR technique based literature on dipolar glasses and the information obtained from them. The NMR nuclei studied in dipolar glasses are 1H, 2H and 87Rb and 31P. 207Pb, 45Sc and 93Nb are other nuclei which are studied in relaxor materials. 

ID NMR parameters such as SLR time and line shape measurements give information about the dynamics of the intra-bond O-H-O motion along with reorien-tational motion of symmetric groups in the dipolar glasses. However, the information obtained from such studies is limited hence reports of such studies are meagre. In the corresponding deuterated analogues much more information... 

There are many questions about relaxors. Are they similar to dipolar glasses where elementary dipoles exist on the atomic scale or is the relaxor state indeed characterized by the presence of nanosized polar cluster of variable sizes and orientations. 

Relaxor ferroelectrics47-49 (RFEs) have attracted considerable attention in recent years due to their unusual physical behaviour. Relaxors are technologically important as transducer/actuator materials. Relaxors are intermediate between dipolar glasses and classical FEs and exhibit both substitutional and charge disorder. They exhibit very large dielectric, piezoelectric, and electromechanical... 

There are always questions about relaxors, that whether, the relaxor transition is a transition to a dipolar glass state or FE state broken up into nanodomains due to quenched RFs. 

---