Local Ordering Processes in Ferroelectric, Glass-like and Modulated phases: An EPR Study

By means of two selected examples it will be shown that electron paramagnetic resonance (EPR) spectroscopy delivers unique microscopic information about crystals with ferroelectric, glass-like and modulated phases. The first example concerns a proton glass. Frustrating interactions between the protons of the hydrogen bond system in mixed crystals of ferroelectric betaine phosphite and antiferroelectric betaine phosphate give rise to a glass-like order of the proton system. Electron nuclear double resonance (ENDOR) and two-dimensional pulsed HYSCORE (hyperfine sublevel correlation spectroscopy) investigations give access to the distribution function of the local proton order parameter and to the Edwards-Anderson glass order parameter. The investigations give evidence for a new cluster-like intermediate phase probably similar to the Griffiths phase in magnetic spin glasses. In the second example a crystal is considered which shows a complex sequence of commensurate and incommensurate phases firstly discovered by EPR investigations. Dimethylammonium gallium sulfate hexahydrate (DMAGaS) proves to be a new model system for a phase sequence paraelectric — ferroelectric — IC/C modulated — antiferroelectric not yet observed for any other crystal. For both examples the experimental results will be discussed in comparison to microscopic model theories.