Broadband Dielectric Spectroscopy of Mixed Betaine Phosphate Betaine Phosphite, CuInP_2(S_xSe_{1-x})_6 crystals and PMN-PSN-PZN Ceramics

TIn the first chapter, the current ideas about crystalline disordered system are surveyed and exemplified by the results allowing for the most straightforward interpretation. Exclusive consideration is intended for broadband dielectric spectroscopy results. Several critical remarks are presented about various predefined forms of the distribution of relaxation times and it physical meaning. The second chapter is devoted to the experimental and theoretical methods of the broadband dielectric spectroscopy, which have been applied by the author to collect the original data personally. The third chapter contains results of investigations of the mixed hydrogen-bond betaine phosphite betaine phosphate phase transitions dynamics. Here distribution of the relaxation times, double well potential parameters, local polarization distributions of the investigated crystals are presented. In the fourth chapter the dielectric spectroscopy results of mixed CuInP2(SxSe1-x)6 crystals are presented, phase diagram of these crystals is discussed. The fifth chapter contains the dielectric spectra of PMN-PSN-PZN relaxor ferroelectrics in a very wide frequency range. Formation of the polar nanoregions below the Burns temperature (700-800 K) is manifested by the appearance of the dielectric relaxation in the THz range and by splitting of polar modes in the infrared spectra. Dielectric spectra are analyzed in terms of distribution of relaxation times using the Tichonov regularization method. The distributions splits in two components near room temperature and the origin of both components are discussed. In the infrared spectra interesting phonon anomalies are observed