Central-peak components and polar soft mode in relaxor PbMg1/3Nb2/3O3 crystals

Dielectric response of relaxor PMN single crystals was investigated in a broad frequency range 100 Hz-100 THz by a combination of dielectric spectroscopy (100 Hz-1 GHz, 10-700 K), time-domain THz spectroscopy and infrared (IR) reflectivity (5-3000 cm(-1) , 10-300 K). Polar TO1 phonon (soft mode) is resolved in THz and IR spectra at all temperatures studied, follows the Cochran law and softens towards the Burns temperature on heating. The central mode (dielectric relaxation) splits below room temperature into two components. First one appears in the THz range and is assigned as IR activated phonon density of states due to the breaking of local symmetry as a consequence of polar nano-clusters. Second relaxation originates from cluster breathing and flipping. It anomalously broadens on cooling and leads to frequency-independent dielectric loss between 100 Hz-100 GHz below 200 K in the non-ergodic phase.