Soft and central mode behaviour inPbMg1/3Nb2/3O3 relaxor ferroelectric

Abstract

The relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) is investigated by means of dielectric and Fourier transform far infraredtransmission spectroscopy in the frequency range from 10 kHz to 15 THz at temperaturesbetween 20 and 900 K using mostly thin films on infrared transparent sapphire substrates.While the thin film relaxors display reduced dielectric permittivity at low frequencies, theirhigh frequency lattice response is shown to be the same as for single-crystal/ceramicspecimens. In contrast to the results of inelastic neutron scattering, the optic soft mode isfound to be underdamped at all temperatures. On heating, the TO1 soft phononfollows the Cochran law with an extrapolated critical temperature of 670 K nearto the Burns temperature. Above 450 K the soft mode frequency levels off near50 cm−1 and above the Burns temperature it slightly hardens. Central-mode-type dispersionassigned to the dynamics of polar nanoclusters appears below the Burns temperature atfrequencies near to but below the soft mode and slows down and broadens dramatically oncooling, finally, below the freezing temperature of 200 K, giving rise to frequencyindependent losses from the microwave range down. A new explanation of thephonon ‘waterfall’ effect in inelastic neutron scattering spectra is proposed.