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From the new infrared (IR) reflectivity and time-domain terahertz (THz)spectra combined with available high-frequency dielectric data above the MHzrange in a broad temperature range of 10-900 K, a full picture of the soft andcentral mode behavior in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3(PMN) is suggested. A detailed comparison is given with the recent hyper-Ramanspectroscopy data (Phys. Rev. Lett. 117, 155501 (2016)), and also with otheravailable experiments based on inelastic light and neutron scattering. Theclosest agreement is with the hyper-Raman data, both techniques yield the samenumber of soft-mode components and the same high-temperature softening towardsthe temperature T* ~ 400 K. In addition to evaluation of the IR-THz data usingfitting with standard factorized form of the dielectric function, we performeda successful fitting of the same data using the effective medium approach(EMA), originally based on the assumption that the mesoscopic structure of PMNconsists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs)with somewhat harder TO polar modes in the direction along the local PND dipole(Phys. Rev. Lett. 96, 027601 (2006)). Evaluation using the Bruggeman EMAmodelling has been successfully applied in the entire investigated temperaturerange. These results suggest that the response perpendicular to the localdipole moment, at high temperatures induced by random fields rather than PNDs,undergoes a classical softening from high temperatures with permittivityobeying the Curie-Weiss law, eps_per = C/(T-Tc), C = 1.7 x 10^5 K and Tc = 380K. Below the Burns temperature ~620 K, a GHz relaxation ascribed to flipping ofthe PNDs emerges from the soft mode response, slows down and broadens,remaining quite strong towards the cryogenic temperatures, where it can beassigned to fluctuations of the PND boundaries.

Infrared, terahertz, and microwave spectroscopy of the soft and central modes in Pb(Mg1/3Nb2/3)O3