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

From the new infrared (IR) reflectivity and time-domain terahertz (THz) spectra combined with available high-frequency dielectric data above the MHz range in a broad temperature range of 10-900 K, a full picture of the soft and central mode behavior in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) is suggested. A detailed comparison is given with the recent hyper-Raman spectroscopy data (Phys. Rev. Lett. 117, 155501 (2016)), and also with other available experiments based on inelastic light and neutron scattering. The closest agreement is with the hyper-Raman data, both techniques yield the same number of soft-mode components and the same high-temperature softening towards the temperature T* ~ 400 K. In addition to evaluation of the IR-THz data using fitting with standard factorized form of the dielectric function, we performed a successful fitting of the same data using the effective medium approach (EMA), originally based on the assumption that the mesoscopic structure of PMN consists 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 EMA modelling has been successfully applied in the entire investigated temperature range. These results suggest that the response perpendicular to the local dipole moment, at high temperatures induced by random fields rather than PNDs, undergoes a classical softening from high temperatures with permittivity obeying the Curie-Weiss law, eps_per = C/(T-Tc), C = 1.7 x 10^5 K and Tc = 380 K. Below the Burns temperature ~620 K, a GHz relaxation ascribed to flipping of the PNDs emerges from the soft mode response, slows down and broadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations of the PND boundaries.