Infrared, terahertz, and microwave spectroscopy of the soft and central modes in Pb(Mg1/3Nb2/3)O3
Résumé
From the new infrared (IR) reflectivity and time-domain terahertz spectra combined with available highfrequencydielectric data above the megahertz range in a broad temperature range of 10 to 900 K, a fullpicture of the soft- and central-mode behavior in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) issuggested. A detailed comparison is made with the recent hyper-Raman spectroscopy data [Hehlen et al., Phys.Rev. Lett. 117, 155501 (2016)] and also with other available experiments based on inelastic light and neutronscattering. It is revealed that each type of experiment provides slightly different data. The closest agreementis with the hyper-Raman data: both techniques yield the same number of soft-mode components and the samehigh-temperature softening towards the temperature T∗ ≈ 400 K. In addition to evaluation of the IR-terahertz datausing fitting with the standard factorized form of the dielectric function, we performed a successful fitting of thesame data using the effective medium approach (EMA), originally based on the assumption that the mesoscopicstructure of PMNconsists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs) with somewhatharder transverse optical polar modes in the direction along the local PND dipole [Hlinka et al., Phys. Rev. Lett.96, 027601 (2006)]. Evaluation using Bruggeman EMA modeling has been successfully applied in the entireinvestigated 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 hightemperatures with permittivity obeying the Curie-Weiss law, ε⊥ = C/(T − TC), where C = 1.7 × 105K andTC = 380 K, whereas the response parallel to it shows no softening. Below the Burns temperature, ∼620 K, agigahertz relaxation ascribed to flipping of the PNDs emerges from the soft-mode response, slows down, andbroadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations ofthe PND boundaries.
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