Abstract-Infrared, terahertz, and microwave spectroscopy of the soft and central modes in Pb ( M g 1 / 3 N b 2 / 3 ) O 3

D. Nuzhnyy, J. Petzelt, V. Bovtun, M. Kempa, S. Kamba, J. Hlinka,  B. Hehlen

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.174113

From the new infrared (IR) reflectivity and time-domain terahertz spectra combined with available high-frequency dielectric data above the megahertz range in a broad temperature range of 10 to 900 K, a full picture of the soft- and central-mode behavior in the classical relaxor ferroelectric $\mathrm{Pb}\left(M{g}_{1/3}N{b}_{2/3}\right)O_3$ (PMN) is suggested. 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 neutron scattering. It is revealed that each type of experiment provides slightly different data. 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 $T^{\ast }\approx 400K$. In addition to evaluation of the IR-terahertz data using fitting with the 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 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 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, ${\varepsilon }_{\perp }=C/(T-T_C)$, where $C=1.7\times {10}^{5}K$ and $T_C=380K$, whereas the response parallel to it shows no softening. Below the Burns temperature, $\sim 620K$, a gigahertz 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.

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