Infrared, terahertz and microwave spectroscopy of the soft and central modes in PMN

Analysis of IR and THz spectra using Bruggeman effective medium approach revealed that the mesoscopic structure of Pb(Mg_1/3Nb_2/3)O₃ (PMN) consists of randomly oriented uniaxially anisotropic polar nanodomains with harder transverse optical polar modes in the direction along the local dipoles.

The lowest-frequency phonon of the E symmetry polarized perpendicular to the local dipole moments undergoes softening towards T* ≈ 400 K, which gives evidence about a local structural phase transition. This softening is also responsible for previously observed high temperature dependence of permittivity, which follows the Curie-Weiss law with the same critical temperature even without any anomaly in the low-frequency permittivity [D. Nuzhnyy et al, Phys. Rev. B 96, 174113 (2017)].

Figure: Temperature dependence of soft-mode components A₁ and E in PMN from our terahertz and IR spectra.