Soft mode driven local ferroelectric transition in lead-based relaxors

Abstract

Recently, we have analyzed the complex infrared (IR) and terahertz (THz) response of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) and suggested a new interpretation of the so-called intermediate temperature T∗ ≈ 400 K, where the split soft phonon mode undergoes incomplete softening. IR-THz reflectivity was fitted using effective medium approximation, which describes the uniaxially anisotropic response of polar nanodomains (PNDs). It indicated that the response perpendicular to the local polarization undergoes a classical Cochran softening toward T∗ with the extrapolated low-frequency permittivity obeying the Curie-Weiss (CW) law with TC ≈ T∗. In this letter, we report analysis performed in a similar way on the published IR-THz response in closely related relaxor Pb(Mg1/3Ta2/3)O3 and have revealed a similar behavior: The lowest-frequency phonon polarized perpendicularly to the local polarization of PNDs softens below ∼900 K according to the Cochran law and the corresponding low-frequency permittivity obeys the CW law with TC = 340 K ≈ T∗ and Curie constant C = 1.16 × 105 K. This is comparable to similar parameters for PMN, TC = 380 K ≈ T∗ and C = 1.7 × 105 K. This explains the high-temperature CW anomaly in the low-frequency dielectric response of both materials and strongly suggests that both relaxors undergo a local ferroelectric phase transition near T∗ which induces a new polarization component perpendicular to the local random electric field.