Bandgap behavior and singularity of the domain-induced light scattering through the pressure-induced ferroelectric transition in relaxor ferroelectric AxBa₁-xNb₂O₆ (A: Sr,Ca)

Abstract

In this letter, we have investigated the electronic structure of AxBa₁−xNb₂O₆ relaxor ferroelectrics on the basis of optical absorption spectroscopy in unpoled single crystals with A = Sr and Ca under high pressure. The direct character of the fundamental transition could be established by fitting the Urbach’s rule to the photon energy dependence of the absorption edge yielding band gaps of 3.44(1) eV and 3.57(1) eV for A = Sr and Ca, respectively. The light scattering by ferroelectric domains in the pre-edge spectral range has been studied as a function of composition and pressure. After confirming with x-ray diffraction the occurrence of the previously observed ferroelectric to paraelelectric phase transition at 4 GPa, the light scattering produced by micro and nano ferroelectric domains at 3.3 eV in Ca₀.₂₈Ba₀.₇₂Nb₂O₆ has been probed. The direct band gap remains virtually constant under compression with a drop of only 0.01 eV around the phase transition. Interestingly, we have also found that light scattering by the polar nanoregions in the paraelectric phase is comparable to the dispersion due to ferroelectric microdomains in the ferroelectric state. Finally we have obtained that the bulk modulus of the ferroelectric phase of Ca₀.₂₈Ba₀.₇₂Nb₂O₆ is B₀ = 222(9) GPa.