Structure evolution mechanism of Na0.5Bi2.5Nb2-x Wx O9+δ ferroelectric ceramics: Temperature-dependent optical evidence and first-principles calculations

The effects of W doping on the structure and optical properties of Aurivillius ferroelectric Na0.5Bi2.5Nb2O9-based ceramics have been investigated by x-ray diffraction, far-infrared reflectance, and spectroscopic ellipsometry (SE). Moreover, the temperature dependent lattice dynamics and electronic properties have been systemically explored by Raman scattering (77-800 K) and SE (200-800 K), respectively. The anomalous temperature dependence of phonon frequencies around 600 K can be ascribed to the existence of an intermediate phase between ferroelectric orthorhombic and paraelectric tetragonal phases, which is also confirmed by temperature dependent electronic transitions extracted by the SE technique. In addition, the intrinsic evolution mechanism of the structure and electronic transitions during the phase transitions has been clarified by first-principles calculations, which indicate the effective application of noncontact optical methods in judging the phase transformations of ferroelectric materials.