Phonon-coupled impurity dielectric modes in Sr1-1.5xBixTiO3

SrTiO3 (ST) doped with Bi shows three Bi-induced dielectric modes (modes A, B, and C) that are superimposed upon the quantum paraelectric behavior of pure ST. In ST doped with low concentration Bi, the modes A and B exhibit dielectric behavior arising from noninteracting polar clusters. The evidence of the dielectric modes coupling with optical phonons is observed from Raman spectra. The dielectric steps induced by Bi doping can be well described by the modified Debye-Langevin model involving the correlation effect of Bi ions via interaction with the optical phonons of the host lattice. With higher Bi concentration, modes A and B merge into a ferroelectric-relaxor mode, mode C, showing an evolution process towards ferroelectric relaxer.