Broad-band dielectric spectroscopy and ferroelectric soft-mode response in theBa0.6Sr0.4TiO3 solid solution

Abstract

Ceramic Ba0.6Sr0.4TiO3 (BST-0.6) samples were studied in the broad spectral range of106–1014 Hz by using several dielectric techniques in between 20 and 800 K. The dominant dielectricdispersion mechanism in the paraelectric phase was shown to be of strongly anharmonicsoft-phonon origin. The whole soft-mode response in the vicinity of the ferroelectrictransition was shown to consist of two coupled overdamped THz excitations, whichshow classical features of a coupled soft and central mode, known from manyferroelectric crystals with a dynamics near the displacive and order–disorder crossover.Similar behaviour has been recently revealed and theoretically simulated in pureBaTiO3 (see Ponomareva et al 2008 Phys. Rev. B 77 012102 and Hlinka et al 2008 Phys. Rev. Lett. 101167402). Also for the BST system, this feature was confirmed by the theory based onmolecular dynamics simulations with an effective first-principles Hamiltonian. In all theferroelectric phases, additional relaxation dispersion appeared in the GHz range, assignedto ferroelectric domain-wall dynamics. The microwave losses were analysed from the pointof view of applications. The paraelectric losses above 1 GHz are comparable with those insingle crystals and appear to be of intrinsic multi-phonon origin. The ceramic BSTsystem is therefore well suited for applications in the whole microwave range.