Large activation energy in aged Mn-doped Sr0.4Ba0.6Nb2O6 ferroelectric ceramics

Abstract

The properties of ferroelectrics can be significantly enhanced through appropriate doping, and also can be strongly influenced by aging treatment. In this paper, we report a slow aging effect with a large activation energy and an enhanced energy storage density in the aged Mn-doped Sr_0.4Ba_0.6Nb₂O₆ ferroelectric ceramics. Mn-doped Sr_0.4Ba_0.6Nb₂O₆ (SBN40-xMn) (x = 0.0, 1.0, 2.0, 4.0, 6.0, 8.0 mol%) ferroelectric ceramics were prepared by the conventional solid state reaction method. The dielectric constant of Mn-doped Sr_0.4Ba_0.6Nb₂O₆ ceramics shows a decrease after aging in the ferroelectric state. The activation energy derived from the dielectric aging curves shows an obviously large value of 1.87 eV and 2.01 eV for SBN40-4Mn and SBN40-6Mn sample, respectively, compared with the value of 0.55 eV in SBN40-2Mn sample. Besides, the large activation energy is much higher than those values reported in previous literatures for perovskite ferroelectrics such as BaTiO₃ and Pb(Ti_0.42Zr_0.58)O₃ based system. The relaxation time (reflecting the aging rate) is also much longer than the cited perovskite systems. The large relaxation time and activation energy both indicate the slow aging process. Moreover, the aging is related to the migration of oxygen vacancies, and the existence of oxygen vacancies in the Sr_0.4Ba_0.6Nb₂O₆ ceramics is proved by the dielectric relaxation at high temperature and the XPS spectra investigation. Based on the longer relaxation time and larger activation energy, a different diffusion path of oxygen vacancy in our tetragonal tungsten bronze structure high Mn doped SBN is proposed: a long distance migration path of oxygen vacancies among neighbor oxygen octahedron, in contrasting to the short migration distance of oxygen vacancies within perovskite unit cells. In consequence of the long path oxygen vacancy diffusion, a typical double hysteresis loop appeared and the maximum energy storage density is achieved in aged SBN40-4Mn sample. These results can provide guidance for understanding the dynamic of aging process and improving the energy storage properties of doped ferroelectric materials with the tungsten bronze structure.