Scaling of conductivity spectra in the acceptor-doped ferroelectric SrBi2Ta2O9

The thermodynamic conductivity data σac(f,T) of the acceptor (Ca2+)-doped SrBi2Ta2O9, a fatigue-free ferroelectric, was subjected to a modified Jonscher's power law σ′(f)=σdc[1+(ffp)n+(ffq)m], and universal behavior. The scaling laws that describe the σac(f,T) of most glasses are found to be applicable also to this crystalline ferroelectric system up to its Curie temperature (TC320°C). Application of the time temperature superposition principle reveals that the scaling that indicates a common physical mechanism deviates from the ideal behavior at T<TC and at the "critical point" (T→TC). The scaling is significantly improved using the random barrier model due to noninteracting mobile charges, especially at T<TC. At T>TC, however, a deviation from the scaling to either model occurs. Interestingly, the value of the exponent m(=0.73) falls in the range of the three-dimensional disordered oxide and silicide glasses. It is believed that the collapse of long-range polar ordering at and above TC induces a hopping motion of the mobile charges, which is different from the one in the ferroelectric state.