Scaling of conductivity spectra in the acceptor-doped ferroelectric SrBi2Ta2O9 -: art. no. 024104

The thermodynamic conductivity data sigma(ac)(f,T) of the acceptor (Ca2+)-doped SrBi2Ta2O9, a fatigue-free ferroelectric, was subjected to a modified Jonscher's power law sigma(')(f)=sigma(dc)[1+(f/f(p))(n)+(f/f(q))(m)], and universal behavior. The scaling laws that describe the sigma(ac)(f,T) of most glasses are found to be applicable also to this crystalline ferroelectric system up to its Curie temperature (T-C approximate to 320 degrees 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 < T-C and at the "critical point" (T -> T-C). The scaling is significantly improved using the random barrier model due to noninteracting mobile charges, especially at T < T-C. At T > T-C, 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 T-C induces a hopping motion of the mobile charges, which is different from the one in the ferroelectric state.