Effect of A-site La and Ba doping on threshold field and characteristic temperatures of PbSc0.5Ta0.5O3 relaxor studied by acoustic emission

The structural transitions in Pb_1-xLa_xSc_(1+x)/2Ta_(1-x)/2O₃, x = 0.08 (PLST) relaxor crystals were studied by means of acoustic emission (AE) under an external electric field (E) and compared with those observed in pure PbSc_0.5Ta_0.5O₃ (PST) and Pb_0.78Ba_0.22Sc_0.5Ta_0.5O₃ (PBST) [E. Dul'kin etal, EPL 94, 57002 (2011)]. Similar to both the PST and PBST compounds, in zero field PLST exhibits AE corresponding to a para-to-antiferroelectric incommensurate phase transition at T_n = 276 K, lying in the vicinity of dielectric temperature maximum (T_m). This AE signal exhibits a nontrivial behavior when applying E resembling the electric-field-dependence of T_n previously observed for both the PST and PBST, namely, T_n initially decreases with the increase of E, attains a minimum at a threshold field E_th = 0.5 kV/cm, accompanied by a pronounced maximum of the AE count rate N˙ = 12 s^-1, and then starts increasing as E enhances. The similarities and difference between PST, PLST, and PBST with respect to T_n, E_th, and N˙ are discussed from the viewpoint of three mechanisms: (i) chemically induced random local electric field due to the extra charge on the A-site ion, (ii) disturbance of the system of stereochemically active lone-pair electrons of Pb²⁺ by the isotropic outermost electron shell of substituting ion, and (iii) change in the tolerance factor and elastic field to the larger ionic radius of the substituting A-site ion due to the different radius of the substituting ion. The first two mechanisms influence the actual values of T_n and E_th, whereas the latter is shown to affect the ormalized N˙, indicating the fractions undergoing a field-induced crossover from a modulated antiferroelectric to a ferroelectric state. Creation of secondary random electric field, caused by doping-induced A-site-O ionic chemical bonding, is discussed.