Frequency dependence of the characteristic temperatures in PbSc _0.5Ta_0.36Nb_0.14O₃ relaxor ferroelectrics crystals seen via acoustic emission

PbSc_0.5Ta_0.36Nb_0.14O₃ relaxor ferroelectrics crystals were investigated in a wide temperature range of 200-700 K using acoustic emission. The intermediate temperature T* as well as the Burns temperature T_d were successfully detected at 480 K and 581 K, respectively. Another acoustic emission burst was detected at Tnl ∼ 211 K, which is below the temperature of the dielectric-permittivity maximum T _m ∼ 257 K but it is accompanied by a slight anomaly in the dielectric permittivity. Tnl is attributed to the low-temperature boundary of an incommensurately modulated antiferroelectric phase transition taking place over a temperature range. All characteristic temperatures exhibit frequency dispersion in the range of 0.1-10 kHz, which is considerably stronger for T* and T_d than for T_m and Tnl. In addition, Tnl depends linearly on frequency, whereas both T* and T_d resemble the non-linear frequency dependence of T_m typical of relaxors. The nature of the frequency dispersion of T* and T_d is discussed from the viewpoint of complex behavior of low-energy polar modes intrinsic of relaxor ferroelectrics.