A comparative dielectric relaxation study of PMN-PT and PMN-PZ ceramics using impedance spectroscopy

AC-impedance spectroscopic studies in the temperature range of 30-400 °C are carried out on solid solutions of lead magnesium niobate (PMN) with lead titanate (PT) and lead zirconate (PZ), both of them in the 65/35 atomic ratio. For PMN-PT this corresponds to the morphotropic phase boundary composition (with normal ferroelectric behaviour), and for PMN-PZ it is near the phase boundary between normal ferroelectric and relaxor ferroelectric compositions. The variation of dielectric permittivity with temperature at different frequencies shows normal ferroelectric and relaxor-like dependence for PMN-PT and PMN-PZ, respectively. Temperature-dependent spectroscopic modulus plots reveal a much broader peak for PMN-PZ compared to that for PMN-PT, which is consistent with the dielectric behaviour. PMN-PT shows nearly ideal Debye behaviour below Tm (the temperature of the permittivity maximum) and the behaviour departs from ideality above Tm, whereas non-ideal Debye behaviour is seen both below and above Tm for PMN-PZ. Complex modulus plots fit well with two depressed semicircles and three depressed semicircles, respectively, for PMN-PT and PMN-PZ. The relaxation observed in the spectroscopic plots around 1 MHz for PMN-PT has been assigned to polarisation relaxation expected for normal-sized domains. No such relaxation could be observed for PMN-PZ around 1 MHz because of the mesoscopic domain sizes.