Canonical-glass-like behavior of the polycrystalline relaxor ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbZrO3:: Heat-capacity study

A solid solution of lead magnesium niobate (PMN), a relaxor ferroelectric, with lead zirconate (PZ), an antiferroelectric, gives rise to a system that behaves like a relaxor ferroelectric for lower concentrations of PZ, and like a normal ferroelectric above 50% substitution by PZ. This paper reports the heat-capacity behavior of (1 - x)PMN-xPZ for the composition range x = 0.30 to 0.95 and. temperature range 300-600 K. It was observed that, although the atomic structure of the material is basically crystalline throughout, with sharp x-ray diffraction peaks, the crossover from normal-ferroelectric behavior to relaxor-ferroelectric, behavior (on decreasing x) is accompanied by a matching crossover from crystalline behavior to glassy behavior, as exhibited in the heat-capacity plots. In other words, the heat-capacity curves for the relaxor compositions bear resemblance to those observed for canonical or conventional glasses, with the glass-transition temperature and the continuous step in specific heat changing gradually as a function of the composition parameter x. However, not all properties match those for canonical glasses. For example, soaking for 24 h at a temperature or 10 to 20 K below the mean glass-transition temperature does not raise the specific heat to a value nearly equal to the value in the unfrozen state. Similarly, the glass-transition temperature (for 0.7PMN-0.3PZ) increases, though only marginally (from 337 K to 343 K), when the rate of heating across the transition is increased by a factor of 50 (from 0.1 K per minute to 5 K per min.). Further, the temperature interval AT over which most of the glass transition occurs in the relaxor ferroelectric is typically as large as 30-40 K, compared to only about 10 K for canonical glasses.