A temperature-composition phase diagram and the phase transition behaviour, especially DPT(diffused phase transition) behavior, in pseudobinary $(1-x)Pb(Fe_{\frac{1}{2}}Nb_{\frac{1}{2}})O_3-xPb(Mg_{\frac{1}{2}}W_{\frac{1}{2}})O_3$ system are investigated.
Specimens were prepared by the conventional mixed-oxide technique; raw-materials powders were mixed by wet-ball milling in a slurry state and calcined at 850℃ for 2 hours, and then compacted into the disc shape under 7000 psi pressure. Compacts were heated at the rate of about 500℃/hour and sintered for 1 hour under $O_2$ atmosphere at various temperatures.
Room temperature X-ray diffraction patterns of annealed sintered compacts show that a perfect solid solution with a perovskite structure is formed in the entire composition range and the degree of ordering at B-sites increases with increasing composition x.
A phase analysis of sintered specimens were conducted by measuring its dielectric properties, E-P hysteresis, and DC resistivity. The character of the phase transition changes from the ferroelectric-paraelectric for the composition range 0.0≤x≤0.5 to the antiferroelectric-paraelectric for the composition range 0.6≤x≤1.0. The ferroelectric-paraelectric transition is relatively sharp for 0.0≤x≤0.1, but becomes diffuse for 0.2≤x≤0.5 ; the antiferroelectric-paraelectric is diffuse for 0.6≤x≤0.88, but becomes sharp for 0.9≤x≤1.0.
For the composition range 0.2≤x≤0.88, DPT behavior is displayed by electrical polarization, piezoelectric characteristics, and frequency dependence of dielectric constants. The broadness σ of permittivity-temperature curves increases with increasing x for x≤0.6, reaches the maximum value at x=0.6, and then decreases with increasing x for 0.6≤x≤1.0. Because the broadness σ is observed to be independent of the ordering at B-sites, the local disorder is likely to have little effect on the DPT phenomena.
Based on the dielectric behaviors of the specimens heat-treated under DC bias and the thermal analysis data obtained by differential scanning calorimetry, the fluctuation responsible for the transition is considered dominantly to be polarization fluctuation ascribed to the small thermodynamic potential difference between the high and low temperature phases near the transition temperature. The critical stability factor $ξ_3c$ defined by Fritzberg's polarization fluctuation model between the sharp and diffused phase transition is found to be about $1.95×10^7$ (v/m).
The experimental evidences suggest that the DPT phenomena are affected by the local disorder or the compositional fluctuation only to a small extent, but they are largely affected by the polarization fluctuation. The sharp transition occurs for the stability factor $ξ_3＞ξ_3c$, where the diffuse transition occurs for $ξ_3≤ξ_3c$.
