Evidence for a tricritical point coinciding with the triple point in (Pb0.94Sr0.06)(ZrxTi1-x)O3 : A combined synchrotron x-ray diffraction, dielectric and Landau theory study

We present here results of high resolution synchrotron x-ray diffraction (SXRD) and dielectric studies in conjunction with Landau theory considerations on (Pb0.94Sr0.06)(ZrxTi1-x)O3 compositions in the vicinity of the morphotropic phase boundary (MPB) to find evidence for the flattening of the free energy surface at the MPB proposed in recent ab-initio studies on strongly piezoelectric ceramics. SXRD results reveal that the tetragonal and pseudorhombohedral monoclinic compositions with x=0.515 and 0.550 transform directly into the cubic paraelectric phase, whereas for 0.520<x<0.545, the pseudotetragonal as well as pseudorhombohedral monoclinic compositions first transform to the tetragonal phase and then to the cubic phase. Our results reveal the existence of a triple point at x~0.550. It is shown that the tetragonal to cubic transition, irrespective of the composition, upto x~0.545 is accompanied with a discontinuous change in the unit cell volume and thermal hysteresis confirming first order nature of this transition. The pseudorhombohedral monoclinic composition for x=0.550, on the other hand, transforms directly into the cubic phase in a second order manner. Our experimental results thus reveal a crossover from first order to second order phase transition through a tricritical point around x=0.550. Landau theory calculations also confirm gradual flattening of the free energy surface on approaching the tricritical composition x=0.550. We conclude that the triple point in the PZT phase diagram is indeed a tricritical point.