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We present here results of high resolution synchrotron x-ray diffraction(SXRD) and dielectric studies in conjunction with Landau theory considerationson (Pb0.94Sr0.06)(ZrxTi1-x)O3 compositions in the vicinity of the morphotropicphase boundary (MPB) to find evidence for the flattening of the free energysurface at the MPB proposed in recent ab-initio studies on stronglypiezoelectric ceramics. SXRD results reveal that the tetragonal andpseudorhombohedral monoclinic compositions with x=0.515 and 0.550 transformdirectly into the cubic paraelectric phase, whereas for 0.520<x<0.545, thepseudotetragonal as well as pseudorhombohedral monoclinic compositions firsttransform to the tetragonal phase and then to the cubic phase. Our resultsreveal the existence of a triple point at x~0.550. It is shown that thetetragonal to cubic transition, irrespective of the composition, upto x~0.545is accompanied with a discontinuous change in the unit cell volume and thermalhysteresis confirming first order nature of this transition. Thepseudorhombohedral monoclinic composition for x=0.550, on the other hand,transforms directly into the cubic phase in a second order manner. Ourexperimental results thus reveal a crossover from first order to second orderphase transition through a tricritical point around x=0.550. Landau theorycalculations also confirm gradual flattening of the free energy surface onapproaching the tricritical composition x=0.550. We conclude that the triplepoint in the PZT phase diagram is indeed a tricritical point.

physical review. b, condensed matter and materials physics

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