Polymorphism and penferroelectricity in PLZT ceramics

The interaction of structure with optical and electrical properties in lanthanum-doped lead zirconate-titanate (PLZT) ceramics is complex. An interpretation is presented which shows the critical dependence of structure and, hence, properties on the state of polarization, the temperature, and the La content. To explain the behavior of PLZT ceramics two new concepts are presented: polymorphism and penferroelectricity. Polymorphism occurs when either electrical or mechanical ordering fields generate a hybrid crystal structure in which an individual grain attains a crystal structure dependent on its orientation with respect to the field. Penferro-electricity refers to the condition in which material is noncubic and polar but with such a low spontaneous polarization that no domains exist. The justification for applying these concepts to PLZT rests on the existing structural, optical, and dielectric data supplemented by new X-ray evidence and new data on the temperature dependence of optical and electrical properties. Emphasis is placed on materials with 65/35 Zr/Ti ratio and 6 < X ≤ 8 where X is the atom % La substituted for Pb. Analysis of existing data disclosed 1) a lack of X-ray evidence for the morphotropic boundary predicted by average remanent birefrigence and planar coupling and 2) a discrepancy in the position of the phase transition located by the Curie point and by planar coupling coefficient. The new X-ray, optical insertion loss, dielectric and piezo-electric data show conclusively that the phase equilibria in 65/35 PLZT depend strongly on the state of polarization. In thermally annealed ceramics a state of penferroelectricity exists between the cubic paraelectric state and the rhombohedral ferroelectric state. Rhombohedral ferroelectric material with 6 < X < 8 transforms to polymorphic material with very strong ferroelectric properties during poling. Because of the large crystal distortions which accompany poling, the optical transmission in the poled state is much reduced. This behavior is the basis for many of the useful device applications of PLZT in this compositional range.