A thermodynamic analysis is presented for the diffusionless phase diagrams of ferroelectric solid solutions that display a morphotropic phase boundary (MPB) separating adjacent tetragonal and rhombohedral phases. Equations are developed for the shape of the MPB, the locations of triple and tricritical points, and for the line along which the anisotropy of polarization vanishes. The appearance of lower symmetry orthorhombic and monoclinic phases is considered and the topologies of energy surfaces in the region of the phase diagram where these phases may stabilize are illustrated. The theory is applied to the solid solution of lead zirconate with lead titanate (PZT) and relationships between polar anisotropy and the transformation strain, dielectric susceptibility and piezoelectric properties, are discussed. The analysis is used to reproduce phase boundary lines for solid solutions of lead titanate with lead magnesium niobate (PMN‐PT) and lead zinc niobate (PZN‐PT) and composition–temperature diagrams along isopleths in the ternary system PMN‐PZT are estimated. The anisotropies of polarization in solid solutions based on lead titanate and barium titanate are contrasted. The results provide a thermodynamic framework useful for guiding experimental investigations of ferroelectric solid solutions and for generating energy functions used in constitutive modeling and phase field simulations of microstructure and properties.

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The electrothermal (electrocaloric and pyroelectric) properties of ferroelectric thin films have many applications in active solid-state cooling and infrared sensing devices. It has been shown experimentally that some thin-film ferroelectrics can produce much larger electrothermal responses than their bulk counterparts. In this work, the electrothermal properties of bulk polar dielectric (ferroelectric and incipient ferroelectric) materials and thin films have been computed using a thermodynamic methodology and the effects of electrical, thermal and mechanical boundary conditions have been illustrated. In particular, the sensitivity of pyroelectric and electrocaloric response to bias and driving fields, lateral clamping and misfit strain, thermal stresses and composition have been demonstrated. The computations show that the electrothermal behavior of ferroelectric materials for practical cooling devices depends on a complex interplay of several related sets of physical phenomena. These include the nature of the ferroelectric transition, the particular dependence of the equilibrium and transport properties on electric field and mechanical boundary constraints, and the orientation and thermal expansion coefficients of the thin film and substrate materials. The combined results provide insights concerning how the composition and orientation of the thin film material, the choice of substrate, the deposition/annealing temperature, and the electrode configuration can be used to optimize the electrothermal properties for particular applications.

A sub-hertz harmonic force was used to investigate the elastic response of ternary (1-y-x)Pb(In1/2Nb1/2)O3-yPb(Mg1/3Nb2/3)O3-xPbTiO3 (PIN-PMN-PT) single crystal with compositions (x = 0.30 and x = 0.33) near the Morphotropic Phase Boundary (MPB) under isothermal and zero-field conditions. The observed instability under mechanical compression is attributable to a ferroelectric rhombohedral FR-ferroelectric orthorhombic FO phase transformation. The associated strain is a function of proximity to MPB and is consistent with the prediction of the classical Devonshire theory. Isothermal resonance-antiresonance frequency sweeps versus temperature revealed a non-vertical MPB with (∂TRT/∂x)<0, where TRT is the rhombohedral-tetragonal transition temperature. These results provide insights into the role of PT content on the stability of PIN-PMN-PT crystals used for electromechanical devices and sound projectors.

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