A first-principles-derived scheme that incorporates ferroelectric and antiferrodistortive degrees of freedom is developed to study finite-temperature properties of $\mathrm{Pb}({\mathrm{Zr}}_{1-x}{\mathrm{Ti}}_x){\mathrm{O}}_3$ solid solution near its morphotropic phase boundary. The use of this numerical technique (i) resolves controversies about the monoclinic ground state for some Ti compositions, (ii) leads to the discovery of an overlooked phase, and (iii) yields three multiphase points that are each associated with four phases. Additional neutron diffraction measurements strongly support some of these predictions.

Ab initio simulations and experimental techniques are combined to reveal that, unlike what was commonly accepted for more than 30 years, perovskites and related materials enhance their ferroelectricity as hydrostatic pressure increases above a critical value. This unexpected high-pressure ferroelectricity is different in nature from conventional ferroelectricity because it is driven by an original electronic effect rather by long-range interactions.

We report a room-temperature high-pressure x-ray and Raman scattering investigation of lead titanate (${\mathrm{PbTiO}}_3$) up to 63 GPa. Three continuous phase transitions at 13, 20, and 45 GPa between tetragonal-like phases occur. As a result, no evidence is found for a pressure-induced morphotropic phase boundary. Our study provides experimental evidence that ${\mathrm{PbTiO}}_3$ presents a complex sequence of phases accommodating pressure through mechanisms involving oxygen octahedra tilting and reentrance of ferroelectricity.

Combining density-functional calculations and x-ray diffraction experiments, we show that ${\text{BiFeO}}_3$ epitaxially grown under compressive strain on cubic substrates evolves from the monoclinic $Cc$ phase (resulting from the strain-induced deformation of the ground-state rhombohedral $R3c$ phase) to the monoclinic $Cm$ phase with increasing misfit, the transition being at about $-4.5\mathrm{\%}/-5.5\mathrm{\%}$. Moreover, the polarization of the $Cc$ phase only rotates (instead of increasing) for misfit strain ranging from 0% to $-4\mathrm{\%}$, due to a strong coupling between polar displacements and oxygen octahedra tilts. This strong interaction is of interest for multiferroics where usually both structural degrees of freedom coexist.

The restored strain energy in $\mathrm{Pb}{\mathrm{Zn}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_3$ (PZN) and $9\%\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$-doped PZN (PZN-9%PT) relaxor crystals has been studied by means of acoustic emission (AE). Two types of AE activity signals have been recorded: (i) related to temperature- or electric-field-induced macroscopic phase transitions and (ii) associated with formation/disappearance of intrinsic polar nanoregions. Monitoring of AE under varying [001] electric fields has allowed a unique in situ observation of a low-field $(1\mathrm{kV}∕\mathrm{cm})$ irreversible orthorhombic-to-$M_C$ phase transition within the morphotropic phase boundary region of PZN-9%PT.

Strain engineering aims to take advantage of the stress field imposed by substrates on thin films. It requires an understanding of the consequences of stress fields on the physical properties of the deposited materials. This is achieved in ferroelectric thin films through the use of misfit-strain phase diagrams that show the stability regions for the possible phases. These encompass bulk phases as well as new ones exhibiting symmetries that are not present in the bulk. For the solid solution lead zirconate–lead titanate, Pb(Zr_1−x Ti _x )O₃, monoclinic phases found in the bulk morphotropic phase boundary region and associated to concentrations exhibiting the highest properties can be stabilized on a wider range of composition in thin films. In addition, phases of lower symmetry can be stabilized through the use of anisotropic biaxial stress fields, generated by orthorhombic substrates for example. Another crucial aspect of the influence of biaxial stress fields is the generation of domain structures. Theoretical tools as well as experimental verifications have provided much insight on the underlying physics. We, therefore, present here an overview of the influence of both iso- and anisotropic biaxial stress fields on the structures and properties of ferroelectric thin films exemplified on Pb(Zr_1−x Ti _x )O₃.