Zeches et al. (Science 2009) discovered that large compressive epitaxial strain stabilizes a tetragonal-like polymorph of the multiferroic perovskite BiFeO3 that coexists with the rhombohedral-like ground-state phase in nanoscale stripes, mimicking the morphotropic phase boundary (MPB) that gives lead-based piezoelectrics their giant electromechanical response, and that the two phases can be reversibly interconverted by a local electric field. The paper framed a three-part program that remains open: (i) whether strain-driven MPB formation is a generic route in perovskites other than BiFeO3; (ii) whether the strain-driven switching mechanism is viable at the device level (macroscopic piezoelectric coefficients, endurance, retention, switching speed, scalability for probe-based storage or actuators); and (iii) the thermodynamic and kinetic limits of the strain-induced two-phase region. Later literature has reproduced the mixed phase extensively within the BiFeO3 family, built a largely complete strain-temperature-thickness phase diagram, and demonstrated 5-14% reversible field-induced strains, but no strain-driven MPB has been demonstrated in another single-composition perovskite, and no device-level qualification of the switching mechanism exists.

Background

Piezoelectric materials interconvert electrical and mechanical energy, and the highest electromechanical couplings arise in ferroelectric solid solutions positioned at a morphotropic phase boundary (MPB): a nearly composition-independent boundary across which two crystallographic polymorphs, typically rhombohedral and tetragonal, are nearly degenerate and can be interconverted by an electric field, producing giant strains. The technologically dominant MPB systems, Pb(Zr,Ti)O3 (PZT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), reach this near-degeneracy by chemical alloying and therefore contain lead. A perovskite is an ABO3 oxide with corner-sharing BO6 octahedra; BiFeO3 (BFO) is a single-composition perovskite multiferroic (simultaneously ferroelectric and antiferromagnetic) whose bulk ground state is a rhombohedral R3c distortion of the perovskite cell (the R3c space group: a rhombohedral cell in which the oxygen octahedra are uniformly rotated about the polarization axis) with a large spontaneous polarization. Epitaxial strain denotes the biaxial in-plane deformation imposed on a thin film by coherent lattice matching to a single-crystal substrate; by choosing substrates with a smaller in-plane lattice parameter one can impose several percent of compressive strain. First-principles calculations had predicted an alternative super-tetragonal polymorph of BFO, and Zeches et al. (Science 326, 977, 2009) showed experimentally that compressive epitaxial strain (about 4-5%, achieved on LaAlO3 and YAlO3 substrates) stabilizes a long-axis tetragonal-like polymorph (out-of-plane lattice parameter c of roughly 4.65 Angstrom, c/a ratio about 1.27; later refined as a monoclinic distortion of the tetragonal structure), that films at intermediate strain exhibit coexistence of this phase with the rhombohedral-like phase in ordered 30-50 nm stripes with 1-2 nm surface height contrast, and that a scanning-probe-applied electric field reversibly converts the mixed phase to a tetragonal-like single phase with nonvolatile nanometer-scale surface displacement, of the scale required for atomic-force-microscope (AFM) probe-based data storage and micro-actuators. A morphotropic phase boundary created by strain rather than by composition would provide a lead-free, chemically simple route to MPB functionality. The 2009 demonstration was, however, purely local: switching was performed quasi-statically with an AFM tip (observed by AFM topography and piezoresponse force microscopy, PFM, which measures surface displacement induced by an AC voltage), on a single film thickness, with no data on endurance, retention, or speed, and the substrate-dependent phase map was qualitative. In the classical MPB context, Khachaturyan (Philos. Mag. 90, 37, 2010) developed the adaptive-phase theory, in which the macroscopic symmetry at an MPB is produced by fine nanoscale textures of the endpoint phases rather than by a homogeneous bridging phase; this framework became the standard reference point for interpreting whether strain-induced mixed phases are true thermodynamic coexistence or nanotextured adaptive states.

Problem Statement

In the framework established by Zeches et al. (Science 2009) for the strain-driven morphotropic phase boundary in epitaxial BiFeO3 films, determine whether epitaxial strain is a generic, reproducible route to MPB-like functionality in single-composition perovskites, and establish the conditions under which that functionality is physically stable and practically usable. Concretely: (i) determine whether strain-driven MPB formation is generic across single-composition perovskites, either by establishing it in at least two distinct perovskite compositions other than BiFeO3 (and its strained or doped variants) in which several percent of biaxial epitaxial strain produces coexistence of two distinct polymorphs with near-degenerate free energies and reversible electric-field-driven interconversion, or by a class-level criterion for whether such strain-driven MPB formation is expected across single-composition perovskites or is instead peculiar to BiFeO3 (equivalently, impossible for other single-composition perovskites); in addition, establish whether the resulting electromechanical response is intrinsic to the two-phase thermodynamics or an adaptive nanotexture/phase-boundary-motion effect; (ii) establish the device-level viability of the strain-driven switching mechanism, that is, measure macroscopic piezoelectric coefficients and quantify endurance under repeated cycling, nonvolatile retention, switching speed, and scalability to the film thicknesses and device geometries required for nanoscale probe-based data storage or micro-actuation; and (iii) establish the kinetic and stability limits of the strain-induced two-phase region, namely the long-term stability of the mixed phase and the dynamics of interphase-boundary motion under repeated electrical cycling, since the strain-temperature-thickness phase diagram has already been substantially advanced by later work (Siemons et al. 2013; Yang et al. 2016; Y. Zhang et al. 2020).

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