Supertetragonal Phase Research Articles

Origin of supertetragonality in BaTiO3

Understanding ferroelectricity is of both fundamental and technological importance to further stimulate the development of new materials designs and manipulations. In this respect, supertetragonal (ST) phases with high $c/a$ ratio of $\ensuremath{\sim}1.3$ have recently resulted in outstanding ferroelectric polarization values, which urges for a microscopic origin of these novel structural phases. Here, we perform an in-depth first-principle study on the well-known ferroelectric barium titanate ${\mathrm{BaTiO}}_{3}$ under a hydrostatic negative pressure, showing an isosymmetric phase transition to such a ST phase. The chemical origin and driving mechanisms of this phase transition are identified as a drastic change of the covalently $\ensuremath{\pi}$-bonded electrons. These findings provide guidance in the search for new ST phases, with great opportunities for novel multiferroic materials, and can be generalized in the understanding of other isosymmetric phase transitions.

Ca Solubility in a BiFeO3-Based System with a Secondary Bi2O3 Phase on a Nanoscale.

In BiFeO3 (BFO), Bi2O3 (BO) is a known secondary phase, which can appear under certain growth conditions. However, BO is not just an unwanted parasitic phase but can be used to create the super-tetragonal BFO phase in films on substrates, which would otherwise grow in the regular rhombohedral phase (R-phase). The super-tetragonal BFO phase has the advantage of a much larger ferroelectric polarization of 130–150 μC/cm2, which is around 1.5 times the value of the rhombohedral phase with 80–100 μC/cm2. Here, we report that the solubility of Ca, which is a common dopant of bismuth ferrite materials to tune their properties, is significantly lower in the secondary BO phase than in the observed R-phase BFO. Starting from the film growth, this leads to completely different Ca concentrations in the two phases. We show this with advanced analytical transmission electron microscopy techniques and confirm the experimental results with density functional theory (DFT) calculations. At the film’s fabrication temperature, caused by different solubilities, about 50 times higher Ca concentration is expected in the BFO phase than in the secondary one. Depending on the cooling rate after fabrication, this can further increase since a larger Ca concentration difference is expected at lower temperatures. When fabricating functional devices using Ca doping and the secondary BO phase, the difference in solubility must be considered because, depending on the ratio of the BO phase, the Ca concentration in the BFO phase can become much higher than intended. This can be critical for the intended device functionality because the Ca concentration strongly influences and modifies the BFO properties.

Evidence for pressure-induced 'morphotropic like' phase transition with concomitant octahedral tilting and emergence of a 're-entrant ferroelectric phase' in super-tetragonal phase of 0.5BiFeO3–0.5PbTiO3 solid solution system

Evidence for pressure-induced 'morphotropic like' phase transition with concomitant octahedral tilting and emergence of a 're-entrant ferroelectric phase' in super-tetragonal phase of 0.5BiFeO₃–0.5PbTiO₃ solid solution system

Properties of stress-induced super tetragonal phase in epitaxial BiFeO3 thin film

Stress-induced structural phase transitions in multiferroic BiFeO3 thin films have attracted much attention for both fundamental research and promising applications in electronic devices. In this work, with the assistance of spherical aberration-corrected transmission electron microscopy, a tetragonal phase with the super large tetragonality (c/a ∼ 1.53) was found in the epitaxial BiFeO3 thin film on a LaAlO3/Si substrate. Analyses of atom positions showed that displacements of an Fe atom in this super tetragonal phase are much smaller than that in a regular tetragonal phase. First-principles calculations revealed that the structural phase transition occurs with the increasing lattice tetragonality c/a, accompanied by transitions in the atomic coordination and magnetic order.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe_{1-x}Co_{x}O_{3} Solid Solutions near Room Temperature.

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe_{1-x}Co_{x}O_{3} solid solutions with 0.25≤x≤0.50 (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1-1.0GPa, depending on the composition, the electric polarization of bulk BFCO increases by ∼150%. The origin of such an exceptional behavior is a phase transformation involving a low-T rhombohedral (R) phase and a high-T supertetragonal (T) phase. Both R and T phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 μ_{B} per formula unit. Contrary to what occurs in either bulk BiFeO_{3} or BiCoO_{3}, the T phase is stabilized over the R by increasing temperature due to its higher vibrational entropy. This extraordinary T-induced R→T phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.

Enhanced transient negative capacitance during inhomogeneous ferroelectric switching

The reversal of polarization in a ferroelectric material involves overcoming an energy barrier, and has been previously proposed and found to yield transient negative capacitance (NC) in the intermediate states of the switching process. Homogeneous switching was assumed to interpret the experimental results of NC; however, inhomogeneous switching is a more abundant mechanism than homogeneous switching, but its possible effect on NC is basically unknown. Here, we use first-principles-based techniques to investigate the occurrence of NC during these two types of switching processes in the supertetragonal phase of ${\mathrm{BiFeO}}_{3}$. We find NC in both cases, but with the magnitude of the inverse of the capacitance being drastically larger in the inhomogeneous (nucleation limited) switching, as compared to the homogeneous switching. We further analyze the origin of the different NC effects, and point to the different energetic trajectories between these two representative switching mechanisms.

Influence of BiFeO3 crystallographic phase on the enhanced interfacial magnetization in BiFeO3/La2/3Sr1/3MnO3 heterostructures

Large induced magnetization is observed in rhombohedral-like BiFeO3 (RBFO) when grown on a ferromagnetic La2/3Sr1/3MnO3 layer. This magnetization survives up to room temperature and is strongly influenced by the deposition conditions and crystallographic phase of BiFeO3. The magnetization in RBFO is induced by the Fe–O–Mn ferromagnetic (FM) superexchange (SE) interaction, after a strong hybridization between Fe and Mn orbitals mediate through oxygen orbitals, which enhances the spin canting in a BFO layer near the interface. The reduction in the magnetization of BFO due to change in its growth conditions is caused by the altered oxygen content in the interfacial region, which has a direct impact on the FM SE Fe–O–Mn coupling at the interface. In marked contrast, the induced magnetization in the super-tetragonal phase (TBFO) of BFO, in which the canting of Fe spins is forbidden by the symmetry, shows a comparatively very small moment, which vanishes below the room temperature (∼200 K). We propose that the ferromagnetic ordering induced in TBFO is limited to the adjoined section only, whereas it extended to a certain degree of bulk in RBFO. A spin-glass like state is also observed in BiFeO3/La2/3Sr1/3MnO3 heterostructure with irreversible temperature ∼100 K.

Strain Engineering of Epitaxial Oxide Heterostructures Beyond Substrate Limitations

Epitaxial strain, imparted by an underlying substrate, is a powerful pathway to drive phase transitions and dramatically alter properties in oxide heterostructures, enabling the emergence of new ground states and the enhancement of ferroelectricity, piezoelectricity, superconductivity and ferromagnetism. However, the limitation of commercially available single-crystal substrates and the lack of continuous strain tunability preclude the ability to take full advantage of strain engineering for further exploring novel properties and exhaustively studying fundamental physics in complex oxides. Here we report an approach for imposing continuously tunable, large epitaxial strain in oxide heterostructures beyond substrate limitations by inserting an interface layer through tailoring its gradual strain relaxation. Taking BiFeO3 as a model system, we demonstrate that the introduction of an ultrathin interface layer allows the creation of a desired strain that can induce phase transition and stabilize a new metastable super-tetragonal phase as well as morphotropic phase boundaries overcoming substrate limitations. Furthermore, continuously tunable strain from tension to compression can be generated by precisely adjusting the thickness of the interface layer, leading to the first achievement of continuous O-R-T phase transition in BiFeO3 on a single substrate. This proposed route could be extended to other oxide heterostructures, providing a platform for creating exotic phases and emergent phenomena.

Polymorphism in Bi-based perovskite oxides: A first-principles study

Under normal conditions, bulk crystals of BiScO$_3$ , BiCrO$_3$, BiMnO$_3$, BiFeO$_3$, and BiCoO$_3$ present three very different variations of the perovskite structure: an antipolar phase, a rhombohedral phase with a large polarization along the space diagonal of the pseudocubic unit cell, and a supertetragonal phase with even larger polarization. With the aim of understanding the causes for this variety, we have used a genetic algorithm to search for minima in the surface energy of these materials. Our results show that the number of these minima is very large when compared to that of typical ferroelectric perovskites like BaTiO$_3$ and PbTiO$_3$ , and that a fine energy balance between them results in the large structural differences seen. As byproducts of our search we have identified charge-ordering structures with low energy in BiMnO$_3$ , and several phases with energies that are similar to that of the ground state of BiCrO$_3$. We have also found that a inverse supertetragonal phase exists in bulk, likely to be favored in films epitaxially grown at large values of tensile misfit strain.

Designer defect stabilization of the super tetragonal phase in >70-nm-thick BiFeO3 films on LaAlO3 substrates

The stability of the “T-like” (T′) phase in BiFeO3 films grown on LaAlO3(001) is investigated. We show that the T′ phase can be stabilized for thicknesses >70 nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600 °C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale “defect pockets”, which apply a local compressive strain of ∼1.8%. We propose that the cumulative effect of local stresses induced by such “designer defects” maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T′ phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness.

Temperature controlled evolution of monoclinic to super-tetragonal phase of epitaxial BiFeO3 thin films on La0.67Sr0.33MnO3 buffered SrTiO3 substrate

Epitaxial BiFeO3 thin films of 130nm were deposited by pulsed laser deposition (PLD) technique on La0.67Sr0.33MnO3 buffered SrTiO3 (001) substrate at various temperatures under different ambient oxygen pressures. Reciprocal space mapping reveals that, with decreasing temperature and oxygen pressure, the broadly reported monoclinic phase (MA) of BiFeO3 thin film initially transforms to a tetragonal phase (T1) with c/a =1.05 (1) in a narrow girth of deposition condition and then to a super-tetragonal phase (T2) with giant c/a = 1.24 (1), as confirmed by reciprocal space mapping using high resolution x-ray diffraction. The surface morphology of the films reveals the island growth of the BiFeO3 films deposited at low temperatures. We propose that the transformation from monoclinic to the super-tetragonal phase is essentially due to the manifestation of excess local strain as a result of the island growth. This study offers a recipe to grow the super-tetragonal phase of BiFeO3, with giant c/a =1.24 (1) which exhibits exceptionally large ferroelectric polarization, on ferromagnetic layer La0.67Sr0.33MnO3. This phase of BiFeO3 can be utilized for the ferroelectric control of magnetism at the interface of BiFeO3 and La0.67Sr0.33MnO3.

Intrinsic polarization switching mechanisms inBiFeO3

A first-principles-based effective Hamiltonian technique is used to investigate the polarization switching mechanisms in two polymorphic phases of ${\mathrm{BiFeO}}_{3}$ having no defects. The switching mechanism is homogeneous for any switching field in the rhombohedral phase, while in the supertetragonal phase it changes from the classical nucleation and domain-wall motion to nucleation-limited switching with virtually no propagation, and then to homogeneous switching with increasing electric field. The first two inhomogeneous switching mechanisms of the supertetragonal phase of ${\mathrm{BiFeO}}_{3}$ are thus intrinsic in nature, and can be well described by the classical and nucleation-limited switching models, respectively. The reason behind their absence in the rhombohedral phase is also indicated. Moreover, the field-induced changes of switching mechanism within the supertetragonal phase are further elucidated from an energetic point of view.

A multiferroic on the brink: Uncovering the nuances of strain-induced transitions in BiFeO3

Bismuth ferrite (BiFeO$_3$) is one of the very few known single-phase multiferroic materials. While the bulk compound is rhombohedral (R), the discovery of an epitaxial strain-induced structural transition into a so-called 'super tetragonal-phase' (T-phase) in this material incited a flurry of research activity focused on gaining an understanding of this phase transition and its possible functionalities. This metastable phase of BiFeO$_3$ is also multiferroic, with giant ferroelectric polarization and coexisting antiferromagnetic order, but above all it is the strain relaxation-induced phase mixtures and their outstanding piezoelectric and magnetoelectric responses which continue to intrigue and motivate the physicist and materials scientist communities. Here, we review the research into the T-phase and mixed-phase BiFeO$_3$ system. We begin with a brief summary of the history of the T-phase and an analysis of the structure of the various phases reported in the literature. We then address important questions regarding the symmetry and octahedral rotation patterns and the (as yet underexplored) important role of chemistry in the formation of the metastable T-phase. We follow by describing the phase transitions in this material, and how these may hold promise for large magnetoelectric responses. Finally we point out some experimental challenges inherent to the study of such a system, and potential pathways for how they may be overcome. It is our intention with this work to highlight important issues that, in our opinion, should be carefully considered by the community in order to use this fascinating materials system for a new paradigm of functionality.

Absence of Metallic Conductivity in Tetragonal and Cubic PbVO3 at High Pressure

Transport properties of PbVO3, a material whose structural and electronic properties bear similarities with high-temperature copper superconductors, were investigated between 2 and 300 K and in the pressure range of 0.1 MPa and 11.3 GPa. There is a structural phase transition from about 2 GPa at room temperature from a super-tetragonal phase to a cubic phase with a significant drop of resistivity by about 5 orders of magnitude. Nevertheless, the cubic phase exhibited a semiconducting behavior of resistivity between 2 and 300 K up to 11.3 GPa. The pressure dependence of lattice parameters of the tetragonal and cubic phases was in very good agreement with results of first-principle calculations from J. Phys.: Condens. Matter 24, 435403 (2012). Fitting with the Birch–Murnaghan equation of states gave a bulk modulus K0 = 38.0(1.1) GPa and a unit cell volume V0 = 67.63(6) A3 for the tetragonal phase and K0 = 179.5(1.4) GPa and V0 = 58.272(14) A3 for the cubic phase.

Strain Engineering Magnetic Frustration in Perovskite Oxide Thin Films

Our first-principles results show that geometric frustration can be induced in thin films of multiferroic BiFeO(3). We find that competing magnetic interactions occur in the so-called supertetragonal phase of this material, which can be grown on strongly compressive substrates. We show that the frustration level can be tuned by appropriately choosing the substrate; in fact, the phase diagram of the films presents a critical line at which the three-dimensional spin order gets annihilated. We argue that these effects are not exclusive to BiFeO(3) and predict that they also occur in BiCoO(3).

A phase transition close to room temperature in BiFeO3 thin films

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by using appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, display a reversible temperature-induced phase transition at about 100 °C, and thus close to room temperature.

Electrically controllable spontaneous magnetism in nanoscale mixed phase multiferroics

Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO₃. Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.