Antiferrodistortive Instabilities Research Articles

Understanding the correlation between intermediate monoclinic phase (Cc) and piezoelectric properties in NaNbO3-BaTiO3-CaZrO3 ternary system with octahedral tilt

Compared with traditional tetragonal (T) P4mm - rhombohedral (R) R3m morphotropic phase boundary (MPB) system, the structure-property relationship of MPB systems with octahedral tilt is still unclear. In this work, the phase structure evolution under composition and electric field modulations in lead-free (0.9-x)NaNbO3-0.1BaTiO3-xCaZrO3 (NN-BT-xCZ) close to the untilt P4mm-tilt R3c MPB was investigated in details by means of the ex- and in-situ x-ray diffraction and various electrical property measurements. Both P4 mm and R3c phases can irreversibly transform into a Cc phase under electric field loading, which should be considered as a result of the cooperative coupling between ferroelectric and antiferrodistortive instabilities. Quantitative analysis indicates that little composition dependent lattice strain and domain switching of P4 mm phase can be observed, and the variation of piezoelectric coefficient is closely related to Cc phase, which exhibits an enhancement of both lattice strain and domain switching within MPB. This suggests that lattice strain and domain switching of Cc phase are coupled. On the contrary, both of them are competed in P4 mm phase because the P4 mm phase is dominated by irreversible domain switching. The experimental results would benefit for deeply understanding the origin of piezoelectric response for many octahedral-tilt MPB piezoelectric systems.

Piezoelectric properties of II–IV/I–V and II–IV/III–III ferroelectric perovskite superlattices

The stability of high-symmetry P4mm polar phase in eleven ferroelectric perovskite superlattices with the polar discontinuity is studied from first principles. In most superlattices, this phase exhibits either the ferroelectric or the antiferrodistortive instability, or both of them. The structure of the ground state and, for a number of systems, also of metastable phases in these superlattices is found. The spontaneous polarization and piezoelectric properties of superlattices are calculated. The appearance of high piezoelectric coefficients (up to 150–270 pC/N) in some superlattices is associated with the strain-induced local rearrangement of certain atomic groups in the primitive cell.

Evidence for pressure-induced polarization rotation, octahedral tilting, and reentrant ferroelectric phase in tetragonal (Pb0.5Bi0.5)(Ti0.5Fe0.5)O3

Despite the technological significance of the tetragonal PbTiO3 for the piezoelectric transducer industry, its high pressure behaviour is quite controversial as two entirely different scenarios, involving pressure induced (1) morphotropic phase boundary (MPB) like structural transition with concomitant rotation of the ferroelectric polarization vector and (2) antiferrodistortive (AFD) phase transition followed by emergence of a reentrant ferroelectric phase, have been proposed in recent theoretical and experimental studies. We have attempted to address these controversies through a high resolution synchrotron x-ray diffraction study of pressure induced phase transitions in the tetragonal phase of a modified PbTiO3 composition containing 50% BiFeO3, where BiFeO3 was added to enhance the AFD instability of PbTiO3. We present here the first experimental evidence for the presence of the characteristic superlattice reflections due to an AFD transition at a moderate pressure pc1 ~2.15 GPa in broad agreement with scenario (2), but the high pressure ferroelectric phase belongs to the monoclinic space group Cc, and not the tetragonal space group I4cm predicted under scenario (2), which permits the rotation of the ferroelectric polarization vector as per scenario (1). We show that the monoclinic distortion angle and ferroelectric polarization of the Cc phase initially decrease with increasing pressure for p < 7 GPa, but start increasing above pc2 ~ 7 GPa due to an isostructural Cc-I to Cc-II transition reminiscent of MA(apc > bpc ~ cpc) to MB(apc< bpc ~ cpc) transition predicted for MPB systems. We also show that octahedral tilting provides an efficient mechanism for accommodating pressure induced volume reduction for the stabilisation of the reentrant ferroelectric phase Cc-II.

Triggered incommensurate transition in PbHfO3

We report a type of structural phase transitions in dielectric materials: the triggered incommensurate (IC) transition. We demonstrate evidence for such a transition in the perovskite antiferroelectric ${\mathrm{PbHfO}}_{3}$ by means of single-crystal x-ray diffraction, diffuse, and inelastic-scattering experiments, which we interpret using the Landau theory of phase transitions. This transition is not driven by an IC soft mode, as is the case for the majority of IC dielectrics, but by the soft mode associated with a different, coexisting order parameter related to antiferrodistortive (AFD) tilts of the oxygen octahedra. When cooling from the high-temperature cubic phase these two order parameters are established simultaneously and discontinuously at ${T}_{\text{IC}}\ensuremath{\approx}468$ K. Two lattice instabilities are present in the cubic phase: AFD instability with critical temperature ${T}_{0}\ensuremath{\approx}441$ K and ferroelectric instability with ${T}_{0}\ensuremath{\approx}408$ K. The IC instability is absent. The analysis of the transition mechanism within the Landau theory of coupled order parameters indicates that the transition is of triggered character, conditioned upon the attractive biquadratic coupling between the IC modulation and the AFD octahedral tilts, and is driven by the AFD soft mode.

Giant suppression of dielectric loss in BaZrO3

We report the effect of precursor's purity and heterovalent substitution on the microstructure and dielectric properties of BaZrO3 ceramics. We find that independent of the purity or raw materials, the dielectric loss of BaZrO3 at microwave frequencies is rather high with a Q factor in the range of 1000–3000 at 10 GHz. All stoichiometric ceramics studied show up to three types of low-T dielectric relaxations in the 2–250 K range. All these dielectric anomalies can be suppressed by partial substitution of Zr for Nb. By alloying of BaZrO3 with a hypothetical BaGa1/2Nb1/2O3 phase the Q × f value of ceramics at microwave frequency can be improved significantly. The origin of this remarkable improvement is attributed to the effect of the donor ions on the random electric fields and antiferrodistortive instability. Ceramic with a chemical composition of BaZr0.96Ga0.02Nb0.02O3 sintered at 1600 °C shows dielectric constant, ε′ = 36.7, temperature coefficient of the resonance frequency, τf = +110 ppm/°C and Q × f = 172 THz.

First-principles study of structural and vibrational properties of SrHfO3 compared to SrZrO3

The structural and vibrational properties of SrHfO3 compared to those of the SrZrO3 were investigated using first-principles calculations. The phonon dispersion curves, reported in the high-symmetry cubic perovskite phase, point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center. Different possible intermediate phases are characterized by comparing their internal energies, giving the same hierarchy of phases as for the SrZrO3. The main features of the ferroelectric instability are also discussed, and, contrary to what was found for SrZrO3, it seems very unlikely to induce the ferroelectricity in the SrHfO3Pnma ground state either by compressive or tensile epitaxial strain.

External-field-induced crystal structure and domain texture in (1−x)Na0.5Bi0.5TiO3–xK0.5Bi0.5TiO3 piezoceramics

Lead-free perovskites based on Na0.5Bi0.5TiO3 (NBT) are being considered as viable alternatives to lead-containing piezoelectric materials. The piezoelectric response of morphotropic compositions of these bismuth-based piezoelectrics during the application of external stimuli are governed by an intrinsic local disorder, ferroelectric and antiferrodistortive instabilities, and domain texturing. Understanding the coupling between these mechanisms is of crucial importance for the development of new, environmentally friendly, piezoelectric materials. In this investigation we applied in-situ transmission electron microscopy and high-energy X-ray diffraction to study the changes in the crystal structure and domain texturing during the application of mechanical stresses and electric fields to the morphotropic composition of (1-x)Na0.5Bi0.5TiO3–xK0.5Bi0.5TiO3 (NBT-KBT) piezoceramics. It was found that the mechanisms involved largely depend on the materials' initial structural state and that phase transitions and domain texturing dominate the polarization- and strain-driven processes.

Metastability effects in strained and stressed SrTiO3 films

The sequence of ground states for SrTiO3 film subjected to epitaxial strain as well as to mechanical stress along the [001] and [110] axes is calculated from first principles within the density functional theory. Under the fixed-strain boundary conditions, an increase in the lattice parameter of a substrate results in the [Formula: see text](II) sequence of ground states. Under the fixed-stress boundary conditions, the phase sequence is different and depends on how the stress is applied. It is revealed that the simultaneous presence of competing ferroelectric and antiferrodistortive instabilities in SrTiO3 gives rise to the appearance of metastable phases, whose number increases dramatically under the fixed-stress conditions. In the metastable phases, the octahedral rotation patterns are shown to differ substantially from those in the ground state. It is suggested that in systems with competing instabilities, each polar phase has its optimal octahedral rotation pattern which stabilizes this phase and creates a potential barrier preventing this phase to be transformed into other structures.

An empirical model for perovskite tetragonality

Tetragonal distortions in perovskites are useful for advanced devices, as they can induce functional properties like capacitance, piezoelectricity, pyroelectricity, and ferroelectricity. These distortions are commonly due to either second-order Jahn-Teller effects or antiferrodistortive instabilities. Accurate empirical composition-structure models can significantly reduce the time and cost involved with developing new functional ceramics; however, until now no such model has existed for tetragonal perovskites, the structures of which are further complicated by extrinsic point defects often used to tune electrical properties. In this work, LeBail refinements of X-ray diffraction patterns obtained from [(PbyBa1−y)(1−3x)La(2x)]TiO3 (P4mm, No. 99) were conducted to measure lattice constants and reveal tetragonality trends. These trends were combined with a model for unit-cell volume in order to derive a general empirical model, based solely on published ionic radii data, for tetragonal lattice constants of perovskites in P4mm and P4/mmm.

Piezoelectricity and rotostriction through polar and non-polar coupled instabilities in bismuth-based piezoceramics.

Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed “rotopolarization”. These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.

Electrostatic engineering of strained ferroelectric perovskites from first principles

Design of novel artificial materials based on ferroelectric perovskites relies on the basic principles of electrostatic coupling and in-plane lattice matching. These rules state that the out-of-plane component of the electric displacement field and the in-plane components of the strain are preserved across a layered superlattice, provided that certain growth conditions are respected. Intense research is currently directed at optimizing materials functionalities based on these guidelines, often with remarkable success. Such principles, however, are of limited practical use unless one disposes of reliable data on how a given material behaves under arbitrary electrical and mechanical boundary conditions. Here we demonstrate, by focusing on the prototypical ferroelectrics PbTiO3 and BiFeO3 as testcases, how such information can be calculated from first principles in a systematic and efficient way. In particular, we construct a series of two-dimensional maps that describe the behavior of either compound (e.g. concerning the ferroelectric polarization and antiferrodistortive instabilities) at any conceivable choice of the in-plane lattice parameter, a, and out-of-plane electric displacement, D. In addition to being of immediate practical applicability to superlattice design, our results bring new insight into the complex interplay of competing degrees of freedom in perovskite materials, and reveal some notable instances where the behavior of these materials depart from what naively is expected.

Investigation of structural, vibrational and ferroic properties of AgNbO3 at room temperature using neutron diffraction, Raman scattering and density-functional theory

We perform a comprehensive study of the structural, lattice dynamical and ferroic properties of the room temperature phase of AgNbO3 using a combination of neutron diffraction, Raman spectroscopy and first-principles density functional theory. The Rietveld analysis of neutron diffraction data indicates that AgNbO3 at room temperature crystallizes in an orthorombic structure with noncentrosymmetric Pmc21 space group symmetry. The zone-center phonon mode frequencies of Pmc21 and Pbcm phases of AgNbO3 are computed and are found to be in good agreement with experimentally obtained Raman mode frequencies. Computed frequencies for both space groups are found to be very close. However, computed modes with frequencies 488 and 846 cm−1 are found to be Raman-active for the Pmc21 phase while Raman inactive for Pbcm phase. The computed potential energy surfaces and Born effective charges of the ions suggest that the primary contribution to ferroic properties of AgNbO3 come from Nb and O ions only. Furthermore the calculated potential energy profiles suggest competing ferroic and antiferrodistortive instabilities in AgNbO3 are primarily due to the off-centering of Nb ions and rotations of NbO6 octahedra, respectively.

Instabilities in the ferro- and antiferroelectric lead perovskites driven by transition metal ion mass: from PbTiO3 via PbZrO3 to PbHfO3

The lattice dynamics of Pb-containing perovskite oxides are investigated theoretically for the transition metal series Ti, Zr, Hf, in order to elucidate their commonalities and their distinctions. For all three compounds, pronounced precursor effects are found to their phase transition temperatures, which get more pronounced the heavier the central transition metal ion is. In addition, a competition between a polar and an antiferrodistortive instability is predicted to take place, which is strongly mass dependent. While in PbTiO3 the polar instability wins, both instabilities are active in PbZrO3, whereas in PbHfO3 the antiferrodistortive phase transition dominates the dynamics. For all three compounds, marked anomalies in the elastic constants are predicted, which are most pronounced in PbHfO3. Experimental results for elastic anomalies preceding the phase transition, which agree qualitatively with the model calculations are presented for PbHfO3.

First-principles study of the lattice dynamical properties of strontium ruthenate

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO3 show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. Our systematic analysis confirms previous expectations and contributes to rationalizing better why many ABO3 perovskites, including metallic compounds, exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties.

Emergent Superstructural Dynamic Order due to Competing Antiferroelectric and Antiferrodistortive Instabilities in BulkEuTiO3

Microscopic structural instabilities of EuTiO3 single crystal were investigated by synchrotron x-ray diffraction. Antiferrodistortive (AFD) oxygen octahedral rotational order was observed alongside Ti derived antiferroelectric (AFE) distortions. The competition between the two instabilities is reconciled through a cooperatively modulated structure allowing both to coexist. The electric and magnetic field effect on the modulated AFD order shows that the origin of large magnetoelectric coupling is based upon the dynamic equilibrium between the AFD - antiferromagnetic interactions versus the electric polarization - ferromagnetic interactions.

Electrical properties of improper ferroelectrics from first principles

We study the interplay of structural and polar distortions in hexagonal YMnO${}_{3}$ and short-period PbTiO${}_{3}$/SrTiO${}_{3}$ (PTO/STO) superlattices by means of first-principles calculations at constrained electric displacement field $D$. We find that in YMnO${}_{3}$ the tilts of the oxygen polyhedra produce a robustly polar ground state, which persists at any choice of the electrical boundary conditions. Conversely, in PTO/STO the antiferrodistortive instabilities alone do not break inversion symmetry, and open-circuit boundary conditions restore a nonpolar state. We suggest that this qualitative difference naturally provides a route to rationalizing the concept of ``improper ferroelectricity'' from the point of view of first-principles theory. We discuss the implications of our arguments for the design of novel multiferroic materials with enhanced functionalities and for the symmetry analysis of the phase transitions.

Multiferroic behavior of Aurivillius Bi4Mn3O12from first principles

The multiferroic behavior of the hypothetical Aurivillius compound Bi${}_{4}$Mn${}_{3}$O${}_{12}$ has been explored on the basis of density functional calculations. We find that the tetragonal paraelectric phase of this material is ferromagnetic, showing ferroelectric and antiferrodistortive instabilities similar to the ones observed in its ferroelectric parent compound Bi${}_{4}$Ti${}_{3}$O${}_{12}$. Our results indicate, however, that the presence of Mn${}^{+4}$ ions at the $B$ sites shrinks the cell volume and, consequently, the unstable polar mode, associated with the ferroelectric polarization, is overcome by an antiferrodistortive distortion. In this way, Bi${}_{4}$Mn${}_{3}$O${}_{12}$ exhibits incipient ferroelectricity at its equilibrium volume. We show that the ferroelectric state can be favored by strain or partial substitution of Mn with Ti.

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (TC) in BaTiO3. However, in multiferroic BiFeO3 thin films an unanticipated strain-driven decrease of TC was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between −2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.

First-principles study of structural and vibrational properties of SrZrO3

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO${}_{3}$. We start from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported. We point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center. We show that the strong antiferrodistortive motions suppress ferroelectricity and are responsible for the orthorhombic ground state as in CaTiO${}_{3}$. The structural properties of possible intermediate phases and of the orthorhombic $Pnma$ ground state are reported. For the latter, an assignment of IR and Raman zone-center phonon modes is proposed. The main features of the ferroelectric instability are also discussed, and we show that a ferroelectric ground state can even be induced in SrZrO${}_{3}$ by strain engineering.

Evidence for Room-Temperature Multiferroicity in a Compound with a Giant Axial Ratio

In the search for multiferroic materials magnetic compounds with a strongly elongated unit-cell (large axial ratio c/a) have been scrutinized intensely. However, none was hitherto proven to have a switchable polarization, an essential feature of ferroelectrics. Here, we provide evidence for the epitaxial stabilization of a monoclinic phase of BiFeO3 with a giant axial ratio (c/a=1.23) that is both ferroelectric and magnetic at room temperature. Surprisingly, and in contrast with previous theoretical predictions, the polarization does not increase dramatically with c/a. We discuss our results in terms of the competition between polar and antiferrodistortive instabilities and give perspectives for engineering multiferroic phases.