Ferroelectric Distortion Research Articles

Unusual Mott transition in multiferroic PbCrO3

The Mott insulator in correlated electron systems arises from classical Coulomb repulsion between carriers to provide a powerful force for electron localization. Turning such an insulator into a metal, the so-called Mott transition, is commonly achieved by "bandwidth" control or "band filling." However, both mechanisms deviate from the original concept of Mott, which attributes such a transition to the screening of Coulomb potential and associated lattice contraction. Here, we report a pressure-induced isostructural Mott transition in cubic perovskite PbCrO3. At the transition pressure of ∼3 GPa, PbCrO3 exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. For the first time to our knowledge, these findings validate the scenario conceived by Mott. Close to the Mott criticality at ∼300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid-gas transition. The anomalously large lattice volume and Coulomb potential in the low-pressure insulating phase are largely associated with the ferroelectric distortion, which is substantially suppressed at high pressures, leading to the first-order phase transition without symmetry breaking.

Magnetic anisotropy energy in disordered [formula omitted

We theoretically analyze the influence of chemical disorder on magnetic anisotropy in Ge1−xMnxTe semiconductor layers known to exhibit carrier-induced ferromagnetism and ferroelectric distortion of rhombohedral crystal lattice. Using DFT method we determine the local changes in the crystal structure due to Mn ions substitution for Ge and due to the presence in Ge1−xMnxTe of very high concentration of cation vacancies. We calculate the effect of this structural and chemical disorder on single ion magnetic anisotropy mechanism and show that its contribution is of the order of magnitude smaller as compared to magnetic anisotropy mechanism originating from the spin polarization induced by Mn ions into neighboring Te and Ge ions. We also discuss magnetic anisotropy effects due to pairs of Mn ions differently allocated in the lattice. The spatial averaging over chemical disorder strongly reduces the strength of this magnetic anisotropy mechanism and restores the global rhombohedral symmetry of magnetic system.

Competition between compressive strain and Mn doping on tuning the structure and magnetic behavior of BiFeO3 thin films

BiFe 1-x Mn x O 3 (BFMO) films (x = 0, 0.05, 0.1) were grown on (001) LaAlO 3 (LAO) substrates to demonstrate the role of chemical pressure on the lattice structure and magnetic behaviors of compressively-strained BiFeO 3 (BFO) films. The structural analyses indicated that the substitution of Jahn–Teller active ion Mn at Fe site tuned the ferroelectric distortion and antiferrodistortive (AFD) rotation in BFO lattice, thus weakened the tetragonal-like (T-like) phase originated from the large substrate clamping. The phase transition mechanism was discussed to reveal the competition between inner lattice distortion and external stress in BFO films. Clear domain variants change from (T-like) phase to rhombohedral-like (R-like) phase provided further evidence for the structure alteration. Meanwhile the gradual relaxation in total lattice strain of BFMO films with increasing x reduced the magnetism contribution from the strong spin and lattice coupling.

Magnetic anisotropy induced by crystal distortion in Ge1−xMnxTe/PbTe//KCl (001) ferromagnetic semiconductor layers

Ferromagnetic resonance (FMR) study of magnetic anisotropy is presented for thin layers of IV-VI diluted magnetic semiconductor Ge1−xMnxTe with x = 0.14 grown by molecular beam epitaxy on KCl (001) substrate with a thin PbTe buffer. Analysis of the angular dependence of the FMR resonant field reveals that an easy magnetization axis is located near to the normal to the layer plane and is controlled by two crystal distortions present in these rhombohedral Ge1−xMnxTe layers: the ferroelectric distortion with the relative shift of cation and anion sub-lattices along the [111] crystal direction and the biaxial in-plane, compressive strain due to thermal mismatch.

Interplay of coupling between strain and rotation in ferroelectric SrZrO3/SrTiO3 superlattices

The combination of oxygen octahedral rotation and epitaxial strain provides a unique opportunity to tune the ferroelectric properties of perovskite superlattices. Here, through first-principles calculations, we demonstrate that the oxygen octahedral rotation predominates the ground state and ferroelectric properties of SrZrO3/SrTiO3 superlattices. The predicted ground state combines the ferroelectric distortion and antiferrodistortive modes simultaneously. The structure–strain phase diagrams of the superlattices are calculated with and without octahedral rotations, which elucidate the interplay of coupling between epitaxial strain and octahedral rotation. It is found that the presence of octahedral rotation not only lowers the energy but also changes the sequence of phase transition from c–r–aa to c–r, in which the coupling of rotation and strain induces an out-of-plane polarization that transforms aa-phase into r-phase.

Metallic-like to nonmetallic transitions in a variety of heavily oxygen deficient ferroelectrics

The coupling between ferroelectric distortions and electron transport is an important factor in understanding ferroelectric/noncentrosymmetric materials with metallic conductivities and ferroelectric-based thermoelectrics. Here, multiple d0 ferroelectrics with a variety of crystal structures are doped via oxygen deficiency, resulting in metallic-like conduction in the paraelectric state. It is found that most of the studied systems show a metallic-like to nonmetallic transition near the paraelectric-ferroelectric transition. The metallic-like to nonmetallic transition temperature can be shifted using mechanisms that shift the paraelectric-ferroelectric transition temperature. It was found that the metallic-like to nonmetallic transition temperature could be shifted from 373 K to 273 K by varying (Ba1−xSrx)TiO3−δ from x = 0 to x = 0.3 and x = 1. The most probable mechanism for ferroelectric-electron transport coupling was determined to be Anderson localization associated with polarization with short-range order.

Analysis of the Mn–O and Y–O bonds in paraelectric and ferroelectric phase of magnetoelectric YMnO3 from the first principles calculations

We report the first-principles band-structure calculations on multiferroic hexagonal YMnO3, realized with objective to study how the Mn–O and Y–O bonds change when the compound undergoes paraelectric (PE) ↔ ferroelectric (FE) phase transition. The compound was simulated in non-magnetic and antiferromagnetic configuration. Exchange–correlation effects were treated by modified Becke-Johnson approach, which enabled successful reproduction of experimental band gap in the FE phase and predicted a small band gap in the PE phase, in agreement with experimental findings. The Mn–O and Y–O bonds were investigated qualitatively, by inspecting charge density maps projected onto suitable planes, and quantitatively, by applying the Bader's topological analysis and comparing electronic densities of states in different phases. Independently of the fact how the Mn ions were treated, as spin-polarized or not, the results indicate that the Mn–O bonds do not suffer significant changes during the PE-FE phase transition, while the Y–O bonds become more covalent, with clear signs of hybridization between Y 4dz2 and the O pz orbitals. The results from present study, therefore, substantiate the Y d0-ness model to explain ferroelectric distortion in hexagonal manganites.

Ferroelectric Graphene-Perovskite Interfaces.

Owing to their record-breaking energy conversion efficiencies, hybrid organometallic perovskites have emerged as the most promising light absorbers and ambipolar carrier transporters for solution-processable solar cells. Simultaneously, due to its exceptional electron mobility, graphene represents a prominent candidate for replacing transparent conducting oxides. Thus, it is possible that combining these wonder materials may propel the efficiency toward the Schokley-Queisser limit. Here, using first-principles calculations on graphene-CH3NH3PbI3 interfaces, we find that graphene suppresses the octahedral tilt in the very first perovskite monolayer, leading to a nanoscale ferroelectric distortion with a permanent polarization of 3 mC/m(2). This interfacial ferroelectricity drives electron extraction from the perovskite and hinders electron-hole recombination by keeping the electrons and holes separated. The interfacial ferroelectricity identified here simply results from the interplay between graphene's planar structure and CH3NH3PbI3's octahedral connectivity; therefore, this mechanism may be effective in a much broader class of perovskites, with potential applications in photovoltaics and photocatalysis.

Impact of Ferroelectric Distortion on Thermopower in BaTiO3

We present a strategy for increasing thermoelectric performance by invoking ferroelectric distortion in a transition-metal oxide. By using an ab initio calculation approach with the Boltzmann transport equation, a large Seebeck coefficient is calculated in n-type BaTiO3. The polar structural distortion causes the peculiar dispersion of the lowest conduction band, which enhances the Seebeck coefficient along the polar direction. A microscopic mechanism of carrier concentration dependence of the Seebeck coefficient is discussed in terms of the band velocity distribution in the Brillouin zone.

Impact of biaxial compressive strain on the heterostructures of paraelectrics KTaO3 and SrTiO3

We have performed density functional theory calculations to explore the impact of biaxial compressive strain on the heterostructures of paraelectrics KTaO3 and SrTiO3. We find that the strain induces strong ferroelectric distortion in KTaO3/SrTiO3 heterostructures and it stabilizes the heterostructures in ferroelectric states.The strain influences the distribution of doped holes and leads to the localization of holes in TiO2 layer. It is very interesting that ferroelectricity and ferromagnetism simultaneously present in the strained heterostructures formed by the paraelectrics KTaO3 and SrTiO3. The reversal of ferroelectric polarization changes the interface magnetization and thus results in magnetoelectric coupling effect in the heterostructures.

Multiferroicity in polar phase LiNbO3 at room temperature

LiNbO3, prepared by ball milling assisted ceramic method, exhibits weak ferromagnetism and ferroelectricity at room temperature. X-ray diffraction pattern reveals the rhombohedral phase of LiNbO3 with hexagonal unit cell symmetry. The weak ferromagnetic behavior, obtained using VSM, has been explained using Dzyaloshinskii–Moriya interaction caused by the ferroelectric distortion in its magnetic order. The P–E loop measurement shows lossy natured ferroelectric loop. Electrical and dielectric properties analyzed using impedance spectroscopy show two thermally activated conduction processes, derived from the Arrhenius plot. A gradual increase in the dielectric constant below 493K and a rapid increase above 493K reveals the contribution of polarization components and Lithium ion hopping.

Enhanced Polarization Fluctuations in PbZr0.72Sn0.28O3 Compared to PbZrO3 Single Crystals Studied by Brillouin Light Scattering

Acoustic properties of antiferroelectric (AFE) PbZr0.72Sn0.28O3 single crystals were investigated in a wide temperature range by Brillouin light scattering spectroscopy. The Brillouin frequency shift of the longitudinal acoustic (LA) mode exhibited three abrupt changes corresponding to the successive phase transitions between the paraelectric, intermediate, antiferroelectric (AFE2) and antiferroelectric (AFE1) phases upon cooling. The change in the LA mode frequency and the damping maximum of PbZr0.72Sn0.28O3 were much higher than those of pure PbZrO3. This was attributed to enhanced polarization fluctuations caused by increased ferroelectric distortions due to Sn substitution and the proximity to the tricritical point on the phase diagram of PbZr1-xSnxO3.

Extraordinary enhancement of Néel transition temperature in nanoparticles of multiferroic tetragonal compositions of (1 − x)BiFeO3-xPbTiO3 solid solutions

The Néel temperature (TN) in conventional antiferromagnets (AFM) decreases with decreasing particle size. In contrast, we present here evidence for an exotic facet of multiferroicity, whereby one can raise the TN by more than 200 K by decreasing the particle size. We illustrate this by taking the example of a tetragonal composition with x = 0.5 in the solid solutions of (1 − x)BiFeO3-xPbTiO3 (BF-xPT). We attribute it to the increase in the strength of the superexchange interaction via a decrease in the ferroelectric distortion in nanocrystalline powder of BF-0.5PT. The BF-0.5PT nanoparticles also exhibit exchange bias effect due to AFM core-FM shell type magnetic nanostructure.

Potentially multiferroic Aurivillius phaseBi5FeTi3O15: Cation site preference, electric polarization, and magnetic coupling from first principles

We study the structural, ferroelectric, and magnetic properties of the potentially multiferroic Aurivillius phase material Bi$_5$FeTi$_3$O$_{15}$ using first principles electronic structure calculations. Calculations are performed both with PBE and PBEsol exchange correlation functionals. We conclude that PBE systematically overestimates the lattice constants and the magnitude of the ferroelectric distortion, whereas PBEsol leads to good agreement with available experimental data. We then assess a potential site preference of the Fe$^{3+}$ cation by comparing 10 different distributions of the perovskite $B$-sites. We find a slight preference for the "inner" site, consistent with recent experimental observations. We obtain a large value of $\sim$55 $\mu$C/cm$^2$ for the spontaneous electric polarization, which is rather independent of the specific Fe distribution. Finally, we calculate the strength of the magnetic coupling constants and find strong antiferromagnetic coupling between Fe$^{3+}$ cations in nearest neighbor positions, whereas the coupling between further neighbors is rather weak. This poses the question whether magnetic long range order can occur in this system in spite of the low concentration of magnetic ions.

Interfacial Nb-substitution induced anomalous enhancement of polarization and conductivity in BaTiO3 ferroelectric tunnel junctions

Using density functional theory (DFT) method combined with non-equilibrium Green’s function approach, we systematically investigated the structural, ferroelectric and electronic transport properties of Pt/BaTiO3/Pt ferroelectric tunnel junctions (FTJ) with the interface atomic layers doped by charge neutral NbTi substitution. It is found that interfacial NbTi substitution will produce several anomalous effects such as the vanishing of ferroelectric critical thickness and the decrease of junction resistance against tunneling current. Consequently, the thickness of the ferroelectric thin film (FTF) in the FTJ can be reduced, and both the electroresistance effect and sensitivity to external bias of the FTJ are enhanced. Our calculations indicate that the enhancements of conductivity and ferroelectric distortion can coexist in FTJs, which should be important for applications of functional electronic devices based on FTJs.

Effect of Ho and Mn co-doping on structural, ferroelectric and ferromagnetic properties of BiFeO3 thin films

Multiferroic pure, Ho doped, Mn doped, as well as Ho, Mn co-doped BiFeO3 thin films were prepared using a sol–gel technique. The effect of Ho and Mn co-doping on the structure, ferroelectric and ferromagnetic properties of BiFeO3 films was studied in detail. It shows that the enhanced ferroelectric polarization and magnetization were obtained after doping Ho and Mn. Such enhanced ferroelectric properties are attributed to ferroelectric distortion derived from the structural transformation and microstructure improvement after doping Ho and Mn. The enhanced ferromagnetic properties are attributed to the uncompensated surface spins and the release of the latent magnetization locked within the cycloid induced by doping Ho and Mn. The present work provides an easy method to enhance the ferroelectric and ferromagnetic properties of BiFeO3 based magnetoelectric multiferroic films.

Pseudo Jahn–Teller origin of ferroelectric instability in BaTiO3 type perovskites: The Green's function approach and beyond

The local origin of dipolar distortions in ABO3 perovskite crystals is reexamined by means of a novel approach, the Green's function method augmented by DFT computations. The ferroelectric distortions are shown to be induced by the pseudo Jahn–Teller effect (PJTE). The latter involves vibronic hybridization (admixture) of the ground state to same-spin opposite-parity excited electronic bands. Similar to numerous molecular calculations, the PJT approach provides a deeper insight into the nature of chemical bonding in the octahedral cluster [BO6] and, in particular, reveals the local origin of its polar instability. This allows predicting directly which transition ions can create ferroelectricity. In particular, the necessary conditions are established when an ABO3 perovskite crystal with an electronic dn configuration of the complex ion [BO6] can possess both proper ferroelectric and magnetic properties. Distinguished from the variety of cluster approaches to local properties, the Green's function method includes the influence of the local vibronic-coupling perturbation on the whole crystal via the inter-cell interaction responsible for creation of electronic and vibrational bands. Calculated Green's functions combined with the corresponding numeric estimates for the nine electronic bands, their density of states, and the local adiabatic potential energy surface (APES) confirm the eight-minimum form of this surface and feasibility of the PJT origin of the polar instability in BaTiO3. We show also that multicenter long-range dipole–dipole interactions critically depend on the PJTE largely determining the magnitude of the local dipoles. DFT calculations for the bulk crystal and its clusters confirm that the dipolar distortions are of local origin, but become possible only when their influence on (relaxation of) the whole lattice is taken into account. The results are shown to be in full qualitative and semiquantitative agreement with the experimental data for this crystal.

Multiferroic properties of o-LuMnO3 controlled by b-axis strain.

Strain is a leading candidate for controlling magnetoelectric coupling in multiferroics. Here, we use x-ray diffraction to study the coupling between magnetic order and structural distortion in epitaxial films of the orthorhombic (o-) perovskite LuMnO(3). An antiferromagnetic spin canting in the E-type magnetic structure is shown to be related to the ferroelectrically induced structural distortion and to a change in the magnetic propagation vector. By comparing films of different orientations and thicknesses, these quantities are found to be controlled by b-axis strain. It is shown that compressive strain destabilizes the commensurate E-type structure and reduces its accompanying ferroelectric distortion.

Perovskites in low dimensional multi-layer structure types

This study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi5PbTi3O14Cl [1] with charge balancing by removing Pb2+for Bi3+. A statistical 1:2 distribution of M3+and Ti4+across all three perovskite layers was found in Bi6Ti2MO14Cl, M = Cr3+, Mn3+, Fe3+, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi5PbTi3O14Cl) and pronounced spin-glass behavior below Tirr(0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi6Ti2FeO14Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi3Sr2Nb2O11Br, based on Bi3Pb2Nb2O11Cl [2], was synthesized by simultaneously replacing Pb2+with Sr2+and Cl-with Br-. Inter-layer mismatch prevented the formation of Bi3Sr2Nb2O11Cl and Bi3Pb2Nb2O11Br. Sr2+doping reduces the impact of the stereochemically active 6s2lone pair found on Pb2+and Bi3+, resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br-. X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi3Sr2Nb2O11Br compared to Bi3Pb2Nb2O11Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.

Ferroelectric phase transition and soft-mode behavior in BaxSr1−xTiO3: a refined treatment of a quasi-harmonic model

The composition and temperature dependences of the soft-mode and ferroelectric phase transition in BaxSr1−xTiO3 (BST) are presented and analyzed within the framework of a refined treatment of a quasi-harmonic model (QHM) for quantum particles. The QHM involves generalized simple microscopic quantum particles with anharmonic on-site double-Morse-type potential and harmonic interactions. The theory uses the variational principle scheme at finite temperature in the mean-field approximation while taking into account the predominant mass effect, the cell volume effect as well as ferroelectric distortion. The soft-mode frequency and the Ba concentration (x) dependence of the ferroelectric phase transition temperature are analyzed and show good agreement with experimental results throughout the full x range (0 ⩽ x ⩽ 1). It is found that BST is paraelectric below the critical impurity concentration xc = 0.01 with a softening of the soft mode. It becomes ferroelectric above this critical concentration (x ⩾ xc), and as the impurity concentration increases the predominant displacive soft-mode behavior stiffens in the ferroelectric phase with much less softening in crystals indicating a possible phase change at high x. Around x′c = 0.5, Tc deviates gradually from linear behavior with a rather slight round stagnation which might characterize an equilibrium ‘region’ where several ferroelectric phases coexist. The proposed x dependence of the soft-mode behavior indicates that the ferroelectric phase transition is of first order in BST with x > x′c, and of second order for low Ba concentrations (xc ⩽ x < x′c), throughout the tricritical point at x = x′c. Finally, the quantum dynamics of the ferroelectric phase transition and the mode softening are also addressed and discussed, suggesting a net increase of quantum effects with a decrease of x for x ⩽ 0.1, and a dominant Curie–Weiss law for x > 0.1.