Rhombohedral BaTiO3 Research Articles

First-principles modelling of the thermoelectric properties of n-type CaTiO3, SrTiO3 and BaTiO3

We benchmark a fully ab initio modelling workflow for predicting the electrical and thermal transport properties and thermoelectric figure of merit ZT on three oxide perovskites, viz. orthorhombic CaTiO3, tetragonal SrTiO3 and rhombohedral BaTiO3.

Raman spectra of fine-grained materials from first principles

Raman spectroscopy is an advantageous method for studying the local structure of materials, but the interpretation of measured spectra is complicated by the presence of oblique phonons in polycrystals of polar materials. Whilst group theory considerations and standard ab initio calculations are helpful, they are often valid only for single crystals. In this paper, we introduce a method for computing Raman spectra of polycrystalline materials from first principles. We start from the standard approach based on the (Placzek) rotation invariants of the Raman tensors and extend it to include the effect of the coupling between the lattice vibrations and the induced electric field, and the electro-optic contribution, relevant for polar materials like ferroelectrics. As exemplified by applying the method to rhombohedral BaTiO3, AlN, and LiNbO3, such an extension brings the simulated Raman spectrum to a much better correspondence with the experimental one. Additional advantages of the method are that it is general, permits automation, and thus can be used in high-throughput fashion.

Influence of tin (IV) doping on structural and optical properties of rhombohedral barium titanate (BaTiO3)

The impact of tin (IV-Sn4+) doping into rhombohedral barium titanate (BaTiO3) towards the enhancement of second harmonic generation (SHG) was studied. XRD analysis confirms retention of rhombohedral symmetry in Sn4+ doped BaTiO3. UV-Vis absorption study reveals that Sn4+ doping has no significant effect on the optical band gap of BaTiO3. Raman spectrum portrays that undoped and Sn4+ doped BaTiO3 nanopowder exhibits Raman active modes of rhombohedral symmetry. The studies displays that Sn4+ doped BaTiO3 has created non-centrosymmetry, which could be utilized for enhancement in SHG and possibility to improve the efficiency in rhombohedral BaTiO3 based solar cells.

Extrinsic permittivity in domain engineered rhombohedral BaTiO3 monocrystal

Arrangement of 90-degree domain walls in the domain structure of the ⟨111⟩pc-oriented single crystal plates of BaTiO3 was intentionally modified by combinations of in-plane and out-of-plane steady state electric poling at ambient temperature. It is shown that the annealed, in-plane, and out-of-plane poled samples exhibit markedly different dielectric properties at low temperatures. The difference in low-temperature permittivity behavior is enhanced by the out-of-plane fast-decayed bipolar electric pulse treatment, but the efficiency of this enhancement is strongly dependent on the initial domain structure. The absence of a clear signature of the pulse treatment in optical observations of the domain structure suggests that it might involve mostly changes in the 180-degree domain wall subsystem.

On the possible internal structure of the ferroelectric Ising lines in BaTiO3

ABSTRACTThis article is devoted to a detailed analysis of the inner structure of the Ising line, a topological defect separating two parts of a ferroelectric Bloch domain wall with opposite helicity, in the framework of the phenomenological Ginzburg–Landau–Devonshire model. Results performed for Ising lines in a ⟨211⟩-oriented 180-degree domain wall of rhombohedral BaTiO3 suggest remarkably strong dependence of the polarization profiles on the form of gradient terms. Profiles of the skyrmion density, vorticity and divergence of the polarization field were calculated numerically in the few-nm vicinity of the Ising line defect.

First-Principles Study of Lattice Dynamics, Structural Phase Transition, and Thermodynamic Properties of Barium Titanate

Abstract Lattice dynamics, structural phase transition, and the thermodynamic properties of barium titanate (BaTiO3) are investigated by using first-principles calculations within the density functional theory (DFT). It is found that the GGA-WC exchange-correlation functional can produce better results. The imaginary frequencies that indicate structural instability are observed for the cubic, tetragonal, and orthorhombic phases of BaTiO3 and no imaginary frequencies emerge in the rhombohedral phase. By examining the partial phonon density of states (PDOSs), we find that the main contribution to the imaginary frequencies is the distortions of the perovskite cage (Ti-O). On the basis of the site-symmetry consideration and group theory, we give the comparative phonon symmetry analysis in four phases, which is useful to analyze the role of different atomic displacements in the vibrational modes of different symmetry. The calculated optical phonon frequencies at Γ point for the four phases are in good agreement with other theoretical and experimental data. The pressure-induced phase transition of BaTiO3 among four phases and the thermodynamic properties of BaTiO3 in rhombohedral phase have been investigated within the quasi-harmonic approximation (QHA). The sequence of the pressure-induced phase transition is rhombohedral→orthorhombic→tetragonal→cubic, and the corresponding transition pressure is 5.17, 5.92, 6.65 GPa, respectively. At zero pressure, the thermal expansion coefficient α V , heat capacity C V , Grüneisen parameter γ, and bulk modulus B of the rhombohedral phase BaTiO3 are estimated from 0 K to 200 K.

Density functional simulation of carbon at the titanium site in perovskite barium titanate

The perovskite family includes many titanates which have been used in various applications. BaTiO3 is interesting due to its room-temperature ferroelectric properties and relative low toxicity. Organic precursors present during growth make carbon a potentially key impurity, which would subsequently impact upon the BaTiO3 properties. This paper presents a density function study of the structural and electronic properties of carbon substituting for Ti in rhombohedral BaTiO3. The local vibrational modes of the defect centre has been calculated and suggested as a possible route to experimental identification.

Ferroelectric properties of RbNbO3 and RbTaO3

Phonon spectra of cubic rubidium niobate and rubidium tantalate with the perovskite structure are calculated from first principles within the density functional theory. Based on the analysis of unstable modes in phonon spectra, symmetries of possible distorted phases are determined, their energies are calculated, and it is shown that R3m is the ground-state structure of RbNbO3. In RbTaO3, the ferroelectric instability is suppressed by zero-point lattice vibrations. For ferroelectric phases of RbNbO3, spontaneous polarization, piezoelectric, nonlinear optical, electro-optical, and other properties as well as the energy band gap in the LDA and GW approximations are calculated. The properties of rhombohedral RbNbO3 are compared with those of rhombohedral KNbO3, LiNbO3, and BaTiO3.

Identification of complex anions in La1 − x A x MnO3 manganites (A = Ca, Sr) from neutron diffraction data and refinement of their structures on the basis of raman spectroscopy data

The consideration of the Mn(-O…Mn)6 structure unit of the LaMnO3 orthorhombic phase constructed on the basis of neutron diffraction data revealed the presence of atomic groups in the form of (MnO2)− complex anions and separate O2− ions. The Raman spectra have demonstrated that complex anions have a nearly linear O=Mn-O− structure. Comparison of the Raman spectra of the rhombohedral CaMnO3 and BaTiO3 phases and pyramidal molecules of the ZXY2 type has shown that the CaMnO3 phase is composed of pyramidal MnO32− molecules with one Mn=O double bond and two Mn-O− ordinary bonds. The complex anions are linked by O=Mn-O− intermolecular bonds to form planar zigzag Mn=O…Mn chains similar to C=C-C planar chains in some organic compounds exhibiting high electrical conductivity. With decreasing temperature, the Mn…O intermolecular distances decrease, and the Mn=O…Mn chains become even more similar to the C=C-C chains and exhibit high electrical conductivity.

Divergence of dielectric permittivity near phase transition within ferroelectric domain boundaries

Phase-field simulations based on Ginzburg–Landau–Devonshire model were utilised to demonstrate and quantify the enhancement of the dielectric permittivity of multidomain rhombohedral BaTiO3 crystal in the vicinity of the Bloch-type to Ising-type phase transition within 180° domain boundaries with crystallographic orientation. The phase transition is driven by compressive mechanical stress applied in the plane perpendicular to the spontaneous polarisation direction. Simulations reveal a classical Curie–Weiss behaviour of the transverse component of dielectric permittivity, expected in the vicinity of a second-order phase transition.

Bloch-type domain walls in rhombohedral BaTiO3

Ferroelectric domain walls (FDWs) are usually considered to be of Ising type, but there have been suggestions in recent years that Bloch-type FDWs are also possible in some cases, e.g., in the rhombohedral phase of BaTiO3. The mechanically compatible and electrically neutral FDWs in rhombohedral BaTiO3 are of 71-degree, 109-degree, and 180-degree type. We have investigated these FDWs based both on first-principles calculations and on a Ginzburg-Landau-Devonshire (GLD) model [P. Marton, I. Rychetsky, and J. Hlinka, Phys. Rev. B 81, 144125 (2010)]. The results from both approaches confirm the Ising nature of the 71-degree FDW and the Bloch nature of the 180-degree FDW, and predict both Ising-type and Bloch-type FDWs are possible for the 109-degree case. Considering the relatively small rhombohedral strain in BaTiO3, the competition between the energies of Bloch and Ising FDWs can be discussed in terms of a picture in which a Bloch wall is regarded as being composed of a pair of smaller-angle Ising ones. A reduction by 40% in the parameters describing the gradient term in the GLD model brings it into better agreement with the first-principles results for detailed properties such as the energies and widths of the FDWs.

Stress-induced phase transition in ferroelectric domain walls of BaTiO3

The seminal paper by Zhirnov (1958 Zh. Eksp. Teor. Fiz. 35 1175–80) explained why the structure of domain walls in ferroelectrics and ferromagnets is drastically different. Here we show that the antiparallel ferroelectric walls in rhombohedral ferroelectric BaTiO3 can be switched between the Ising-like state (typical for ferroelectrics) and a Bloch-like state (unusual for ferroelectric walls but typical for magnetic ones). Phase-field simulations using a Ginzburg–Landau–Devonshire model suggest that this symmetry-breaking transition can be induced by a compressive epitaxial stress. The strain-tunable chiral properties of these domain walls promise a range of novel phenomena in epitaxial ferroelectric thin films.

Structure of BaTiO3 phases is studied by comparing neutron diffraction and Raman spectroscopy data

Considering the structural unit of the rhombohedral BaTiO3 phase in the form of Ti(-O...Ti)6 which was constructed by neutron diffraction data, we revealed an atomic group in the form of a pyramidal complex anion TiO32−. Inspection of the Raman spectra of this phase showed symmetry group Cs for this complex anion. From the similarity of the Raman spectra of all ferroelectric BaTiO3 phases, we inferred that the orthorhombic phase and tetragonal phase are also built of TiO32− complex anions. In the paraelectric cubic phase, TiO32− complex anions decompose to oxygen ions O2− and Ti2+(O−)2 molecules, which are randomly oriented over 12 possible positions. Average Ti-O distances derived from neutron diffraction data were used to calculate Ti-O− and Ti=O bond lengths in TiO32− complex anions of the ferroelectric BaTiO3 phases. Two types of Ti-O...Ti bridges were found to exist; they belong to weak and strong intermolecular bonds, which are similar to weak and strong hydrogen bonds (H-bonds) in ferroelectrics. Weak bonds exist in all of the three ferroelectric phases; strong bonds are only in the orthorhombic phase and the tetragonal phase. As a result of a low potential barrier height in strong bonds, an oxygen atom persistently hops from one potential well to an adjacent one, which is responsible for bands appearing below 100 cm−1 in the Raman spectra of the tetragonal phase and orthorhombic phase. The data obtained on the structure of the ferroelectric BaTiO3 phases were used to interpret their spontaneous polarization, repolarization, and permittivity.

Phase–field modelling of 180° “Bloch walls” in rhombohedral BaTiO3

This article describes investigations of 180° walls in rhombohedral BaTiO3 in the framework of generalized Ginzburg–Landau–Devonshire model, using phase-field simulation approach. It is demonstrated that the 180° wall with the normal parallel to [1−10] direction has a Bloch-like character. Its domain profile can be understood as a pair of very close 71° and 109° walls, which can be continuously separated by a strong transverse electric field. The intermediate region can be considered as a few nanometers thick monoclinic domain.

Pressure-Induced Anomalous Phase Transitions and Colossal Enhancements of Piezoelectricity in Ground-State BaTiO3

Rhombohedral BaTiO3 under hydrostatic pressure is investigated by first principles calculations. Our results show that just like tetragonal perovskites, as pressure increases, this material first becomes para-electric at low pressures, then transfers to another ferroelectric phase at much higher pressures. We also find a giant enhancement of piezoelectricity near the phase-transition regions, due to large atomic displacements along different directions in response to the applied pressures.

Evolution of Ferroelectric Relaxor Behavior in (1-x)BaTiO3-x(Na1/4Bi3/4)(Mg1/4Ti3/4)O3 Ceramics

(1-x)BaTiO3-x(Na1/4Bi3/4)(Mg1/4Ti3/4)O3 (0 < x ≤ 0.2) ceramics were synthesized by the solid-state reaction and verified to form the perovskite structure. Three dielectric anomalies corresponding to the three phase transitions of BaTiO3 were observed for samples with 0.1 ≤ x < 0.2 and one of those dielectric anomalies exhibit relaxation behavior. At x = 0.2, three phase transitions are pinched, and a distinct ferroelectric relaxor-like behavior dominates. Hysteresis loops were observed for the compositions with 0 < x ≤ 0.2. For 0.14 ≤ x < 0.2 partial linear P-E behaviors are observed. Coexistence of pseudo-cubic (Na1/4Bi3/4)(Mg1/4Ti3/4)O3 and rhombohedral BaTiO3 phases depict a typical relaxor behavior in this system.

Lattice, elastic, polarization, and electrostrictive properties of BaTiO3 from first-principles

Predicting the domain structures and properties in both bulk single crystal and thin film ferroelectrics using the phase-field approach requires the knowledge of fundamental mechanical, electrical, and electromechanical coupling properties of a single-domain state. In this work, the elastic properties and structural parameters of cubic single crystals as well as tetragonal, orthorhombic, and rhombohedral BaTiO3 single domain states are obtained using first-principles calculations under the local density approximation. The calculated lattice constants, bulk modulus, and elastic constants are in good agreement with experiments for both the cubic paraelectric phase and the low-temperature ferroelectric phases. Spontaneous polarizations for all three ferroelectric phases and the electrostrictive coefficients of cubic BaTiO3 are also computed using the Berry’s phase approach, and the results agree well with existing experimentally measured values.

Identification of molecules in inorganic compounds from X-ray or neutron diffraction data and correction of their structures on the basis of vibrational spectroscopy data

A method of identification of molecules in inorganic crystals with asymmetric M-X...M bridges has been suggested on the basis of analysis of interatomic distances in the structure unit M (−X...M)n, which includes the atoms of the first and second coordination spheres. This analysis makes it possible to discern molecules (complex anions or cations) as a groups of atoms linked with each other by short M-X bonds, whereas the atoms of neighboring groups are linked by long M...X bonds. The symmetry of such a group is often lower than it follows from X-ray or neutron diffraction data. Studying vibrational spectra affords information on the true symmetry of a molecule. The use of the method is exemplified by the rhombohedral BaTiO3 phase.

THEORETICAL STUDIES OF THE SPIN HAMILTONIAN PARAMETERS AND LOCAL STRUCTURE FOR THE TETRAGONALRh2+CENTER IN RHOMBOHEDRALBaTiO3

The spin Hamiltonian parameters and local structure for the tetragonal Rh2+center in rhombohedral BaTiO3are theoretically studied from the perturbation formulas of these parameters for a 4d7ion with low spin (S=1/2) in tetragonally elongated octahedra. This center is ascribed to substitutional Rh2+at the Ti4+site in BaTiO3. The [ RhO6]10-cluster suffers relative elongation (characterized by the elongation parameter ρ ≈ 0.9%) along the [100] axis due to the Jahn–Teller effect. The tetragonal elongation can entirely depress the slight trigonal distortion of the original Ti4+site in rhombohedral BaTiO3. The calculated spin Hamiltonian parameters based on the above Jahn–Teller elongation show good agreement with the experimental results.

Spin-Hamiltonian parameters and off-center displacements for Rh4+ and Ir4+ ions in rhombohedral BaTiO3

By means of the formulae based on the first-order theory of resonance for a low-spin 2T2 term in trigonal symmetry, the spin-Hamiltonian parameters (g factors g //, g ⊥ and hyperfine structure constants A //, A ⊥) for Rh4+(4d5) and Ir4+(5d5) ions in the low-temperature rhombohedral phase of BaTiO3 are calculated. The calculated results are in reasonable agreement with the experimental values. From the calculations, the off-center displacements of Rh4+ and Ir4+ in the oxygen octahedra of BaTiO3 crystal are acquired. These impurity displacements are much smaller than that of the replaced host ion Ti4+. This point is analogous to the displacements of 3d n impurities Fe3+ and Co2+ in rhombohedral BaTiO3 obtained by analyzing their spin-Hamiltonian parameters.