Electric field-induced irreversible relaxor to ferroelectric phase transformations in Na0.5Bi0.5TiO3-NaNbO3 ceramics

(1-x)Na_0.5Bi_0.5TiO₃-xNaNbO₃ (x = 0.02, 0.04, 0.06, and 0.08) ceramics were fabricated by solid-state reaction. High-resolution synchrotron x-ray powder diffraction (SXPD) data, coupled with macroscopic electromechanical measurements, reveal the occurrence of an electric field-induced irreversible crystallographic transformation for x = 0.02 and 0.04, from a pseudo-cubic non-ergodic relaxor to a rhombohedral or coexisting rhombohedral-tetragonal long range-ordered ferroelectric phase, respectively. The highest unipolar electrostrain, corresponding to an effective longitudinal piezoelectric strain coefficient of approximately 340 pm V⁻¹, was obtained for x = 0.04; this effect is attributed to enhanced domain switching as a result of the co-existing rhombohedral and tetragonal phases for this composition, which is critical for piezoelectric actuator applications.     

Thermally‐induced phase transformations in Na0.5Bi0.5TiO3–KNbO3 ceramics

The structures and functional properties of Na_0.5Bi_0.5TiO₃–xKNbO₃ (NBT‐xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high‐resolution synchrotron X‐ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long‐range ordered ferroelectric domains in the poled state. In contrast, the near‐cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of ?175°C. Upon heating, the field‐induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, T_ST, which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field‐induced rhombohedral phase in the poled specimens, the pseudo‐cubic lattice parameter, a_p, exhibited an anomalous reduction while the inter‐axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature‐dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature.     

Electrical properties and relaxor phase evolution of Nb-Modified Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-SrTiO3 lead-free ceramics

In this work, the crystalline phase, domain structure, and electrical properties of [Bi_0.5(Na_0.84K_0.16)_0.5]_0.96Sr_0.04Ti_1-xNb_xO₃ (x = 0.010–0.030) ceramics are investigated. Increasing the Nb content induces the phase transition from coexistent rhombohedral and tetragonal phases to a single pseudo-cubic phase, and the lamellar ferroelectric domains evolve into polar nanoregions. Decreased ferroelectric-to-relaxor transition temperature and enhanced frequency dispersion are found in the temperature-dependent dielectric constant and loss, implying a transition from the non-ergodic to ergodic relaxor state. The Nb substitution significantly degrades the long-range ferroelectric order with sharply decreased piezoelectric coefficients from ? 140 to ? 1 pC/N. However, a large strain of 0.32% at 5 kV/mm (normalized strain of 640 pm/V) is obtained around the critical composition of x = 0.0225. The composition of x = 0.030 shows good temperature insensitivity of the strain response, characterized by 308 pm/V with less than 15% reduction from 25 °C to 125 °C.     

New Lead‐Free (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)ZrO3 Ceramics with High Piezoelectricity

For enhancing the piezoelectric properties of ceramics (Bi_0.5Na_0.5)ZrO₃ (BNZ) was used to partially substitute (K_0.5Na_0.5)NbO₃ (KNN). The addition of BNZ changes the symmetry of KNN ceramics from orthorhombic to tetragonal, and finally to rhombohedral phase. A new phase boundary with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature is identified for KNN–0.050BNZ ceramics, where optimum electrical properties were obtained: d₃₃ = 360 pC/N, k_p = 32.1%, ε_r = 1429, tanδ = 3.5%, and T_C = 329°C. The results indicated a new method for designing high‐performance lead‐free piezoelectric materials.     

Giant electric field-induced strain at room temperature in LiNbO3-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3

We report experimental investigation on the ferroelectricity and electric field-induced strain response in LiNbO₃-doped 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ (BNT-BT) piezoelectric ceramics. At room temperature, a large strain of 0.6% (at 70 kV/cm) is achieved in the 2.5%-LiNbO₃-doped BNT-BT, higher than that of commercially-utilized Pb(Zr,Ti)O₃. The corresponding piezoelectric coefficient d^*₃₃ reaches 857 pm/V, which is high among these of BNT-based ceramics at room temperature. Further study indicates that the superior piezoelectric properties are realized at the ferroelectric-relaxor transition temperature T_F-R, which is pushed to room temperature with 2.5% LiNbO₃ doping. This indicates that large electromechanical response can be induced via delicate mixing of the ferroelectric rhombohedral phase and the polar nanoregions (PNRs) relaxor-ferroelectric tetragonal phase.     

Structure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of BiFeO3‐BaTiO3‐Bi0.5Na0.5TiO3 Lead‐Free Multiferroic Ceramics

Lead‐free multiferroic ceramics of BiFeO₃‐BaTiO₃‐Bi_0.5Na_0.5TiO₃ have been prepared by a conventional ceramic technique. The microstructure, multiferroic, and piezoelectric properties of the ceramics have been studied. The ceramics sintered at 1000°C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and tetragonal phases is formed at x = 0.02. After the addition of Bi_0.5Na_0.5TiO₃, two dielectric anomalies are observed at high temperatures (T_m ~ 510°C–570°C and T₂ ~ 720°C). The phase transition around T_m becomes wider gradually with increasing x. The ferroelectricity, piezoelectricity, and ferromagnetism of the ceramics are significantly improved after the addition of Bi_0.5Na_0.5TiO₃. High resistivity (~1.3 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 27.4 μC/cm²), good piezoelectricity (d₃₃ =140 pC/N, k_p = 31.4%), and weak magnetic properties (M_r =0.19 emu/g) are observed.     

Phase‐dependent radiation‐resistant behavior of BaTiO3: An in situ X‐ray diffraction study

The radiation‐resistant response of BaTiO₃ in the tetragonal and rhombohedral phases on exposure to 100 MeV Ag⁷⁺ ion irradiation was investigated by in situ X‐ray diffraction (XRD) at room temperature (300 K) and low temperature (25 K), respectively. This study revealed that the BaTiO₃ in rhombohedral phase retained crystallinity up to an ion fluence of 1×10¹⁴ ions/cm², whereas tetragonal phase amorphized at much lower fluence viz. 1×10¹³ ions/cm². The in situ XRD along with Raman spectroscopy studies revealed that BaTiO₃ in rhombohedral phase is more radiation resistant than that of tetragonal phase. The density functional theory (DFT) calculations confirmed higher bond strength of rhombohedral phase as compared to tetragonal phase, which supported the experimental result of higher radiation stability of rhombohedral phase. The theoretical predictions on high‐temperature phase will be of relevance to the nuclear waste applications.     

Structural phase transition,electrical and photoluminescent properties of Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 lead-free ferroelectric thin films

Lead-free ferroelectric Pr³⁺-doped (1-x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x?=?0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO₂/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700?°C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr³⁺ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.     

Structural,dielectric and ferroelectric properties of lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics prepared by Plasma Activated Sintering

Lead-free Ba_0.85Ca_0.15Zr_0.10Ti_0.90O₃ (BCZT) ceramics were prepared by Plasma Activated Sintering (PAS). The influence of PAS sintering temperature on the crystalline phase, microstructure, and, dielectric and ferroelectric properties of BCZT ceramics were studied. The phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral and tetragonal phases and then to tetragonal phase as the sintering temperature increased. Microstructural characterization of BCZT ceramics indicated that PAS can obtain a compact microstructure at lower temperatures of 1150–1300 °C compared with that from common pressureless sintering. The BCZT ceramics showed different degrees of diffuseness with increased temperature, and the diffuseness exponents C are all approximately on the order of 10⁵ °C. The dielectric and ferroelectric properties of BCZT ceramics were enhanced with increased sintering temperature. BCZT ceramics sintered at 1250 °C exhibited optimum properties of room-temperature ε_r=2863, ε_m=6650, and 2P_r=25.24 μC/cm², resulting from the relatively higher tetragonal phase content of the MPB between tetragonal and rhombohedral phases together with a compact microstructure.     

Effect of BiMeO3 on the Phase Structure,Ferroelectric Stability,and Properties of Lead‐Free Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics

The effects of BiMeO₃ (Me = Fe, Sc, Mn, Al) addition on the phase transition and electrical properties of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT20) lead‐free piezoceramics were systematically investigated. Results showed that addition of BiFeO₃ into BNKT20 induces a phase transition from tetragonal–rhombohedral coexisted phases to a tetragonal phase with the observation of enhanced piezoelectric properties (d₃₃ = 150 pC/N for 0.02BiFeO₃). BiScO₃, BiMnO₃, and BiAlO₃ substitutions into BNKT20 induce a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point T_F–R to room temperature. Accordingly, large accompanying normalized strains of 0.34%–0.36% are obtained near the ferroelectric–relaxor phase boundary, and the mergence of large strain response can be ascribed to a reversible field‐induced ergodic relaxor‐to‐ferroelectric phase transformation. Moreover, our study also revealed that the composition located at the ferroelectric–relaxor phase boundary where the strain response is consistently derivable shifts to a BNKT20‐rich composition as the tolerance factor t of the end‐member BiMeO₃ increases, and this relationship is expected to provide a guideline for designing high‐performance (Bi_0.5Na_0.5)TiO₃‐based materials by searching the ferroelectric–relaxor phase boundary.     

Enhanced pyroelectric response from domain-engineered lead-free (K0.5Bi0.5TiO3-BaTiO3)-Na0.5Bi0.5TiO3 ferroelectric ceramics

Enhanced pyroelectric response is achieved via domain engineering from [001] grain-oriented, tetragonal-phase, lead-free 0.2(2/3K_0.5Bi_0.5TiO₃-1/3BaTiO₃)-0.8Na_0.5Bi_0.5TiO₃ (KBT-BT-NBT) ceramics prepared by a templated grain growth method. The [001] crystallographic orientation leads to large polarization in tetragonal symmetry; therefore, texturing along this direction is employed to enhance the pyroelectricity. X-ray diffraction analysis revealed a Lotgering factor (degree of texturing) of 93 % along the [001] crystallographic direction. The textured KBT-BT-NBT lead-free ceramics showed comparable pyroelectric figures of merit to those of lead-based ferroelectric materials at room temperature (RT). In addition to the enhanced pyroelectric response at RT, an enormous enhancement in the pyroelectric response (from 1750 to 90,900 μC m^?2 K^?1) was achieved at the depolarization temperature because of the sharp ferroelectric to antiferroelectric phase transition owing to coherent 180° domain switching. These results will motivate the development of a wide range of lead-free pyroelectric devices, such as thermal sensors and infra-red detectors.     

A novel ((Bi0.5Na0.5)0.94Ba0.06)1-x (K0.5Nd0.5)xTiO3 lead-free relaxor ferroelectric ceramic with large electrostrains at wide temperature ranges

Novel ((Bi_0.5Na_0.5)_0.94Ba_0.06)_1-x(K_0.5Nd_0.5)_xTiO₃(x = 0.0, 0.02, 0.04, 0.06) lead-free ceramics (BNBT–xKN) were prepared by the solid-state reaction method. The effects of A-site (K_0.5Nd_0.5)²⁺ complex-ion substitution on their phase structure, dielectric, piezoelectric, and electromechanical properties were studied. The X-ray diffraction results indicate that all compositions are located in the morphotropic phase boundary (MPB) region where the tetragonal phase coexists with the rhombohedral phase. In addition, as the KN content increases, the ferroelectric order transform to relaxor order, which is characterized by a degeneration of maximum polarization, remnant polarization and correspondingly adjusts the ferroelectric-relaxor transformation temperature (T_F-R) to room temperature. Interestingly, the disruption of ferroelectric phase caused a significant improvement of strains. A maximum strain of ~ 0.52% corresponding to normalized strain of ~ 612 pm/V appeared at 85 kv/cm for the x = 0.04 composition. Particularly, the composition of x = 0.04 exhibited high electrostrains of temperature insensitivity, which remained above 0.4% and kept within 10% from ambient temperature up to 110 °C. It can be ascribed to the coexistence of non-ergodic and ergodic states in the relaxor region. As a result, the systematic investigations on the BNBT–xKN ceramics can benefit the developments of temperature-insensitive “on-off” actuators.     

Enhanced electromechanical properties of CaZrO3-modified (K0.5Na0.5)NbO3-based lead-free ceramics

Pairing of large strain response and high d₃₃ with high T_c in (K_0.5Na_0.5)NbO₃-based materials is of high significance in practical applications for piezoelectric actuators. Here, we report remarkable enhancement in the electromechanical properties for (1-x)(K_0.52Na_0.48) (Nb_0.95Sb_0.05)O₃-xCaZrO₃ (KNNS-xCZ) lead-free ceramics through the construction of a rhombohedral (R)-tetragonal (T) phase boundary. We investigated the correlation between the composition-driven phase boundary and resulting ferroelectric, piezoelectric, and strain properties in KNNS-xCZ ceramics. The KNNS-xCZ ceramics with x=0.02 exhibited a large strain response of 0.23% while keeping a relatively large d₃₃ of 237pC/N, which was mainly ascribed to the coexistence of R and T phases confirmed by the XRD and dielectric results. It was found that pairing of large strain response and high d₃₃ in KNN-based materials was achieved. As a consequence, we believe that this study opens the possibility to achieve high-performance lead-free electromechanical compounds for piezoelectric actuators applications.     

Phase transition and energy storage properties of Bi0.5Na0.5TiO3–Bi(Mg2/3Nb1/3)O3 lead-free ceramics

Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics have been extensively studied due to their excellent dielectric, piezoelectric and ferroelectric properties. The phase structure and functionalities of BNT can be feasibly adjusted by doping/forming solid solutions with other elements/components. In this work, Bi(Mg_2/3Nb_1/3)O₃ (BMN) was introduced into BNT by a conventional solid-state reaction to form a homogeneous solid solution of (1-x)(Bi_0.5Na_0.5)TiO₃-xBi(Mg_2/3Nb_1/3)O₃ (BNT-xBMN) with a perovskite structure. With the increase of BMN content, a phase transition from rhombohedral R3c to tetragonal P4bm has been confirmed by XRD, along with shifting the ferroelectric-paraelectric phase transition temperature to lower temperatures with broadening dielectric peaks. Furthermore, an optimized recoverable energy density of 1.405 J/cm³ was achieved for BNT-0.10BMN ceramics under a low applied electric field of 140 kV/cm, which is mainly attributed to the transformation from ferroelectric to ergodic relaxor phase.     

Structural properties of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric ceramics

A systematic XRD investigation of poled and unpoled ceramics of the system (1 ? x) Bi_0.5Na_0.5TiO₃–x BaTiO₃ (0 ≤ x ≤ 0.2) (BNBT) was performed. The variation of the lattice parameters confirms the existence of a morphotropic phase boundary at 0.06 ≤ x ≤ 0.08; however, significant differences in unit cell parameters between poled and unpoled states appear. Lattice distortions of the rhombohedral and tetragonal phases are significantly increased in poled samples. Dramatic changes in peak intensities of the pseudo-cubic (2 0 0) reflections between poled and unpoled samples reveal a strong enhancement of the tetragonal volume fraction in the poled state. Temperature-dependent XRD studies confirm a transition into a cubic high-temperature phase. This transition is rather smooth in the unpoled state. In poled samples, the tetragonal distortion and the tetragonal volume fraction display a different temperature variation and tetragonal regions seem to persist into the cubic phase field.     

Structure and ferroelectric behaviour of Na0.5Bi0.5TiO3-KNbO3 ceramics

Lead-free piezoelectric ceramics, (1?x)Na_0.5Bi_0.5TiO₃-xKNbO₃ (NBT-xKN), with x?=?0.02–0.08 were fabricated by solid-state reaction and sintering. The crystal structures and dielectric properties were measured for different KN contents. All compositions in the unpoled, as-sintered state were found to be single-phase pseudo-cubic. However, typical ferroelectric behaviour, with well-saturated polarisation-electric field hysteresis loops, was observed for certain compositions at high electric field levels. It is shown using high-energy synchrotron X-ray diffraction that the application of the electric field induced an irreversible structural transformation from the nano-polar pseudo-cubic phase to a ferroelectric rhombohedral phase. The changes in lattice elastic strain and crystallographic texture of a poled NBT-0.02KN specimen as a function of the grain orientation, ψ, conform well to those expected for a conventional rhombohedrally distorted perovskite ferroelectric ceramic. The dielectric permittivity-temperature relationships for all compositions exhibit two transition temperatures and a frequency-dependent behaviour that is typical of a relaxor ferroelectric. The transition temperatures and grain size decrease with the increasing KN content.     

Ferroelectric P4mm to relaxor P4bm transition and temperature-insensitive large strains in Bi(Mg0.5Ti0.5)O3-modified tetragonal 0.875Bi0.5Na0.5TiO3-0.125BaTiO3 lead-free ferroelectric ceramics

Tetragonal phase (1–x)(0.875Bi_0.5Na_0.5TiO₃–0.125BaTiO₃)–xBi(Mg_0.5Ti_0.5)O₃ lead-free ferroelectrics were designed and fabricated by a conventional solid state route. All the specimens exhibit a tetragonal perovskite structure, and undergo a phase evolution from ferroelectric P4mm to antiferroelectric relaxor P4bm as the BMT addition increases. The critical composition x?=?0.04 makes a bridge between the both tetragonal phases, and gives a large field-induced strain of 0.30% and an adequately-large electrostrictive coefficient Q₃₃ of 0.0254?m⁴/C². To be highlighted, the field-induce strain of the composition x?=?0.04 shows an almost constant value over the temperature range of 18–100?°C, illustrating a temperature-insensitive behavior, which could be attributed to the widened gap between T_R–E and T_F–R. The temperature-insensitive large strain of the tetragonal BNT–BT–BMT composition give a promising potential for application in precision position actuators.     

Structure,dielectric, and ferroelectric properties of (1-x)Bi0.5Na0.5TiO3 – X K0.5Na0.5NbO3 (0 ≤ x ≤ 0.1) solid solution

The correlation of the phase structure, dielectric, and ferroelectric properties of lead-free (1-x)(Na_0.5Bi_0.5)TiO₃–xK_0.5Na_0.5NbO₃ (NBTKNx) (0 = x ≤ 0.1) polycrystalline ceramics, fabricated via a solid state reaction technique, were investigated. The Rietveld refinement allowed identifying the crystallographic transformation from a rhombohedral to a coexisting rhombohedral-tetragonal or tetragonal long range-ordered ferroelectric (FE) phase. The dielectric investigations showed an increase of the dielectric diffuseness (1.53 = γ ≤ 1.73) and a clear shift of the depolarization temperature (T_d) to a lower temperature while increasing substitution. More importantly, the lattice disorder also generated a plateau-like dielectric anomaly, leading to a thermally stable ϵ_r ∼2859 ± 20% (120–500 °C) and ∼3112 ± 10% (120–420 °C) for x = 0.075 and 0.1 samples, respectively. At room temperature (RT), Raman spectroscopy investigations revealed a downshift of the frequencies as a function of the composition with an inhomogeneous broadening of the Raman lines. On heating, Raman spectra showed changes in the region where the dielectric transitions are observed. Moreover, the composition dependence of the current peaks in the I-E loops confirmed the occurrence of a phase transition from a non-ergodic polar phase to an ergodic weakly polar after the applying of an electric field of 60 kV/cm⁻¹.     

Electric‐Field‐Induced Domain Switching and Domain Texture Relaxations in Bulk Bismuth Ferrite

Bismuth ferrite, BiFeO₃, is an important multiferroic material that has attracted remarkable attention for potential applications in functional devices. While thin films of BiFeO₃ are attractive for applications in nanoelectronics, bulk polycrystalline BiFeO₃ has great potential as a lead‐free and/or high‐temperature actuator material. However, the actuation mechanisms in bulk BiFeO₃ are still to be resolved. Here we report the microscopic origin of electric‐field‐induced strain in bulk BiFeO₃ ceramic by means of in situ high‐energy X‐ray diffraction. Quantification of intrinsic lattice strain and extrinsic domain switching strain from diffraction data showed that the strain response in rhombohedral bulk BiFeO₃ is primarily due to non‐180° ferroelectric domain switching, with no observable change in the phase symmetry, up to the maximum field used in the study. The origin of strain thus differs from the strain mechanism previously shown in thin film BiFeO₃, which gives a similar strain/field ratio as rhombohedral bulk BiFeO₃. A strong post‐poling relaxation of switched non‐180° ferroelectric domains has been observed and hypothesized to be due to intergranular residual stresses with a possible contribution from the conductive nature of the domain walls in BiFeO₃ ceramics.     

Uniaxial compressive stress and temperature dependent mechanical behavior of (1-x)BiFeO3-xBaTiO3 lead-free piezoelectric ceramics

The mechanical behavior of polycrystalline lead-free (1-x)BiFeO₃-xBaTiO₃ (BF-BT) piezoelectric ceramics was investigated under uniaxial compressive stress from room temperature up to 400 °C with macroscopic stress-strain measurements and in situ stress-dependent neutron diffraction. Stress-strain curves revealed a changing mechanical response with BaTiO₃ content and temperature. With decreasing BaTiO₃ content there was an increase in the coercive stress, which reduced the remanent strain and hysteresis. Full pattern structural refinement of the neutron data reveals both rhombohedral distortion and magnetic moment decreases with increasing BaTiO₃ content. In situ stress-dependent neutron diffraction experiments showed that accommodation of external stress occurs through the changes in tilt magnitude and anisotropy of oxygen octahedra at room temperature. The origin of stress-induced strain at room temperature is a lattice deformation without any apparent change in average crystallographic symmetry or domain switching. Temperature-dependent in situ stress-induced measurement of BF-30BT showed maximum strain close to the rhombohedral - pseudocubic transition temperature, which has been proposed to be due to the lattice deformation as well as to the differing degree of tilting of the (Fe/Ti)O₆ octahedra.