Structure–Property Relationships in Solid Solutions of Noncentrosymmetric Aurivillius Phases, Bi4–xLaxTi3O12 (x = 0–0.75)

Enhanced Electrical Conductivity of (001) Oriented Sr0.9La0.1TiO3 Microplatelets for Thermoelectric Applications

Two-dimensional perovskite microplatelets have played an important role in various applications, especially acting as a template to guide grains' epitaxial growth in the preparation of textured ceramics. The (001) oriented Sr0.9La0.1TiO3 microplatelets with a high aspect ratio of ∼20 were synthesized and obtained from Aurivillius Bi4Ti3O12 precursors. To reveal the mechanism of topochemical microcrystal conversion of Bi4Ti3O12 to Sr0.9La0.1TiO3, the reaction interface, morphology development, and phase composition evolution of the (001) oriented Sr0.9La0.1TiO3 microplatelets were investigated. When the temperature of the molten salt is above 753 °C, multiple Sr0.9La0.1TiO3 topological nucleation events took place. At 950 °C, the polycrystalline aggregate of (001)-oriented Sr0.9La0.1TiO3 crystallites grew in place of the original single crystal Bi4Ti3O12 platelets. When the temperature reached 1150 °C, the Sr0.9La0.1TiO3 platelets preserved the shape of a high aspect ratio and exhibited not only enhanced electrical conductivity with a carrier concentration of 3.518 × 1020 cm-3 and carrier mobility of 8.460 cm2·V-1·s-1 but also significantly decreased thermal conductivity ranging from 5.65 W·m-1·K-1 at 300 K to 2.54 W·m-1·K-1 at 1073 K. It can be widely applied in the field of template grain growth methods for preparing textured thermoelectric ceramics to improve their thermoelectric properties.

Annealing-Dependent Morphotropic Phase Boundary in the BiMg0.5Ti0.5O3-BiZn0.5Ti0.5O3 Perovskite System

The annealing behavior of (1-x)BiMg0.5Ti0.5O3−xBiZn0.5Ti0.5O3 [(1-x)BMT−xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT−BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT−BZT system promising for the development of environmentally friendly piezoelectric ceramics.

Lead-Free Aurivillius Phase Bi2LaNb1.5Mn0.5O9: Structure, Ferroelectric, Magnetic, and Magnetodielectric Effects

Aurivillius phase Bi₂LaNb_1.5Mn_0.5O₉, derived from ferroelectric PbBi₂Nb₂O₉ by simultaneous substitution of the A-site and B-site cations, was synthesized using a molten-salt method. Here, we discuss the structure-property relationships in detail. X-ray and neutron diffraction show that Bi₂LaNb_1.5Mn_0.5O₉ adopts an A2₁am orthorhombic crystal structure. Rietveld refinement analysis, supported by Raman spectroscopy, indicates that the Bi³⁺ ions occupy the bismuth oxide blocks, La³⁺ ions occupy the perovskite A-site, and Nb⁵⁺/Mn³⁺ ions occupy the perovskite B-site. Ferroelectric ordering takes place at 535 K, which coexists with local ferromagnetic order below 65 K. The cation disorder on the B-site results in relaxor-ferroelectric behavior, and the short-range ferromagnetic order can be attributed to Mn³⁺/Mn⁴⁺ double-exchange. Magnetodielectric coupling measured at 5 K and 100 kHz in a field of 5 T suggests the existence of intrinsic spin-lattice coupling with a magnetodielectric coefficient of 0.20%. These findings will provide significant impetus for further research into potential devices based on the magnetodielectric effect in Aurivillius materials.

Synergistic Polarization Engineering on Bulk and Surface for Boosting CO2 Photoreduction

Sluggish charge kinetics and low CO₂ affinity seriously inhibit CO₂ photoreduction. Herein, the synchronous promotion of charge separation and CO₂ affinity of Bi₄ Ti₃ O₁₂ is realized by coupling corona poling and surface I-grafting. Corona poling enhances ferroelectric polarization of Bi₄ Ti₃ O₁₂ by aligning the domains direction, which profoundly promotes charge transfer along opposite directions across bulk. Surface I-grafting forms a surface local electric field for further separating charge carriers and provides abundant active sites to enhance CO₂ adsorption. The two modifications cooperatively further increase the ferroelectric polarization of Bi₄ Ti₃ O₁₂ , which maximize the separation efficiency of photogenerated charges, resulting in an enhanced CO production rate of 15.1 μmol g^-1  h^-1 (nearly 9 times) with no sacrificial agents or cocatalysts. This work discloses that ferroelectric polarization and surface ion grafting can promote CO₂ photoreduction in a synergistic way.

Noncentrosymmetric (NCS) solid solutions: elucidating the structure-nonlinear optical (NLO) property relationship and beyond

A systematic approach toward the discovering of novel functional noncentrosymmetric (NCS) materials revealing technologically useful applications is an ongoing challenge. This Frontiers article investigates a series of NCS solid solutions with respect to their crystal structure and second-harmonic generation (SHG) response. The solid solutions include NCS polar aluminoborates, Al_5-xGa_xBO₉ (0.0 ≤ x ≤ 0.5), rare earth element-doped bismuth tellurites, Bi_2-xRE_xTeO₅ (RE = Y, Ce, and Eu; 0.0 ≤ x ≤ 0.2), layered perovskites, Bi_4-xLa_xTi₃O₁₂ (0.0 ≤ x ≤ 0.75: Aurivillius phases) and CsBi_1-xEu_xNb₂O₇ (0.0 ≤ x ≤ 0.2: Dion-Jacobson phases), calcium bismuth oxides, Ca₄Bi_6-xLn_xO₁₃ (Ln = La and Eu; x = 0, 0.06 and 0.12), and sodium lanthanide iodates, NaLa_1-xLn_x(IO₃)₄ (Ln = Sm and Eu; 0 ≤ x ≤ 1). The origin of SHG for all the NCS solid solutions is discussed and the detailed structure-nonlinear optical (NLO) property relationships are elucidated. In addition, photoluminescence (PL) properties and subsequent energy transfer mechanisms for NCS solid solutions are provided.

Determination of crystal symmetry for Bi4Ti3O12-based ferroelectrics by using electron diffraction

In recent years, inconsistent space groups of monoclinicB1a1 and orthorhombicB2cbhave been reported for the room-temperature ferroelectric phases of both Bi4Ti3O12and lanthanide-substituted Bi4Ti3O12. In this article, the electron diffraction technique is employed to unambiguously clarify the crystal symmetries of ferroelectric Bi4Ti3O12and Bi3.15Nd0.85Ti3O12single crystals at room temperature. All the reflections observed from the two crystals match well with those derived fromB1a1, but the observed reflections 010, 030, {\overline 2}10 and {\overline 2}30 should be forbidden in the case ofB2cb. This fact indicates that both the ferroelectrics are of the space groupB1a1 rather thanB2cb, which is confirmed by convergent-beam electron diffraction observations. On the basis of the monoclinic space groupB1a1, the lattice parameters of both the ferroelectrics were calculated by the Rietveld refinement of powder X-ray diffraction data.