The spontaneous polarization of perovskite layer structured Sr2Nb2O7 ferroelectrics (FEs) is originated from mainly the oxygen octahedral rotations and partially the displacement of Sr2+ ions. However, there is FE anomaly of showing the typical characteristics of antiferroelectric (AFE)-like behavior with double polarization-electric field hysteresis loops of Sr2Nb2O7 ceramics by Pb doping. Here, combinations of low frequency and sub-terahertz band dielectric measurements under applied DC field reveal field-induced transition from AFE to FE structure. Temperature dependence of dielectric constant suggested a second-order phase transition near 215 degrees C, which further supported by the lattice parameters and thermal expansion coefficient extracted from the variable temperature X-ray diffraction in the heating and cooling processes. The selected area electron diffraction results show no new superlattice spots are observed along [1 0 0] zone axis related to c direction. We proposed a model based on octahedral tilting/rotation that accounted for the incommensurate lattice modulation in the c direction for Sr2Nb2O7 system by Pb doping. All the results show AFE-like behavior be likely to origin from the electron structures of Pb2+ with a 6s(2) lone pair. Our results gave us a new concept provide possibilities for the design of AFE-like materials in layer structured compounds with super high FE Curie point.

The sodium (Na) and Ce co-doped calcium bismuth titanate (CBT; CaBi4Ti4O15) Aurivillius ceramics in a Ca1-x(Na0.5Ce0.5)xBi(4)Ti(4)O(15) (CNCBT; doping content (x) = 0, 0.03, 0.05, 0.08 and 0.12) system were synthesized by the conventional solid-state sintering method. All compositions show a single-phase orthorhombic (space group: A2(1)am) structure at room temperature. The shift of the Curie point (T-C) towards lower temperatures (T) on doping results from the increased tolerance factor (t). The substitution-enhanced ferroelectric performance with large maximum polarization (P-m) and facilitated domain switching is evidenced by the developed electrical polarization-electric field (P-E) and electrical current-electric field (I-E) hysteresis loops. The piezoelectric coefficient (d(33) = 20.5 +/- 0.1 pC/N) of the x = 0.12 sample is about four times larger than that of pure CBT. The improved piezoelectric properties can be attributed to the high remanent polarization (P-r) and relatively high dielectric permittivity (epsilon'). In addition, multi-sized (micron and sub-micron) domain structures were observed in the CNCBT ceramics by the piezoresponse force microscope (PFM). The multiple-sized ferroelectric domain structure with smaller domains is beneficial to the easy domain switching, enhanced ferroelectric performance, and improved piezoelectric properties of the CNCBT ceramics. The designed Aurivillius-phase ferroelectric ceramics with the T-C around 765 degrees C and high piezoelectric coefficient (d(33)) are suitable for high-temperature piezoelectric applications.

The BIMEVOXes are among the best oxide ion conductors at low and intermediate temperatures. Their high conductivity is associated with local defect structure. In this work, the local structures of two BIMEVOX compositions, Bi2V0.9Ge0.1O5.45 and Bi2V0.95Sn0.05O5.475, are examined using total neutron and X-ray scattering methods, with both compositions exhibiting the ordered α-phase at 25 °C and the disordered γ-phase at 700 °C. While the diffraction data for the α-phase do not allow for the polar (C2) and nonpolar (C2/m) structures to be readily distinguished, measurements of dielectric permittivity suggest the α-phase is weakly ferroelectric in character, consistent with calculations of spontaneous polarization based on a combination of density functional calculations and machine learning methodology. Reverse Monte Carlo (RMC) analysis of total scattering data reveals Ge preferentially adopts tetrahedral geometry at both temperatures, while Sn is found to predominantly adopt octahedral coordination in the α-phase and tetrahedral coordination in the γ-phase. In all cases, V polyhedra are found to consist of tetrahedral, pentacoordinate, and octahedral geometries, as also predicted by the crystallographic analysis and confirmed by 51V solid state NMR spectroscopy. Although similar long-range structures are observed at room temperature, the oxide ion vacancy distributions were found to be quite different between the two studied compositions, with a nonrandom deficiency in vacancy pairs in the second-nearest shell along the ⟨100⟩ tetragonal direction for BIGEVOX10, compared with a long-distance (>8.0 Å) ordering of equatorial vacancies for BISNVOX05. This is attributed to the differences in the preferred coordination geometries of the substituent cations in the two systems. Impedance spectroscopy measurements reveal both compositions show high conductivity in the order of 10-1 S cm-1 at 600 °C.

Multiferroics are materials that exhibit two or more primary ferroic properties within the same phase and have potential applications in sensors, spintronics and memory devices. Here, the dielectric, ferroelectric and mag-netic properties of novel multiferroics derived from BaTi1_x(Fe0.5Nb0.5)xO3 (BTFN, 0.01 <= x <= 0.10) ceramics are investigated. Multiferroism in these ceramics is manifested by the coexistence of ferroelectric long-range ordering and antiferromagnetism. With increasing x-value, there is a structural evolution from a tetragonal perovskite to a mixture of tetragonal and cubic phases, accompanied by a decrease in the temperature of maximum permittivity. At room temperature, ferroelectric behaviour is evidenced by the presence of current peaks corresponding to domain switching in the current-electric field loops, while the observation of non-linear narrow magnetic hysteresis loops suggests dilute magnetism. The results indicate that in the x = 0.07 compo-sition the antiferromagnetic order is established through an indirect super-exchange interaction between adja-cent Fe ions.

•Room-temperature single phase multiferroic materials was obtained.•Magnetoelectric coupling was observed at microwave (GHz) frequency.•The coupling suggests a dynamic interaction between magnetic moments and ferroelectric polarization.

Solventless mechanochemical synthesis by manual grinding was applied to grow organic proton conducting salts, imidazole-succinic acid (C3H4N2-HOOC(CH2)2COOH) and imidazole-glutaric acid (C3H4N2-HOOC(CH2)3COOH). This synthesis method induces crystallization and provides the phase-pure compounds. The compounds exhibit different electric conducting behavior and activation energies Ea compared with the reported single crystals obtained from the solution method. The difference in conducting property can be related to intrinsic defects and structural disorder introduced by mechanochemical grinding, indicating that the mechanochemical method bears strong capability for tuning conductivities. Moreover, complete deuteration of the organic salts is achieved by the method. The mechanochemical synthesis of organic salts also holds high potential for the actual industrialized large-scale production.

Total neutron and X-ray scattering and 51 V solid-state NMR reveal details of the local structure in the BIGEVOX oxide ion conductor. A non-random deficiency in next-nearest-neighbour vacancy pairs in the 〈100〉 tetragonal direction is found.

While BIMEVOX systems have attracted attention for their fast oxide-ion conductivity at intermediate temperatures, there are only a limited number of reports concerning their local structure. In this work, both long-range and local structures in the Bi2V1-xGaxO5.5-x-delta (BIGAVOX, 0.025 <= x <= 0.40) system are investigated using X- ray powder diffraction (XRD) and a combination of X-ray total scattering, V-51 and Ga-71 solid-state nuclear magnetic resonance (NMR), and Raman spectroscopy, supported by electrical measurements using a.c. impedance spectroscopy. The three main BIMEVOX polymorphs, alpha, beta, and gamma, are observed at room temperature over the compositional ranges 0.025 <= x < 0.10, 0.10 <= x < 0.20, and 0.20 <= x < 0.40, respectively. Above x = 0.10, as more Ga is introduced into the lattice, a general growth of the distance between bismuthate and vanadate layers is observed, indicating increasing ionicity in the interaction between these layers. Ga is found to adopt octahedral and tetrahedral geometries, while V polyhedra include tetrahedral, pentacoordinate, and octahedral geometries. With increasing x-value, as the vacancy concentration increases, more octahedral V polyhedra transform to lower coordinate geometries, resulting in a decrease in the average V-O bond length. Reversible alpha <-> beta and beta <-> alpha phase transitions are observed on heating the x = 0.05 composition, while the beta <-> gamma and gamma '<-> gamma phase transitions are observed on heating the x = 0.15 and 0.20 compositions, respectively. The gamma-BIGAVOX compositions (x = 0.20 and 0.25) generally show a high conductivity of similar to 10(-2) S cm(-1) at 600 degrees C.

A single phase four-layer Aurivillius structured ferroelectric ceramic, (Ca0.2Sr0.2Ba0.2Pb0.2Nd0.1Na0.1)Bi4Ti4O15 (6ABTO) was obtained using a high entropy design concept. The material, which has orthorhombic symmetry in space group A21am at room temperature, has Ca2+, Sr2+, Ba2+, Pb2+, Nd3+ and Na+ distributed not only on the A sites of the perovskite layer but also in the bismuthate layer. 6ABTO shows complex ferroelectric behavior, with a Curie point of 557 °C. Four current peaks are observed in the current - electric field curve. These peaks are attributed to a combination of a field induced phase transition and domain wall switching, which is the first reported occurrence of such current - electric field behavior for an Aurivillius structured ferroelectric material. Despite the level of disorder between the A site cations in the perovskite layer and the Bi positions in the bismuthate layer, 6ABTO does not show the relaxor ferroelectric behavior, that is commonly observed in cases of such disorder. This suggests that relaxor behavior in ferroelectrics, may be more associated with the thermal stability of dipoles, rather than the presence of polar nano-regions formed as a result of chemical disorder. The nature of the high entropy effect in 6ABTO is discussed through comparison of results from isostructural compositions containing 5, 4 and 3 of the component cations

The production of high‐quality BiFeO3 thin films on cost‐effective transparent electrodes for visible light harvesting applications and devices remains a challenge. Here, the production of single‐phase nanostructured BiFeO3 thin films via chemical solution deposition (CSD) on transparent conductive fluorine‐doped tin oxide FTO glass substrates is reported. It is shown that BiFeO3 is of high purity using a variety of analytical tools and that the as‐obtained BiFeO3 thin films have a single‐grain single‐domain structure exhibiting ferroelectric switching under poling. The BiFeO3 samples show visible light absorption with a bandgap of 2.7 eV under all processing conditions. By changing the annealing atmosphere, it was possible to modify the photocurrent produced, which were (at 1.23 VNHE) 0.07 (O2‐annealed), 0.02 (air‐annealed), and 0.01 mA cm−2 (Ar‐annealed). This indicates a change in the mobile carriers available. The results show that it is possible to produce single‐phase BiFeO3 on a transparent conductive electrode system with controllable photoconductivity.

The concept of high entropy ceramics opens up the possibility to optimise the properties of relaxor ferroelectrics with Aurivilius phase structures. In this paper, two new multi-element substituted Aurivillius phase ceramics, (Ca0.25Sr0.25Ba0.25Pb0.25)Bi2Nb2O9 and (Ca0.2Sr0.2Ba0.2Pb0.2Nd0.1Na0.1)Bi2Nb2O9 are prepared. Both ceramics are single phase, exhibiting orthorhombic symmetry with space group A21am at room temperature. The frequency dependence of the temperature of the dielectric permittivity peak, Tm, shows relaxor behaviour for both ceramics. Multi-domain configurations, including long range ordered ferroelectric domains and nano domains, were observed using piezoresponse force microscopy (PFM). Under an applied DC field, the nano domains irreversibly transformed to micro-meter sized domains, which is consistent with the Vogel-Fulcher Law fitted results that show that the freezing temperatures of the two ceramics are above room temperature. Their improved piezoelectric constant is attributed to their low coercive fields, which are related to the multi domain configurations in the Aurivillius phase relaxor ferroelectrics.

Grain size can have significant effects on the properties of electroceramics for dielectric, piezoelectric, and ferroelectric applications. Here, we systematically investigate the effect of grain size on the structure and properties of Mn-modified 0.67BiFeO3-0.33BaTiO3 ceramics, an important lead-free piezoelectric ceramic that exhibits both a high piezoelectric coefficient and a high Curie point. Ceramics with average grain sizes ranging from 0.46 to 6.85 μm were prepared using conventional and spark plasma sintering. It was found that the morphotropic phase boundary compositions are composed of two polar structures, rhombohedral and tetragonal, with DC poling inducing an increase in the fraction of the rhombohedral phase. All ceramics show relaxor behavior and their freezing temperature moves to higher temperatures with increasing grain size, although their Burns temperature is independent of grain size. In fine-grained ceramics, which show pronounced relaxor behavior, significant grain size dependency is seen in dielectric, piezoelectric, and ferroelectric properties, which is attributed to the presence of single ferroelectric domains and high concentrations of polar nanoregions. In coarse-grained ceramics, a critical grain size of 2.83 μm yields the highest dielectric permittivity at room temperature, with the piezoelectric coefficient plateauing at this grain size, which can be attributed to the contribution of both polar nanoregions and high domain wall density.

Ferroelectric domain walls (DWs) are important nanoscale interfaces between two domains. It is widely accepted that ferroelectric domain walls work idly at terahertz (THz) frequencies, consequently discouraging efforts to engineer the domain walls to create new applications that utilize THz radiation. However, the present work clearly demonstrates the activity of domain walls at THz frequencies in a lead-free Aurivillius phase ferroelectric ceramic, Ca0.99Rb0.005Ce0.005Bi2Nb2O9, examined using THz-time-domain spectroscopy (THz-TDS). The dynamics of domain walls are different at kHz and THz frequencies. At low frequencies, domain walls work as a group to increase dielectric permittivity. At THz frequencies, the defective nature of domain walls serves to lower the overall dielectric permittivity. This is evidenced by higher dielectric permittivity in the THz band after poling, reflecting decreased domain wall density. An elastic vibrational model has also been used to verify that a single frustrated dipole in a domain wall represents a weaker contribution to the permittivity than its counterpart within a domain. The work represents a fundamental breakthrough in understanding the dielectric contributions of domain walls at THz frequencies. It also demonstrates that THz probing can be used to read domain wall dielectric switching.

A WC/RbBi2Ti2NbO10 nanocomposite exhibits excellent photocatalytic behaviour, which is attributed to its ferroelectric nature and the loaded WC as a cocatalyst.

A high entropy dopant approach has been used to prepare a new BIMEVOX ceramic system, Bi2V1x(Mg0.25Cu0.25Ni0.25Zn0.25)(x)O5.5-3x/2. Structures were investigated using a combination of X-ray and neutron powder diffraction, with electrical characterisation by A.C. impedance spectroscopy. A gamma-type phase is observed at room temperature over the compositional range 0.10 <= x <= 0.30, the upper limit of which is beyond that seen for all the single substituted systems based on these substituents, apart from BIMGVOX. No stabilisation of the fully disordered gamma-phase is seen at room temperature over this compositional range, with only the incommensurately ordered gamma-phase evident below around 450 degrees C. Changes in defect structure are used to explain an apparent transition in the compositional variation of lattice parameters. The HE dopant approach has no detrimental effect on ionic conductivity, with values comparable to those of the single substituted systems based on the component oxides.

The development of lead-free ferroelectric ceramics exhibiting a high Curie point is regarded as a key aspect of improving the overall performance of high temperature sensing devices. Two-layer Dion-Jacobson phases have great potential for these applications, but the dielectric properties of many of these compounds have not been investigated and several are still erroneously considered to be non-polar. RbPrNb2O7, a two-layer Dion-Jacobson phase, has previously been identified as being non-polar at room temperature, with an orthorhombic crystal structure in space group Imma. Here, the structure of RbPrNb2O7 is re-examined and its ferroelectric (FE) character confirmed through ferroelectric, dielectric and piezoelectric measurements, using dense bulk ceramics. In addition, the microstructure and phase evolution of the compound were analysed by transmission electron microscopy, variable temperature X-ray powder diffraction and differential scanning calorimetry. RbPrNb2O7 is a ferroelectric at room temperature, exhibiting orthorhombic symmetry in space group I2cm. It undergoes a reversible FE ↔ FE phase transition at around 260 °C, with evidence for a diffuse apolar transition between 800 and 900 °C. Details of the subtle structural changes occurring at these transitions are discussed. RbPrNb2O7 possesses a high Curie point of ~1090 °C, making it a potential candidate for use in high temperature piezoelectric sensing.

Dielectric materials, with high tunability at microwave frequencies, are key components in the design of microwave communication systems. Dense Ba0.6Sr0.4TiO3 (BST) ceramics, with different grain sizes, were prepared in order to optimise the dielectric tunability via polar nano cluster effects. Dielectric permittivity and loss measurements were carried at both high and low frequencies and were supported by results from X-ray powder diffraction, scanning and transmission electron microscopies, Raman spectroscopy and piezoresponse force microscopy. The concentration of polar nano clusters, whose sizes are found to be in the range 20 to 50 nm, and the dielectric tunability increase with increasing grain size. A novel method for measurement of the microwave tunability in bulk dielectrics is presented. The highest tunability of 32% is achieved in ceramics with an average grain size of 10 um. The tunability of BST ceramics with applied DC field is demonstrated in a prototype small resonant antenna.

In order to improve the stability of silicon-based heavy metals adsorbents and reduce ions release (Zn, Mg, etc) in the ion exchange process, a novel adsorbent with excellent heavy metal adsorption was synthesized by loading sodium isobutyl xanthate (SIBX) onto the surface of silicon-zinc mesoporous (SZM) material. For comparison, two silicon-zinc materials using silicate (Na2SiO3, inorganic silicon) and tetraethyl orthosilicate (C8H20O4Si, organic silicon) were synthesized separately. Among them, inorganic silicon-zinc mesoporous (SZM-1) material was used to verify its ions exchange properties, organic silicon-zinc mesoporous (SZM-2) material acts as matrix and modified with SIBX. The copper sulfate solution was employed to evaluate the adsorption ability of as-prepared adsorbents. Above materials were characterized by a set of complementary techniques: SEM-EDS, XRD, TEM, BET, FT-IR, and XPS. The analyses prove that the SIBX can be attached to the surface of the SZM-2, the presence of SIBX can simultaneously immobilize Zinc atoms and adsorb Copper ions.

Nanomaterials play a significant role in adsorption treatment of dye wastewater, but irreversible aggregation of nanoparticles poses a significant problem. In this work, nanomesoporous zinc-doped silicate (NMSZ) was prepared by an in situ method. To prevent agglomeration, NMSZ was covalently bonded to graphene oxide (GO) sheets to form a nano-silica/zinc/graphene oxide composite (GO-NMSZ), aimed at removal of cationic dye methylene blue (MB). For comparison, undoped mesoporous silica (MS) was also synthesized and modified to obtain a silica/graphene oxide composite (GO-MS). The materials were characterized by powder XRD, SEM, FTIR spectroscopy, TEM, nitrogen sorption, and X-ray photoelectron spectroscopy (XPS). Preservation of the oxygen-containing groups of GO in the composites led to higher adsorption capacities. The best GO-NMSZ composite exhibited an enhanced adsorption capacity of 100.4 mg g(-1) for MB compared to those of undoped GO-MS (80.1 mg g(-1)) and nongrafted NMSZ (55.7 mg g(-1)). The nonselective character of GO-NMSZ is demonstrated by effective adsorption of anionic dye Congo red (127.4 mg g(-1)) and neutral dye isatin (289.0 mg g(-1)). The adsorption kinetics, adsorption isotherms, and a thermodynamic study suggested that MB adsorption occurs by chemisorption and is endothermic in nature.