The effect of cation size on structure and properties of Ba-based tetragonal tungsten bronzes Ba4M2Nb10O30 (M = Na, K or Rb) and Ba4M2Nb8Ti2O30 (M = Ca or Sr).

The second largest family of oxide ferroelectrics, after perovskites, are the tetragonal tungsten bronzes (TTB) with the general formula A24A12C4B12B28O30. Cation disorder in TTBs is known to occur if the size difference between cations is small, but the impact of cation disorder on structure and properties has not yet been extensively addressed. In this study we investigate the effect of the size of the M cation, including cation disorder, on the crystal structure and dielectric properties in the two series Ba4M2Nb10O30 (BMN, A = Na, K and Rb) and Ba4M2Nb8Ti2O30 (BMNT, M = Ca, Sr). Dense and phase pure ceramics in the two series were prepared by a two-step solid state synthesis route. The crystal structures of the materials were characterized by powder X-ray diffraction combined with Rietveld refinement. A close to linear relation between the in-plane lattice parameter (a) and the size of the M-cation were observed. Ba4M2Nb8Ti2O30 was shown to possess cation disorder on the A-sites in line with previous work on Ba4M2Nb10O30. Thermodynamic calculations from density functional theory also indicated a drive for cation disorder in the three BMN compositions. Non-ambient temperature X-ray diffraction revealed contraction of the in-plane (a) and expansion of the out-of-plane (c) lattice parameters at the ferroelectric phase transition for Ba4M2Nb10O30. The ferroelectric transition temperature acquired by dielectric spectroscopy showed a systematically increasing TC with decreasing size of the M-cation within both compositional series studied. The compositional dependence of TC is discussed with respect to the size of the M-cation, cation disorder and the tetragonality, as well as the Ti-content. The relaxor to ferroelectric properties observed by polarization-electric field hysteresis loops are discussed in relation to the relative size of cations on the on A1 and A2 sites and the Ti-content.

Cation Disorder in Ferroelectric Ba4M2Nb10O30 (M = Na, K, and Rb) Tetragonal Tungsten Bronzes.

The crystal structure of tetragonal tungsten bronzes, with the general formula A12A24C4B12B28O30, is flexible both from a chemical and structural viewpoint, resulting in a multitude of compositions. The A1 and A2 lattice sites, with different coordination environments, are usually regarded to be occupied by two different cations such as in Ba4Na2Nb10O30 with Na+ and Ba2+ occupying the A1 and A2 sites, respectively. Here, we report on a systematic study of the lattice site occupancy on the A1 and A2 sites in the series Ba4M2Nb10O30 (M = Na, K, and Rb). The three compounds were synthesized by a two-step solid-state method. The site occupancy on the A1 and A2 sites were investigated by a combination of Rietveld refinement of X-ray diffraction patterns and scanning transmission electron microscopy with simultaneous energy-dispersive spectroscopy. The two methods demonstrated consistent site occupancy of the cations on the A1 and A2 sites, rationalized by the variation in the size of the alkali cations. The cation order-disorder phenomenology in the tungsten bronzes reported is discussed using a thermodynamic model of O'Neill and Navrotsky, originally developed for cation interchange in spinels.

Understanding the Hydrothermal Formation of NaNbO3: Its Full Reaction Scheme and Kinetics.

Sodium niobate (NaNbO3) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb2O5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa7Nb6O19·15H2O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na2Nb2O6·H2O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO3. The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution-precipitation mechanism.

Experimental setup for high-temperature in situ studies of crystallization of thin films with atmosphere control.

Understanding the crystallization process for chemical solution deposition (CSD) processed thin films is key in designing the fabrication strategy for obtaining high-quality devices. Here, an in situ sample environment is presented for studying the crystallization of CSD processed thin films under typical processing parameters using near-grazing-incidence synchrotron X-ray diffraction. Typically, the pyrolysis is performed in a rapid thermal processing (RTP) unit, where high heating rates, high temperatures and atmosphere control are the main control parameters. The presented in situ setup can reach heating rates of 20°C s-1 and sample surface temperatures of 1000°C, comparable with commercial RTP units. Three examples for lead-free ferroelectric thin films are presented to show the potential of the new experimental set-up: high temperature, for crystallization of highly textured Sr0.4Ba0.6Nb2O6 on a SrTiO3 (001) substrate, high heating rate, revealing polycrystalline BaTiO3, and atmosphere control with 25% CO2, for crystallization of BaTiO3. The signal is sufficient to study a single deposited layer (≥10 nm for the crystallized film) which then defines the interface between the substrate and thin film for the following layers. A protocol for processing the data is developed to account for a thermal shift of the entire setup, including the sample, to allow extraction of maximum information from the refinement, e.g. texture. The simplicity of the sample environment allows for the future development of even more advanced measurements during thin-film processing under non-ambient conditions.

Controlled Growth of Srx Ba1-x Nb2 O6 Hopper- and Cube-Shaped Nanostructures by Hydrothermal Synthesis.

Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material Srx Ba1-x Nb2 O6 (x=0.32-0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube-shaped to hollow-ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X-ray total scattering and PDF analysis was used to reveal the structure of the Nb-acid precursor, showing Lindqvist-like motifs. The changing growth mechanism, from layer-by-layer growth forming cubes to hopper-growth giving hollow-ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow-ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth.

A Fast, Low-Temperature Synthesis Method for Hexagonal YMnO3 : Kinetics, Purity, Size and Shape as Studied by In Situ X-ray Diffraction.

The reaction mechanisms, phase development and kinetics of the hydrothermal synthesis of hexagonal-YMnO3 from Y2 O3 and Mn2 O3 using in situ X-ray diffraction are reported under different reaction conditions with temperatures ranging from 300 to 350 °C, and using 1, 5 and 10 m KOH, and 5 m NaOH mineraliser. Reactions initiated with Y2 O3 hydrating to Y(OH)3 , which then dehydrated to YO(OH). Higher temperatures and KOH concentrations led to faster, more complete dehydrations. However, 1 m KOH led to YO(OH) forming concurrently with Y(OH)3 before Y(OH)3 fully dehydrated but yielded a very low phase purity of hexagonal-YMnO3 . Using NaOH mineraliser, no YO(OH) was observed. Dehydration also initiated at a higher temperature in the absence of Mn2 O3 . The evolution of Rietveld refined scale factors was used to determine kinetic information and approximate activation energies for the reaction. The described hydrothermal synthesis offers a fast, low-temperature method for producing anisometric h-YMnO3 particles.

Structural Evolution of Ferroelectric and Ferroelastic Barium Sodium Niobate Tungsten Bronze.

The crystal structure of the ferroelastic and ferroelectric tungsten bronze Ba2NaNb5O15 (BNN) has been debated. Here, we re-examine the crystal structure of BNN by ambient powder X-ray diffraction combined with density functional theory calculations. We demonstrate that the room temperature space group is Cmm2 with significant cation disorder on the Ba and Na cation sublattices. Density functional theory calculations reveal a relatively flat energy landscape between structures of different symmetries, including the energetics of cation disorder. We also study the structural evolution and the ferroelectric and ferroelastic phase transitions by high-temperature X-ray diffraction and dilatometry. The ferroelectric phase transition at 570 °C is of first order and cause the cell to expand in the c direction, while the ferroelastic distortion starting at 270 °C takes place in the ab plane and does not affect the polarization. The phase transitions are not coupled, which means that BNN is a ferroic material with two primary and uncoupled order parameters.

Tracer diffusion of 96Zr and 134Ba in polycrystalline BaZrO3.

Cation tracer diffusion in polycrystalline cubic BaZrO3 perovskites was studied using the stable isotopes 134Ba and 96Zr in air at 1015-1200 and 1300-1500 °C, respectively. Thin films of 134BaO and 96ZrO2 were deposited on polished BaZrO3 pellets by drop casting of aqueous precursor solutions containing the tracers. Isotope distribution profiles were recorded using secondary ion mass spectrometry. All the depth profiles exhibited two distinct regions, which enabled the assessment of both lattice and grain boundary diffusion using Fick's second law and Whipple-Le Clair's equation. The grain boundary diffusion of both cations was several orders of magnitude higher than the lattice diffusion. The lattice diffusion of Ba2+ was found to be significantly faster than the lattice diffusion of Zr4+, while the activation energies were comparable, respectively 395 ± 44 and 435 ± 67 kJ mol-1. Activation energies for the diffusion of Ba2+ and Zr4+ through a Ba2+ vacancy were calculated by density functional theory using the nudged elastic band method. The calculated and experimental activation energies were in excellent agreement. The cation diffusion data in BaZrO3 are compared to previous data on A and B-site diffusivity in perovskites. Finally, the diffusivity of Zr4+ in compounds with perovskite and fluorite crystal structures is discussed in relation to the chemical stability of BaZrO3-based materials.

Direct observation of ferroelectric field effect and vacancy-controlled screening at the BiFeO3/LaxSr1-xMnO3 interface.

The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.

On the energetics of cation ordering in tungsten-bronze-type oxides.

Oxides with the tetragonal tungsten bronze (TTB) structure are well-known ferroelectrics that show a large flexibility both with respect to chemical composition and cation ordering. Two of the simplest compounds in this family are lead metaniobate (PbNb2O6 or PN) and strontium barium niobate (SrxBa1-xNb2O6 or SBN). While PN is a classical ferroelectric, SBN goes from ferroelectric to relaxor-like with increasing Sr content, with a polar direction different from that in PN. The partially occupied sublattices in both systems give the possibility for cation order-disorder phenomena, but it is not known if or how this influences the polarization and ferroelectricity. Here, we use density functional theory (DFT) calculations to investigate how cation and cation vacancy ordering influences the energetics of these compounds, by comparing both the energy differences and the barriers for transition between different cation configurations. We extend the thermodynamic model of O'Neill and Navrotsky, originally developed for cation interchange in spinels, to describe the order-disorder phenomenology in TTB oxides. The influence of order-disorder processes on the functional properties of PN and SBN is discussed.

1D oxide nanostructures from chemical solutions.

Nanotechnology has motivated a tremendous effort in the synthesis approaches to grow free standing or hierarchical nanomaterials such as nanowires and nanorods. Bottom-up approaches based on chemistry are an important approach to produce nanomaterials, and here the concepts of growing oxide 1D nanostructures from chemical solutions are reviewed. The thermodynamic and kinetic aspects of the nucleation and growth of oxide compounds in solutions are presented with emphasis on hydrothermal and molten salt synthesis. The importance of solubility of precursors, the precursor chemistry, role of organic additives as well as the chemical complexity and dimensionality and symmetry of the crystal structure of the compound grown are highlighted.

Revisiting the crystal structure of rhombohedral lead metaniobate.

Lead metaniobate (PbNb2O6) can exist both as a stable rhombohedral and a metastable orthorhombic tungsten-bronze-type polymorph. Although the orthorhombic is a well-known ferroelectric material, the rhombohedral polymorph has been far less studied. The crystal structure and energetic stability of the stable rhombohedral polymorph of lead metaniobate is re-examined by powder X-ray diffraction and powder neutron diffraction in combination with ab initio calculations. We show that this structure is described by the polar space group R3, in contradiction to the previously reported space group R3m. The crystal structure is unusual, consisting of edge-sharing dimers of NbO(6/2) octahedra forming layers with 6- and 3-fold rings of octahedra and lead ions in channels formed by these rings. The layers are connected by corner-sharing between octahedra. Finally, the crystal structure is discussed in relation to other AB2O6 compounds with B = Nb, Ta.

Oxidation Kinetics of Nanocrystalline Hexagonal RMn1-xTixO3 (R = Ho, Dy).

Hexagonal manganites, RMnO3 (R = Sc, Y, Ho-Lu), are potential oxygen storage materials for air separation due to their reversible oxygen storage and release properties. Their outstanding ability to absorb and release oxygen at relatively low temperatures of 250-400 °C holds promise of saving energy compared to current industrial methods. Unfortunately, the low temperature of operation also implies slow kinetics of oxygen exchange in these materials, which would make them inefficient in applications such as chemical looping air separation. Here, we show that the oxidation kinetics of RMnO3 can be improved through Ti4+-doping as well as by increasing the rare earth cation size. The rate of oxygen absorption of nanocrystalline RMn1-xTixO3 (R = Ho, Dy; x = 0, 0.15) was investigated by thermogravimetric analysis, X-ray absorption near-edge structure, and high-temperature X-ray diffraction (HT-XRD) with in situ switching of atmosphere from N2 to O2. The kinetics of oxidation increases for larger R and even more with Ti4+ donor doping, as both induce expansion of the ab-plane, which reduces the electrostatic repulsion between oxygen in the lattice upon oxygen ion migration. Surface exchange rates and activation energies of oxidation were determined from changes in lattice parameters observed through HT-XRD upon in situ switching of atmosphere.

High Velocity, Low-Voltage Collective In-Plane Switching in (100) BaTiO3 Thin Films.

Ferroelectrics are being increasingly called upon for electronic devices in extreme environments. Device performance and energy efficiency is highly correlated to clock frequency, operational voltage, and resistive loss. To increase performance it is common to engineer ferroelectric domain structure with highly-correlated electrical and elastic coupling that elicit fast and efficient collective switching. Designing domain structures with advantageous properties is difficult because the mechanisms involved in collective switching are poorly understood and difficult to investigate. Collective switching is a hierarchical process where the nano- and mesoscale responses control the macroscopic properties. Using chemical solution synthesis, epitaxially nearly-relaxed (100) BaTiO3 films are synthesized. Thermal strain induces a strongly-correlated domain structure with alternating domains of polarization along the [010] and [001] in-plane axes and 90° domain walls along the [011] or [01 1 ¯ $\bar{1}$ ] directions. Simultaneous capacitance-voltage measurements and band-excitation piezoresponse force microscopy revealed strong collective switching behavior. Using a deep convolutional autoencoder, hierarchical switching is automatically tracked and the switching pathway is identified. The collective switching velocities are calculated to be ≈500 cm s-1 at 5 V (7 kV cm-1 ), orders-of-magnitude faster than expected. These combinations of properties are promising for high-speed tunable dielectrics and low-voltage ferroelectric memories and logic.

Removing Fluoride-Terminations from Multilayered V2CT x MXene by Gas Hydrolyzation.

Two-dimensional MXenes have shown great promise for many different applications, but in order to fully utilize their potential, control of their termination groups is essential. Here we demonstrate hydrolyzation with a continuous gas flow as a method to remove F-terminations from multilayered V2CT x particles, in order to prepare nearly F-free and partly bare vanadium carbide MXene. Density functional theory calculations demonstrate that the substitution of F-terminations is thermodynamically feasible and presents partly nonterminated V2CO as the dominating hydrolyzation product. Hydrolyzation at elevated temperatures reduced the F content but only subtly changed the O content, as inferred from spectroscopic data. The ideal hydrolyzation temperature was found to be 300 °C, as a degradation of the V2CT x phase and a transition to vanadium oxycarbides and V2O3 were observed at higher temperature. When tested as electrodes in Li-ion batteries, the hydrolyzed MXene demonstrated a reduced polarization compared with the pristine MXene, but no change in intercalation voltage was observed. Annealing in dry Ar did not result in the same F reduction, and the importance of water vapor was concluded, demonstrating hydrolyzation as a new and efficient method to control the surface terminations of multilayered V2CT x post etching. These results also provide new insights on the thermal stability of V2CT x MXene in hydrated atmospheres.

Structures and Role of the Intermediate Phases on the Crystallization of BaTiO3 from an Aqueous Synthesis Route.

Carbonate formation is a prevailing challenge in synthesis of BaTiO3, especially through wet chemical synthesis routes. In this work, we report the phase evolution during thermal annealing of an aqueous BaTiO3 precursor solution, with a particular focus on the structures and role of intermediate phases forming prior to BaTiO3 nucleation. In situ infrared spectroscopy, in situ X-ray total scattering, and transmission electron microscopy were used to reveal the decomposition, pyrolysis, and crystallization reactions occurring during thermal processing. Our results show that the intermediate phases consist of nanosized calcite-like BaCO3 and BaTi4O9 phases and that the intimate mixing of these along with their metastability ensures complete decomposition to form BaTiO3 above 600 °C. We demonstrate that the stability of the intermediate phases is dependent on the processing atmosphere, where especially enhanced CO2 levels is detrimental for the formation of phase pure BaTiO3.

Hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 below 250 °C.

The hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 are reported using high KOH mineraliser concentrations (>10 M) and low temperatures (<240 °C). The relation between reaction parameters and resulting phase purity were mapped by ex situ and in situ X-ray diffraction. Excess Y2O3 resulted in two-phase product with hexagonal YMnO3 with different lattice parameters. An unusual microstructure was observed in which particles have a hexagonal shape with a highly crystalline edge and either a hollow or polycrystalline interior. An Ostwald ripening mechanism was proposed to explain this phenomenon. Solid-state reactions and density functional theory calculations were performed to determine plausible defect chemistry which can lead to the observed phases with different lattice parameters.

In Vitro Biocompatibility of Piezoelectric K0.5Na0.5NbO3 Thin Films on Platinized Silicon Substrates.

Lead-free piezoelectric ceramics like K0.5Na0.5NbO3 (KNN) represent an emerging class of biomaterials for medical technology, as they can be used as components in implantable microelectromechanical systems (MEMS) and bioactive scaffolds for tissue stimulation. Such functional materials can act as working components in future in vivo devices, and their addition to current implant designs can greatly improve the biological interaction between host and implant. Despite this, only a few reports have studied the biocompatibility of these materials with living cells. In this work, we investigate the biological response of two different cell lines grown on KNN thin films, and we demonstrate excellent biocompatibility of the KNN films with the cells. Undoped and 0.5 mol % CaTiO3-doped KNN thin films with nanometer-sized roughness were deposited on platinized silicon (SiPt) substrates, and cell proliferation, viability, and morphology of human 161BR fibroblast cells and rat Schwann cells grown on the KNN films and SiPt substrates were investigated and compared to glass control samples. The results show that proliferation rates for the cells grown on the KNN thin films were equally high or higher than those on the glass control samples, and no cytotoxic effect from either the films or the substrate was observed. The work demonstrates that KNN thin films on SiPt substrates are very promising candidates for components in implantable medical devices.

Synthesis of KNbO3 nanorods by hydrothermal method.

Potassium niobate (KNbO3) nanorods were prepared from Nb2O5 powder by hydrothermal synthesis in KOH solution at 180 degrees C for 48 h using sodium dodecyl sulfate surfactant. The products were characterized by X-ray diffraction as well as scanning and transmission electron microscopy. The KNbO3 nanorods were shown to have orthorhombic crystal structure and were 100-300 nm in diameter and up to 5 microm long. The addition of surfactant changed the product morphology from agglomerated particles to nanorods. A possible mechanism for the formation of the KNbO3 nanorods is briefly discussed.

La(Ni(2/3)Nb(1/3))O(3) by neutron powder diffraction.

Lanthanum nickel niobium trioxide has been synthesized and its structure refined for the first time. It was found to be a member of the family of technologically important ;double perovskites', crystallizing in the monoclinic space group P2(1)/n. The structure is characterized by a strong orthorhombic pseudosymmetry and a concurrent exhibition of both 1:1 B-cation ordering and a(-)a(-)c(+)-type tilting of the (Ni/Nb)O(6) structural units. Trivalent lanthanum resides on the perovskite A site, which is strongly distorted owing to the tilting of the (Ni/Nb)O(6) sublattice. Ordering of divalent nickel and pentavalent niobium on the B sublattice is described in terms of two twofold special positions (2c and 2d), with nickel taking almost complete occupancy of the 2d site and the 2c position being occupied by a statistical distribution of nickel and niobium.