Revisiting the structures and phase transitions of Ba2NaNb5O15.

The room-temperature and low-temperature structure(s) of Ba2NaNb5O15 (BNN) have been debated since the structure was proposed in the 1960s. This work revisits the structures and phase transitions of BNN, combining high-resolution X-ray and neutron powder diffraction with density functional theory calculations. Temperature-dependent high-resolution X-ray powder diffraction patterns are collected from 4 to 918 K, and sequential batch Rietveld refinement using a symmetry mode approach to describe the structure is used to extract the main structural changes as a function of temperature. The data show that the average structure of BNN is best described by the Ama2 space group, and no other structural phase transitions were observed below the ferroelastic transition. The symmetry mode analysis, combining results from diffraction and density functional theory, shows significant octahedral tilting and corrugations of both the A1 and A2 sites along the c direction. A strong correlation between the spontaneous strain and the octahedral tilting was observed, and a potential connection with emerging microstructure at low temperatures is proposed, all enabled by the symmetry mode approach used in this work.

A-Site Cation Disorder in SrxBa1-xNb2O6 (x = 0.25, 0.33, 0.50, 0.61) Studied by High-Resolution Resonant X-ray Powder Diffraction.

The dielectric and ferroelectric properties of SrxBa1-xNb2O6 (SBN, 0.2 < x < 0.8) are known to be affected by the Sr fraction and can be further controlled by various quenching schemes. Changes in A-site cation configuration are believed to be linked to these changes in properties. In this work, we study the A-site cation disorder in SBN by the use of high-resolution resonant X-ray powder diffraction. The experimental results show that the larger Ba2+ is found exclusively on the larger A2 site, while Sr2+ is found on both the A1 and A2 sites, with an increasing amount on A2 with an increasing Sr fraction. At elevated temperatures, a small migration of Sr2+ from A1 to A2 is observed for SBN50 and SBN61. Linking this change in occupancies to changes in the average cation size on the A1 and A2 sites allows for rationalization of the property changes observed for quenched samples. Furthermore, SBN25 is shown to deviate from the tetragonal P4bm structure and is found to be orthorhombic with a Cmm2 structure.

ID22 - the high-resolution powder-diffraction beamline at ESRF.

Following Phase 2 of the upgrade of the ESRF in which the storage ring was replaced by a new low-emittance ring along with many other facility upgrades, the status of ID22, the high-resolution powder-diffraction beamline, is described. The beamline has an in-vacuum undulator as source providing X-rays in the range 6-75 keV. ID22's principle characteristics include very high angular resolution as a result of the highly collimated and monochromatic beam, coupled with a 13-channel Si 111 multi-analyser stage between the sample and a Dectris Eiger2 X 2M-W CdTe pixel detector. The detector's axial resolution allows recorded 2θ values to be automatically corrected for the effects of axial divergence, resulting in narrower and more-symmetric peaks compared with the previous fixed-axial-slit arrangement. The axial acceptance can also be increased with increasing diffraction angle, thus simultaneously improving the statistical quality of high-angle data. A complementary Perkin Elmer XRD1611 medical-imaging detector is available for faster, lower-resolution data, often used at photon energies of 60-70 keV for pair-distribution function analysis, although this is also possible in high-resolution mode by scanning up to 120°â€…2θ at 35 keV. There are various sample environments, allowing sample temperatures from 4 K to 1600°C, a capillary cell for non-corrosive gas atmospheres in the range 0-100 bar, and a sample-changing robot that can accommodate 75 capillary samples compatible with the temperature range 80 K to 950°C.

Incorporation of nanogels within calcite single crystals for the storage, protection and controlled release of active compounds.

Nanocarriers have tremendous potential for the encapsulation, storage and delivery of active compounds. However, current formulations often employ open structures that achieve efficient loading of active agents, but that suffer undesired leakage and instability of the payloads over time. Here, a straightforward strategy that overcomes these issues is presented, in which protein nanogels are encapsulated within single crystals of calcite (CaCO3). Demonstrating our approach with bovine serum albumin (BSA) nanogels loaded with (bio)active compounds, including doxorubicin (a chemotherapeutic drug) and lysozyme (an antibacterial enzyme), we show that these nanogels can be occluded within calcite host crystals at levels of up to 45 vol%. Encapsulated within the dense mineral, the active compounds are stable against harsh conditions such as high temperature and pH, and controlled release can be triggered by a simple reduction of the pH. Comparisons with analogous systems - amorphous calcium carbonate, mesoporous vaterite (CaCO3) polycrystals, and calcite crystals containing polymer vesicles - demonstrate the superior encapsulation performance of the nanogel/calcite system. This opens the door to encapsulating a broad range of existing nanocarrier systems within single crystal hosts for the efficient storage, transport and controlled release of various active guest species.

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.

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.

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.

Structure and Optical Properties of Titania-PDMS Hybrid Nanocomposites Prepared by In Situ Non-Aqueous Synthesis.

Organic-inorganic hybrid materials are attractive due to the combination of properties from the two distinct types of materials. In this work, transparent titania-polydimethylsiloxane hybrid materials with up to 15.5 vol. % TiO2 content were prepared by an in situ non-aqueous method using titanium (IV) isopropoxide and hydroxy-terminated polydimethylsiloxane as precursors. Spectroscopy (Fourier transform infrared, Raman, Ultraviolet-visible, ellipsometry) and small-angle X-ray scattering analysis allowed to describe in detail the structure and the optical properties of the nanocomposites. Titanium alkoxide was successfully used as a cross-linker and titania-like nanodomains with an average size of approximately 4 nm were shown to form during the process. The resulting hybrid nanocomposites exhibit high transparency and tunable refractive index from 1.42 up to 1.56, depending on the titania content.