Multiscale structure of LaAlO3 from single-crystal X-ray diffraction.

A domain-resolved synchrotron single-crystal X-ray diffraction study of a LaAlO3 pseudo-merohedral twin crystal was successfully carried out in combination with powder diffraction data from the same sample. Multiscale structure information ranging from micro- to nano- to atomic scale was determined from one single crystal. There is almost no change of domain ratios at temperatures of less than 400 K indicating no movement of the domain wall. The changes in domain ratio indicating domain-wall movement were observed in the temperature range of 450 to 700 K, which is consistent with the result of the previous mechanical measurement. It is also found that the ratio of four twin components becomes equal (25%), just below phase transition temperature. These findings are important for domain engineering and theoretical studies related to LaAlO3. The temperature dependence of domain ratio was preserved in the heating and cooling cycle except for the first heating process to 840 K. Therefore, the domain structure after heating to 840 K is intrinsic to the crystal. Accurate structure parameters were determined through unit-cell parameter calibration and domain-resolved structure analysis. The method for calibration of unit-cell parameters from twin crystal data was derived and used to solve the inconsistent unit-cell parameters between single crystal and powder data in the present and previous studies.

Fusion Growth and Extraordinary Distortion of Ultrasmall Metal Oxide Nanoparticles.

Ultrasmall metal oxide nanoparticles (<5 nm) potentially have new properties, different from conventional nanoparticles. The precise size control of ultrasmall nanoparticles remains difficult for metal oxide. In this study, the size of CeO2 nanoparticles was precisely controlled (1.3-9.4 nm) using a continuous-flow hydrothermal reactor, and the atomic distortion that occurs in ultrasmall metal oxides was explored for CeO2. The crystalline nanoparticles grow rapidly like droplets via coalescence, although they reach a critical particle size (∼3 to 4 nm), beyond which they grow slowly and change shape through ripening. In the initial growth stage, the ultrasmall nanoparticles exhibit disordered atomic configurations, including stacking faults. In ultrasmall CeO2 nanoparticles (<3 to 4 nm), unusual electron localization occurs on Ce 4f orbitals (Ce3+) as a result of O disordering, regardless of O vacancy concentration. This behavior differs from ordinary electron localization caused by the presence of O vacancies. The ultrasmall metal oxides have extraordinary distortion states, making them promising for use in nanotechnology applications. Furthermore, the proposed synthesis method can be applied to various other metal oxides and allows exploration of their properties.

Pressure-induced reversal of Peierls-like distortions elicits the polyamorphic transition in GeTe and GeSe.

While polymorphism is prevalent in crystalline solids, polyamorphism draws increasing interest in various types of amorphous solids. Recent studies suggested that supercooling of liquid phase-change materials (PCMs) induces Peierls-like distortions in their local structures, underlying their liquid-liquid transitions before vitrification. However, the mechanism of how the vitrified phases undergo a possible polyamorphic transition remains elusive. Here, using high-energy synchrotron X-rays, we can access the precise pair distribution functions under high pressure and provide clear evidence that pressure can reverse the Peierls-like distortions, eliciting a polyamorphic transition in GeTe and GeSe. Combined with simulations based on machine-learned-neural-network potential, our structural analysis reveals a high-pressure state characterized by diminished Peierls-like distortion, greater coherence length, reduced compressibility, and a narrowing bandgap. Our finding underscores the crucial role of Peierls-like distortions in amorphous octahedral systems including PCMs. These distortions can be controlled through pressure and composition, offering potentials for designing properties in PCM-based devices.

Pair Distribution Function from Liquid Jet Nanoparticle Suspension using Femtosecond X-ray Pulses.

X-ray scattering data measured on femtosecond timescales at the SACLA X-ray Free Electron Laser (XFEL) facility on a suspension of HfO2 nanoparticles in a liquid jet were used for pair distribution function (PDF) analysis. Despite a non-optimal experimental setup resulting in a modest Qmax of ~8 Å-1 , a promising PDF was obtained. The main features were reproduced when comparing the XFEL PDF to a PDF obtained from data measured at the PETRA III synchrotron light source. Refining structural parameters such as unit cell dimension and particle size from the XFEL PDF provided reliable values. Although the reachable Qmax limited the obtainable information, the present results indicate that good quality PDFs can be obtained on femtosecond timescales if the experimental conditions are further optimized. The study therefore encourages a new direction in ultrafast structural science where structural features of amorphous and disordered systems can be studied.

Synchrotron X-ray powder diffraction under high pressures up to 33†MPa for mechanoresponsive materials.

Equipment for synchrotron X-ray diffraction at high pressures up to 33 MPa with an accuracy of ±0.1 MPa using a liquid as a pressure-transmitting medium has been developed. This equipment enables atomic-scale observation of the structural change of mechanoresponsive materials under applied pressures. The validity of the equipment is demonstrated by observation of the pressure dependence of the lattice parameters of copper. The observed bulk modulus of copper was found to be 139 (13) GPa which is a good agreement with the literature value. The developed equipment was subsequently applied to a repeatable mechanoluminescence material, Li0.12Na0.88NbO3:Pr3+. The bulk modulus and compressibility along the a and c axes were determined as 79 (9) GPa, 0.0048 (6) GPa-1 and 0.0030 (9) GPa-1, respectively, for the R3c phase. The advance of high-pressure X-ray diffraction will play an important role in understanding mechanoresponsive materials towards their atomic-scale design.

Towards pump-probe single-crystal XFEL refinements for small-unit-cell systems.

Serial femtosecond crystallography for small-unit-cell systems has so far seen very limited application despite obvious scientific possibilities. This is because reliable data reduction has not been available for these challenging systems. In particular, important intensity corrections such as the partiality correction critically rely on accurate determination of the crystal orientation, which is complicated by the low number of diffraction spots for small-unit-cell crystals. A data reduction pipeline capable of fully automated handling of all steps of data reduction from spot harvesting to merged structure factors has been developed. The pipeline utilizes sparse indexing based on known unit-cell parameters, seed-skewness integration, intensity corrections including an overlap-based combined Ewald sphere width and partiality correction, and a dynamically adjusted post-refinement routine. Using the pipeline, data measured on the compound K4[Pt2(P2O5H2)4]·2H2O have been successfully reduced and used to solve the structure to an R1 factor of ∼9.1%. It is expected that the pipeline will open up the field of small-unit-cell serial femtosecond crystallography experiments and allow investigations into, for example, excited states and reaction intermediate chemistry.

Multicomponent Crystals with Competing Intermolecular Interactions: In Situ X-ray Diffraction and Luminescent Features Reveal Multimolecular Assembly under Mechanochemical Conditions.

Supramolecular chemistry under mechanochemical conditions has drawn attention because it can induce low-solubility molecules to self-assemble, although most of the reported examples have been limited to two-component systems. We applied mechanochemical synthesis to achieve multimolecular self-assembly in more challenging three-component systems. The produced crystals showed multicolor solid-state luminescence depending on the components when exposed to UV light. Optical outputs and X-ray diffraction studies were used to examine the self-assembling behavior in greater depth. Using synchrotron radiation, in situ X-ray diffraction permitted direct observation of the milling process, which started the self-assembly process within 1 min. This research emphasizes the importance of multicomponent molecules with optical functions and self-assembling behavior and offers the possibility of developing more complicated multicomponent crystals and organic solid solutions for advanced materials.

Atomic-Scale Visualization of Ultrafast Bond Breaking in X-Ray-Excited Diamond.

Ultrafast changes of charge density distribution in diamond after irradiation with an intense x-ray pulse (photon energy, 7.8 keV; pulse duration, 6 fs; intensity, 3×10^{19} W/cm^{2}) have been visualized with the x-ray pump-x-ray probe technique. The measurement reveals that covalent bonds in diamond are broken and the electron distribution around each atom becomes almost isotropic within ∼5 fs after the intensity maximum of the x-ray pump pulse. The 15 fs time delay observed between the bond breaking and atomic disordering indicates nonisothermality of electron and lattice subsystems on this timescale. From these observations and simulation results, we interpret that the x-ray-induced change of the interatomic potential drives the ultrafast atomic disordering underway to the following nonthermal melting.

Probing the validity of the spinel inversion model: a combined SPXRD, PDF, EXAFS and NMR study of ZnAl2O4.

Spinels are of essential interest in the solid-state sciences with numerous important materials adopting this crystal structure. One defining feature of spinel compounds is their ability to accommodate a high degree of tailorable point defects, and this significantly influences their physical properties. Standard defect models of spinels often only consider metal atom inversion between octahedral and tetrahedral sites, thereby neglecting other defects such as interstitial atoms. In addition, most studies rely on a single structural characterization technique, and this may bias the result and give uncertainty about the correct crystal structure. Here we explore the virtues of multi-technique investigations to limit method and model bias. We have used Pair Distribution Function analysis, Rietveld refinement and Maximum Entropy Method analysis of Powder X-ray Diffraction data, Zn edge Extended X-ray Absorption Fine Structure, and solid-state 27Al Nuclear Magnetic Resonance to study the structural defects in ZnAl2O4 spinel samples prepared by either microwave hydrothermal synthesis, supercritical flow synthesis, or spark plasma sintering. In addition, the samples were subjected to thermal post treatments. The study demonstrates that numerous synthesis dependent defects are present and that the different synthesis pathways allow for defect tailoring within the ZnAl2O4 structure. This suggests a pathway forward for optimization of the physical properties of spinel materials.

Thermosalience in Macrocycle-Based Soft Crystals via Anisotropic Deformation of Disilanyl Architecture.

We describe here the preparation of soft crystals using disilanyl macrocycle C4 possessing four p-phenylenes circularly connected by four flexible disilane bonds. Single crystals of C4 exhibited a reversible thermal single-crystal-to-single-crystal (SCSC) phase transition behavior between two crystal phases accompanied by remarkable mechanical motion (thermosalient effect), as revealed by thermal analyses and X-ray diffraction measurements. Detailed structural analyses implied that flexibility of the parallelogram disilanyl architecture and molecular packing mode via weak intermolecular interactions facilitated a concerted structural transformation (parallel crank motion) of macrocycles in the crystal, thus resulting in the SCSC phase transition accompanied by anisotropic shrinking/elongation of the cells to induce the thermosalient effect. This work explores a new area of organosilicon chemistry and presents the potential utility of disilanyl macrocycles as soft crystals.

Tetragonality induced superconductivity in anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with Bi square nets.

We report a series of layered superconductors, anti-ThCr2Si2-type RE2O2Bi (RE = rare earth), composed of electrically conductive Bi square nets and magnetic insulating RE2O2 layers. Superconductivity was induced by separating the Bi square nets as a result of excess oxygen incorporation, irrespective of the presence of magnetic ordering in RE2O2 layers. Intriguingly, the transition temperature of all RE2O2Bi including nonmagnetic Y2O2Bi was approximately scaled by unit cell tetragonality (c/a), implying a key role in the relative separation of the Bi square nets to induce superconductivity.

Effects of Substituents on the Blue Luminescence of Disilane-Linked Donor‒Acceptor‒Donor Triads.

A series of disilane-linked donor‒acceptor‒donor triads (D‒Si‒Si‒A‒Si‒Si‒D) was synthesized to investigate the effects of substituents on the photophysical properties. The triads were prepared by metal-catalyzed diiodosilylation of aryl iodides using a Pd(P(t-Bu)3)2/(i-Pr)2EtN/toluene system that we previously developed. Optical measurements, X-ray diffraction analysis, and density functional theory calculations revealed relationships between the photophysical properties and molecular structures of these triads in solution and in the solid state. The compounds emitted blue to green fluorescence in CH2Cl2 solution and in the solid state. Notably, compound 2 showed fluorescence with an absolute quantum yield of 0.17 in the solid state but showed no fluorescence in CH2Cl2. Our findings confirmed that the substituent adjacent to the disilane moiety affects the conformations and emission efficiencies of compounds in solution and in the solid state.

Tightly binding valence electron in aluminum observed through X-ray charge density study.

Accurate and high reciprocal resolution experimental structure factors of aluminum were determined from a synchrotron powder X-ray diffraction data measured at 30 K with sin θ/λ < 2.31 Å-1. The structure factors have small deviations from independent atom model in sin θ/λ < 0.83 Å-1. Theoretical structure factors were prepared using density functional theoretical calculations by full potential linearized augmented plane wave method. The deviation between experimental and theoretical data was also observed at around sin θ/λ ≈ 0.4 Å-1. The charge density was determined by an extended Hansen-Coppens multipole modeling using experimental and theoretical structure factors. Charge density maxima at tetrahedral site were observed in both experimental and theoretical deformation density. The charge-density difference peaks indicating directional bonding formation were observed in the difference density between experiment and theory. The present study reveals tight binding like character of valence electron of aluminum. The fact will provide a crucial information for development of high-performance aluminum alloy.

X-ray electron density investigation of chemical bonding in van der Waals materials.

Van der Waals (vdW) solids have attracted great attention ever since the discovery of graphene, with the essential feature being the weak chemical bonding across the vdW gap. The nature of these weak interactions is decisive for many extraordinary properties, but it is a strong challenge for current theory to accurately model long-range electron correlations. Here we use synchrotron X-ray diffraction data to precisely determine the electron density in the archetypal vdW solid, TiS2, and compare the results with density functional theory calculations. Quantitative agreement is observed for the chemical bonding description in the covalent TiS2 slabs, but significant differences are identified for the interactions across the gap, with experiment revealing more electron deformation than theory. The present data provide an experimental benchmark for testing theoretical models of weak chemical bonding.

Retreat from Stress: Rattling in a Planar Coordination.

Thermoelectric devices convert heat flow to charge flow, providing electricity. Materials for highly efficient devices must satisfy conflicting requirements of high electrical conductivity and low thermal conductivity. Thermal conductivity in caged compounds is known to be suppressed by a large vibration of guest atoms, so-called rattling, which effectively scatters phonons. Here, the crystal structure and phonon dynamics of tetrahedrites (Cu,Zn)12 (Sb,As)4 S13 are studied. The results reveal that the Cu atoms in a planar coordination are rattling. In contrast to caged compounds, chemical pressure enlarges the amplitude of the rattling vibration in the tetrahedrites so that the rattling atom is squeezed out of the planar coordination. Furthermore, the rattling vibration shakes neighbors through lone pairs of the metalloids, Sb and As, which is responsible for the low thermal conductivity of tetrahedrites. These findings provide a new strategy for the development of highly efficient thermoelectric materials with planar coordination.

Aggregation-Induced Emission Enhancement from Disilane-Bridged Donor-Acceptor-Donor Luminogens Based on the Triarylamine Functionality.

Six novel donor-acceptor-donor organic dyes containing a Si-Si moiety based on triarylamine functionalities as donor units were prepared by Pd-catalyzed arylation of hydrosilanes. Their photophysical, electrochemical, and structural properties were studied in detail. Most of the compounds showed attractive photoluminescence (PL) and electrochemical properties both in solution and in the solid state because of intramolecular charge transfer (ICT), suggesting these compounds could be useful for electroluminescence (EL) applications. The aggregation-induced emission enhancement (AIEE) characteristics of 1 and 3 were examined in mixed water/THF solutions. The fluorescence intensity in THF/water was stronger in the solution with the highest ratio of water because of the suppression of molecular vibration and rotation in the aggregated state. Single-crystal X-ray diffraction of 4 showed that the reduction of intermolecular π-π interaction led to intense emission in the solid state and restricted intramolecular rotation of the donor and acceptor moieties, thereby indicating that the intense emission in the solid state is due to AIEE. An electroluminescence device employing 1 as an emitter exhibited an external quantum efficiency of up to 0.65% with green light emission. The emission comes solely from 1 because the EL spectrum is identical to that of the PL of 1. The observed luminescence was sufficiently bright for application in practical devices. Theoretical calculations and electrochemical measurements were carried out to aid in understanding the optical and electrochemical properties of these molecules.

Accurate charge densities from powder X-ray diffraction - a new version of the Aarhus vacuum imaging-plate diffractometer.

In recent years powder X-ray diffraction has proven to be a valuable alternative to single-crystal X-ray diffraction for determining electron-density distributions in high-symmetry inorganic materials, including subtle deformation in the core electron density. This was made possible by performing diffraction measurements in vacuum using high-energy X-rays at a synchrotron-radiation facility. Here we present a new version of our custom-built in-vacuum powder diffractometer with the sample-to-detector distance increased by a factor of four. In practice this is found to give a reduction in instrumental peak broadening by approximately a factor of three and a large improvement in signal-to-background ratio compared to the previous instrument. Structure factors of silicon at room temperature are extracted using a combined multipole-Rietveld procedure and compared with ab initio calculations and the results from the previous diffractometer. Despite some remaining issues regarding peak asymmetry, the new diffractometer yields structure factors of comparable accuracy to the previous diffractometer at low angles and improved accuracy at high angles. The high quality of the structure factors is further assessed by modelling of core electron deformation with results in good agreement with previous investigations.

Revealing the slow decomposition kinetics of type-I clathrate Ba8Ga16Ge30.

Inconsistencies in high temperature thermoelectric property measurements of Ba8Ga16Ge30 have prompted our study on the thermal stability of this heavily studied inorganic clathrate. Using X-ray diffraction, thermal analysis, and imaging techniques on both powder and spark plasma sintered pelletized samples, we probe the structure and decomposition characteristics of this important high temperature thermoelectric material. We demonstrate that the decomposition of Ba8Ga16Ge30 is extremely dependent on the heating conditions employed and, as a result of the slow decomposition kinetics of the clathrate, reveal that the true stability of this system has been overlooked in the extensive literature available. Loss of Ga and Ge from the clathrate cage is evident in all high temperature experiments under both air and inert environments. This study serves to highlight that the underlying structural chemistry and stability of thermoelectric materials at high temperature needs to be considered in parallel with the thermoelectric properties which constitute the figure of merit. Only then will reliable thermoelectric modules for real applications be realized.

High-Temperature Crystal Structure and Chemical Bonding in Thermoelectric Germanium Selenide (GeSe).

The discovery of the ultra-high thermoelectric figure of merit of 2.6 in SnSe has drawn attention to other lead-free IV-VI orthorhombic semiconductors. GeSe has been predicted to possess thermoelectric performances comparable to SnSe. Here, a complete structural study is reported of GeSe with temperature by means of high-resolution synchrotron powder X-ray diffraction. In the orthorhombic phase, the evolution of the bond distances with temperature is shown to deviate significantly with respect to SnSe. Analysis of the chemical bonding within the Quantum Theory of Atoms in Molecules shows that GeSe is ionic with van der Waals interlayer interactions. The signature of the N shell lone pair of Ge is also evident from both the electron density Laplacian and the ELF topologies.

Low-Temperature Anharmonicity in Cesium Chloride (CsCl).

Anharmonic lattice vibrations govern heat transfer in materials, and anharmonicity is commonly assumed to be dominant at high temperature. The textbook cubic ionic defect-free crystal CsCl is shown to have an unexplained low thermal conductivity at room temperature (ca. 1 W/(m K)), which increases to around 13  W/(m K) at 25 K. Through high-resolution X-ray diffraction it is unexpectedly shown that the Cs atomic displacement parameter becomes anharmonic at 20 K.