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.

Thermally Reentrant Crystalline Phase Change in Perovskite-Derivative Nickelate Enabling Reversible Switching of Room-Temperature Electrical Resistivity.

Reversible switching of room-temperature electrical resistivity due to crystal-amorphous transition is demonstrated in various chalcogenides for development of non-volatile phase change memory. However, such reversible thermal switching of room-temperature electrical resistivity has not reported in transition metal oxides so far, despite their enormous studies on the electrical conduction like metal-insulator transition and colossal magnetoresistance effect. In this study, a thermally reversible switching of room-temperature electrical resistivity is reported with gigantic variation in a layered nickelate Sr2.5 Bi0.5 NiO5 (1201-SBNO) composed of (Sr1.5 Bi0.5 )O2 rock-salt and SrNiO3 perovskite layers via unique crystalline phase changes between the conducting 1201-SBNO with ordered (O-1201), disordered Sr/Bi arrangements in the (Sr1.5 Bi0.5 )O2 layer (D-1201), and insulating oxygen-deficient double perovskite Sr2 BiNiO4.5 (d-perovskite). The O-1201 is reentrant by high-temperature annealing of ≈1000 °C through crystalline phase change into the D-1201 and d-perovskite, resulting in the thermally reversible switching of room-temperature electrical resistivity with 102 - and 109 -fold variation, respectively. The 1201-SBNO is the first oxide to show the thermally reversible switching of room-temperature electrical resistivity via the crystalline phase changes, providing a new perspective on the electrical conduction for transition metal oxides.

Charge transfer engineering to achieve extraordinary power generation in GeTe-based thermoelectric materials.

By the fine manipulation of the exceptional long-range germanium-telluride (Ge─Te) bonding through charge transfer engineering, we have achieved exceptional thermoelectric (TE) and mechanical properties in lead-free GeTe. This chemical bonding mechanism along with a semiordered zigzag nanostructure generates a notable increase of the average zT to a record value of ~1.73 in the temperature range of 323 to 773 K with ultrahigh maximum zT ~ 2.7. In addition, we significantly enhanced the Vickers microhardness numbers (Hv) to an extraordinarily high value of 247 Hv and effectively eliminated the thermal expansion fluctuation at the phase transition, which was problematic for application, by the present charge transfer engineering process and concomitant formation of microstructures. We further fabricated a single-leg TE generator and obtained a conversion efficiency of ~13.4% at the temperature difference of 463 K on a commercial instrument, which is located at the pinnacle of TE conversion.

Reversible Transition between Discrete and 1D Infinite Architectures: A Temperature-Responsive Cu(I) Complex with a Flexible Disilane-Bridged Bis(pyridine) Ligand.

A thermoresponsive structural change based on a disilane-bridged bis(pyridine) ligand and CuI is reported. Single-crystal X-ray analysis revealed that there are two polymorphs in the Cu(I) complex: octanuclear copper(I) complex at 20 °C and 1D staircase copper(I) polymer complex at -173 °C. The formation of these polymorphs is due to the flexibility of the ligand. Cu-I bond formation is observed upon cooling the sample from -10 °C to -170 °C. The temperature-induced phase transition progression was clarified by DSC, VT-PXRD, and VT-photoluminescence measurements and indicated a reversible temperature-controlled crystal-to-crystal phase transition. Observation on a VT-stage using a high-speed camera showed crystal cracking during single-crystal to single-crystal transitions between these polymorphic forms.

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.

Dynamic Lone Pair Expression as Chemical Bonding Origin of Giant Phonon Anharmonicity in Thermoelectric InTe.

Loosely bonded ("rattling") atoms with s2 lone pair electrons are usually associated with strong anharmonicity and unexpectedly low thermal conductivity, yet their detailed correlation remains largely unknown. Here we resolve this correlation in thermoelectric InTe by combining chemical bonding analysis, inelastic X-ray and neutron scattering, and first principles phonon calculations. We successfully probe soft low-lying transverse phonons dominated by large In1+ z-axis motions, and their giant anharmonicity. We show that the highly anharmonic phonons arise from the dynamic lone pair expression with unstable occupied antibonding states induced by the covalency between delocalized In1+ 5s2 lone pair electrons and Te 5p states. This work pinpoints the microscopic origin of strong anharmonicity driven by rattling atoms with stereochemical lone pair activity, important for designing efficient materials for thermoelectric energy conversion.

Electron Density Analysis of Metal-Metal Bonding in a Ni4 Cluster Featuring Ferromagnetic Exchange.

We present a combined experimental and theoretical study of the nature of the proposed metal-metal bonding in the tetranuclear cluster Ni4(NPtBu3)4, which features four nickel(I) centers engaged in strong ferromagnetic coupling. High-resolution single-crystal synchrotron X-ray diffraction data collected at 25 K provide an accurate geometrical structure and a multipole model electron density description. Topological analysis of the electron density in the Ni4N4 core using the quantum theory of atoms in molecules clearly identifies the bonding as an eight-membered ring of type [Ni-N-]4 without direct Ni-Ni bonding, and this result is generally corroborated by an analysis of the energy density distribution. In contrast, the calculated bond delocalization index of ∼0.6 between neighboring Ni atoms is larger than what has been found for other bridged metal-metal bonds and implies direct Ni-Ni bonding. Similar support for the presence of direct Ni-Ni bonding is found in the interacting quantum atom approach, an energy decomposition scheme, which suggests the presence of stabilizing Ni-Ni bonding interactions with an exchange-correlation energy contribution approximately 50% of that of the Ni-N interactions. Altogether, while the direct interactions between neighboring Ni centers are too weak and sterically constrained to bear the signature of a topological bond critical point, other continuous measures clearly indicate significant Ni-Ni bonding. These metal-metal bonding interactions likely mediate direct ferromagnetic exchange, giving rise to the high-spin ground state of the molecule.

Thermally responsive morphological changes of layered coordination polymers induced by disordering/ordering of flexible alkyl chains.

The search for a method to control structural transformations of layered coordination polymers is highly desirable to modulate their properties and functions. Herein, we report the construction of a novel coordination polymer named ZnC16 with Zn(II) ions coordinated to isophthalate ligands bearing an n-hexadecyloxy chain (C162-). Its structure consists of a layer-by-layer structure of a rigid two-dimensional coordination network and an assembly of alkyl chains as a thermally responsive moiety. Single crystals of ZnC16 exhibit a thermal crystal-to-crystal phase transition behaviour dominated by disordering/ordering of alkyl chains, which induces the expansion and shrinkage of the distance within the rigid 2D coordination networks. Microscopic observation revealed that the thermal phase transition of ZnC16 induces a significant change in their crystal morphology: a reversible macroscopic elongation/shrinkage of crystal dimensions driven by the displacement of interlayer distances and an irreversible delamination and polycrystal slippage driven by constraints generated from this phase transition. Our result provides a new direction to modulate the dynamic behaviour and related properties and functions of layered coordination polymers where the thermally responsive character of flexible alkyl chains plays an important role in tuning interlayer interactions.

Luminescence color change of [3,4-difluoro-2,6-bis(5-methyl-2-pyridyl)phenyl-κ3N,C1,N']cyanidoplatinum(II) by aggregation.

We have investigated the color and luminescence color changes of novel Pt(L)CN (L = 4,6-difluoro-1,3-di(2-(4-methyl)pyridyl)benzene) in solution and crystalline states that resulted from aggregation-induced emission (AIE). In the solution state, the AIE results from excimer and trimer formation in the excited states at high concentrations. We determined the emission lifetimes of the excimer and trimer to be τE = 1.72 µs and τT = 0.43 µs, respectively, and the emission quantum yields to be ϕE = 20% and ϕT = 12%, respectively, which are slightly smaller yet comparable to τM = 8.85 µs and ϕM = 67% of the monomeric species. In the crystalline state, the purple color of Pt(L)CN with no solvent of crystallization changes to red upon exposure to chloroform vapor, and the invisible emission turns to bright red emission. This phenomenon can be applied to inexpensive devices for the fast chloroform detection. The exposure of purple crystals to dichloromethane vapor causes a further redshift of the invisible emission and blue coloration, which suggests the capability of the discrimination of chloroform from dichloromethane.

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.

Synchrotron X-ray Electron Density Analysis of Chemical Bonding in the Graphitic Carbon Nitride Precursor Melamine.

Melamine is a precursor and building block for graphitic carbon nitride (g-CN) materials, a group of layered materials showing great promise for catalytic applications. The synthetic pathway to g-CN includes a polycondensation reaction of melamine by evaporation of ammonia. Melamine molecules in the crystal organize into wave-like planes with an interlayer distance of 3.3 Šsimilar to that of g-CN. Here we present an extensive investigation of the experimental electron density of melamine obtained from modelling of synchrotron radiation X-ray single-crystal diffraction data measured at 25 K with special focus on the molecular geometry and intermolecular interactions. Both intra- and interlayer structures are dominated by hydrogen bonding and π-interactions. Theoretical gas-phase optimizations of the experimental molecular geometry show that bond lengths and angles for atoms in the same chemical environment (C-N bonds in the ring, amine groups) differ significantly more for the experimental geometry than for the gas-phase-optimized geometries, indicating that intermolecular interactions in the crystal affects the molecular geometry. In the experimental crystal geometry, one amine group has significantly more sp3 -like character than the others, hinting at a possible formation mechanism of g-CN. Topological analysis and energy frameworks show that the nitrogen atom in this amine group participates in weak intralayer hydrogen bonding. We hypothesize that melamine condenses to g-CN within the layers and that the unique amine group plays a key role in the condensation process.

Delayed Onset and Directionality of X-Ray-Induced Atomic Displacements Observed on Subatomic Length Scales.

Transient structural changes of Al_{2}O_{3} on subatomic length scales following irradiation with an intense x-ray laser pulse (photon energy: 8.70 keV; pulse duration: 6 fs; fluence: 8×10^{2} J/cm^{2}) have been investigated by using an x-ray pump x-ray probe technique. The measurement reveals that aluminum and oxygen atoms remain in their original positions by ∼20 fs after the intensity maximum of the pump pulse, followed by directional atomic displacements at the fixed unit cell parameters. By comparing the experimental results and theoretical simulations, we interpret that electron excitation and relaxation triggered by the pump pulse modify the potential energy surface and drives the directional atomic displacements. Our results indicate that high-resolution x-ray structural analysis with the accuracy of 0.01 Å is feasible even with intense x-ray pulses by making the pulse duration shorter than the timescale needed to complete electron excitation and relaxation processes, which usually take up to a few tens of femtoseconds.

Reversible and Stepwise Single-Crystal-to-Single-Crystal Transformation of a Platinum(II) Complex with Vapochromic Luminescence.

The vapochromic single-crystal-to-single-crystal (SCSC) transformation of a highly luminescent PtII complex bearing an N-heterocyclic carbene [Pt(CN)2 (tBu-impy)] (tBu-impyH+ =1-tert-butyl-3-(2-pyridyl)-1H-imidazolium) is reported. The trihydrate form of the complex, which exhibits blue 3 MMLCT emission owing to weak Pt⋅⋅⋅Pt interactions, changed its luminescence color from blue to yellowish-green upon the desorption of water molecules while keeping the high emission quantum yield of more than 0.45. Variable-temperature and continuous in-situ tracking of single-crystal X-ray diffraction measurements revealed that the SCSC transformation proceeds reversibly by the release and reabsorption of water molecules, thereby changing the stacked structure slightly. As a result, the dynamics of vapor-induced SCSC transformation were elucidated: that the anhydrous form returned to the original trihydrate form in a two-step process under a water vapor atmosphere. In addition, the PtII complex exhibited a similar SCSC response accompanied by a luminescence color change in the presence of methanol vapor, while being inactive toward ethanol vapor.

Structure Factors and Charge Density Description of Aluminum: A Quantum Crystallographic Study.

Static structure factors and charge density for metallic aluminum were investigated by periodic calculations using atom-centered Gaussian-type basis sets with the Perdew-Burke-Ernzerhof (PBE) functional implemented in the CRYSTAL14 package and X-ray constrained wave function (XCW) fitting. The effects of additional diffuse d and f basis functions on structure factors were compared with synchrotron powder X-ray diffraction and quantitative convergent electron beam diffraction data. Changes in structure factors from an independent atom model at 022, 113, and 222 reflections introduced d and f basis functions similar to those of the experimental data. The XCW fitting was applied to different sizes of aluminum clusters. The charge density features for a 50-atom cluster clearly demonstrated electron accumulation at tetrahedral sites and electron depletion at octahedral sites. The resolution dependence of the XCW study suggests that structure factors of the five lowest resolution reflections with 0.1% accuracy were indispensable for determining the detailed bonding description in the case of metallic aluminum.

X-ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal-Organic Frameworks.

Glass-forming metal-organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X-ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass-forming Zn-ZIF-zni and the isostructural thermally decomposing Co-ZIF-zni. Electron density analysis shows that the Zn-N bonds are more ionic than the Co-N bonds, which have distinct covalent features. Variable-temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co-ZIF-zni above 673 K. Melting can be controlled by tuning the metal-ligand and imidazole bonding strength as shown from thermal analysis of nine solid-solution Cox Zn1-x -ZIF-zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co-doping into Zn-ZIF-zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal-ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.

Direct observation of one-dimensional disordered diffusion channel in a chain-like thermoelectric with ultralow thermal conductivity.

Structural disorder, highly effective in reducing thermal conductivity, is important in technological applications such as thermal barrier coatings and thermoelectrics. In particular, interstitial, disordered, diffusive atoms are common in complex crystal structures with ultralow thermal conductivity, but are rarely found in simple crystalline solids. Combining single-crystal synchrotron X-ray diffraction, the maximum entropy method, diffuse scattering, and theoretical calculations, here we report the direct observation of one-dimensional disordered In1+ chains in a simple chain-like thermoelectric InTe, which contains a significant In1+ vacancy along with interstitial indium sites. Intriguingly, the disordered In1+ chains undergo a static-dynamic transition with increasing temperature to form a one-dimensional diffusion channel, which is attributed to a low In1+-ion migration energy barrier along the c direction, a general feature in many other TlSe-type compounds. Our work provides a basis towards understanding ultralow thermal conductivity with weak temperature dependence in TlSe-type chain-like materials.