Search

Understanding the nucleation and growth mechanisms of nanocrystals under hydro- and solvothermal conditions is key to tailoring functional nanomaterials. High-energy and high-flux synchrotron radiation is ideal for characterization by powder X-ray diffraction and X-ray total scattering in real time. Different versions of batch-type cell reactors have been employed in this work, exploiting the robustness of polyimide-coated fused quartz tubes with an inner diameter of 0.7 mm, as they can withstand pressures up to 250 bar and temperatures up to 723 K for several hours. Reported here are recent developments of the in situ setups available for general users on the P21.1 beamline at PETRA III and the DanMAX beamline at MAX IV to study nucleation and growth phenomena in solvothermal synthesis. It is shown that data suitable for both reciprocal-space Rietveld refinement and direct-space pair distribution function refinement can be obtained on a timescale of 4 ms.

The electron density and thermal motion of diamond are determined at nine temperatures between 100 K and 1000 K via synchrotron powder X-ray diffraction (PXRD) data collected on a high-accuracy detector system. Decoupling of the thermal motion from the thermally smeared electron density is performed via an iterative Wilson–Hansen–Coppens–Rietveld procedure using theoretical static structure factors from density functional theory (DFT) calculations. The thermal motion is found to be harmonic and isotropic in the explored temperature range, and excellent agreement is observed between experimental atomic displacement parameters (ADPs) and those obtained via theoretical harmonic phonon calculations (HPC), even at 1000 K. The Debye temperature of diamond is determined experimentally to be ΘD = 1883 (35) K. A topological analysis of the electron density explores the temperature dependency of the electron density at the bond critical point. The properties are found to be constant throughout the temperature range. The robustness of the electron density confirms the validity of the crystallographic convolution approximation for diamond in the explored temperature range.

The mixed ionic-electronic conductor beta-Zn4Sb3 is a cheap and high-performing thermoelectric material, but under operating conditions with a temperature gradient and a running current, the material decomposes as Zn readily migrates in the structure. Here, we report an improved stability of beta-Zn(4)Sb(3)by introducing ion-blocking interfaces of stainless steel to segment the sample, produced by a rapid one-step spark plasma sintering synthesis. The stability of the samples is tested under temperature gradients and electric currents, which reveals significantly improved stability of the segmented samples compared to unsegmented samples. The segmented samples are stable under a temperature gradient from 250 degrees C to room temperature with no external current, whereas the unsegmented sample decomposes into ZnSb and Zn under the same conditions. The thermoelectric figure of merit, zT, of the segmented sample is slightly reduced, mainly due to the increased thermal conductivity. In conclusion, a rapid one-step synthesis of segmented beta-Zn4Sb3 is developed, which successfully improves the long-term operational stability by blocking the Zn ion migration.

Magnetic materials with the spinel structure (A2+B3+2O4) form the core of numerous magnetic devices, and ZnFe2O4 constitutes a peculiar example where the nature of the magnetism is still unresolved. Susceptibility measurements revealed a cusp around Tc = 13 K resembling an antiferromagnetic transition, despite the positive Curie–Weiss temperature determined to be ΘCW = 102.8(1) K. Bifurcation of field-cooled and zero-field-cooled data below Tc in conjunction with a frequency dependence of the peak position and a non-zero imaginary component below Tc shows it is in fact associated with a spin-glass transition. Highly structured magnetic diffuse neutron scattering from single crystals develops between 50 K and 25 K revealing the presence of magnetic disorder which is correlated in nature. Here, the 3D-mΔPDF method is used to visualize the local magnetic ordering preferences, and ferromagnetic nearest-neighbor and antiferromagnetic third nearest-neighbor correlations are shown to be dominant. Their temperature dependence is extraordinary with some flipping in sign and a strongly varying correlation length. The correlations can be explained by orbital interaction mechanisms for the magnetic pathways and a preferred spin cluster. This study demonstrates the power of the 3D-mΔPDF method in visualizing complex quantum phenomena thereby providing a way to obtain an atomic-scale understanding of magnetic frustration.

The effect of substituting linkers with electron-donating moieties for part of the conventional ones on the melting and glass transition behaviours of ZIF-62 was investigated by calorimetry and X-ray diffraction. Specifically, substituting 5,6-dimethylbenzimidazole for benzimidazole in ZIF-62 increases both Tm and Tg. The structural origin of this effect was explained.

High-entropy alloys (HEAs) present a promising class of materials due to extraordinary properties and vast possibilities for tailoring product characteristics through changes in stoichiometry. Nanosized HEAs are especially interesting with respect to catalysis of multistep reactions and replacement of scarce, expensive, and/or toxic elements. Finding suitable methods for the synthesis of nano-HEAs remains a severe challenge, and typical methods are limited in productivity, compatibility of different elements, substrate types, and/or specialized laboratory equipment. Here, we report a simple solvothermal method for the synthesis of eight-component Pt-Ir-Pd-Rh-Ru-Cu-Ni-Co HEA nanoparticles with a high potential production capacity. The method relies on simple autoclaves heated to a moderate temperature. In situ X-ray scattering experiments show that the individual metal components form from one-element precursors at higher reaction temperatures, suggesting that the formation is auto-catalyzed, that is, the particles catalyze their own growth, in correspondence with previous findings for a five element (Pt-Ir-Rh-Pd-Ru) noble-metal HEA. This paper extends the method to cheaper and more abundant 3d metals (Co, Ni, and Cu) despite the differences in atomic radii and chemical reactivity and thereby adds significantly to the synthetic chemistry and understanding of HEA nanomaterials.

Nanoscale 15(11), 5284 - 5292 (2023). doi:10.1039/D2NR07128B, Gallium oxides are of broad interest due to their wide band gaps and attractive photoelectric properties. Typically, the synthesis of gallium oxide nanoparticles is based on a combination of solvent-based methods and subsequent calcination, but detailed information about solvent based formation processes is lacking, and this limits the tailoring of materials. Here we have examined the formation mechanisms and crystal structure transformations of gallium oxides during solvothermal synthesis using in situ X-ray diffraction. $γ$-Ga$_2$O$_3$ readily forms over a wide range of conditions. In contrast, $β$-Ga$_2$O$_3$ only forms at high temperatures (T > 300 °C), and it is always preceded by $γ$-Ga$_2$O$_3$, indicating that $γ$-Ga$_2$O$_3$ is a crucial part of the formation mechanism of $β$-Ga$_2$O$_3$. The activation energy for formation of $β$-Ga$_2$O$_3$ from $γ$-Ga$_2$O$_3$ is determined to be 90–100 kJ mol$^{−1}$ in ethanol, water and aqueous NaOH based on kinetic modelling of phase fractions obtained from multi-temperature in situ X-ray diffraction data. At low temperatures GaOOH and Ga$_5$O$_7$OH form in aqueous solvent, but these phases are also obtained from $γ$-Ga$_2$O$_3$. Systematic exploration of synthesis parameters such as temperature, heating rate, solvent and reaction time reveal that they all affect the resulting product. In general, the solvent based reaction paths are different from reports on solid state calcination studies. This underlines that the solvent is an active part of the solvothermal reactions and to a high degree determines different formation mechanisms., Published by RSC Publ., Cambridge

Magnetic materials with the spinel structure (A

High entropy alloy (HEA) nanoparticles hold promise in heterogeneous catalysis, and recently, simple and benign solvothermal synthesis was achieved for the equimolar PtIrPdRhRu. Here we experimentally explore the available composition space in this system, and we find that single-phase products can be obtained at significant deviations from the equimolar case.

A series of small, middle, and large anatase TiO2 particles were synthesized through the hydrolysis of titanium tetraisopropoxide (TTIP) to investigate the size-related chemical bond length and strength variation. Unit cell volume contraction with decreasing particle size is identified from Rietveld refinement of high-resolution synchrotron powder X-ray diffraction (PXRD) patterns. More titanium vacancies are also found for smaller anatase particles. Contrary to the variation in unit cell volume, a larger Debye temperature Theta(D)(TiO2) derived from the linear and nonlinear fitting of atomic displacement parameters (U-iso(TiO2)) as a function of temperature is revealed for smaller anatase particles. The length of the Ti-O bond is also shorter for smaller anatase particles. Furthermore, optical phonon frequencies blue-shifting with the decrease in anatase particle size are determined by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis rules out the presence of a large amount of Ti3+, while optical diffuse reflectance measurement eliminates the existence of a large number of oxygen vacancies in all particles. Combining the analysis results of PXRD, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR), more structural and surface hydroxyls (-OH) appear to exist in smaller anatase particles. It is the structural and surface -OH that are responsible for the size-related chemical bond length and strength variation in the as-synthesized anatase particles.

A series of small, middle, and large anatase TiO

The structures of metal ions in solution constitute essential information for obtaining chemical insight spanning from catalytic reaction mechanisms to formation of functional nanomaterials. Here, we explore Zr

N-type Mg

N-type Mg-3(Sb,Bi)(2) compounds have recently been demonstrated as promising low-cost efficient thermoelectric materials in low and intermediate temperature ranges; however, the thermal stability of this type of material still poses a great challenge for practical applications. In this work, we conduct a systematic investigation of the thermal stability of several high performing n-type Mg-3(Sb,Bi)(2)-based thermoelectric materials in both bulk and powdered forms using X-ray and neutron diffraction. It is found that the bulk sample exhibits a much slower degradation rate based on the evolution of the secondary Bi/Sb phase in comparison with the powdered sample, revealing a clear kinetic effect. Moreover, the surface of the bulk sample will gradually become Mg-poor or Bi-rich even at room temperature when exposed to air for a long time, highlighting the importance of surface encapsulation for applications. An underlying mechanism based on the Mg loss/migration is proposed to account for the property degradation. Importantly, to address the property degradation, we discuss possible solutions and propose Mg-vapor annealing as an effective approach to enhance thermal stability by suppressing the Mg loss/migration through saturating grains and grain boundaries with elemental Mg. We expect a combination of the Mg-vapor annealing and surface coating to further improve the long-term thermal stability. This work sheds light on the strategies for enhancing the long-term stability of n-type Mg3Sb2-based thermoelectrics for practical applications.

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 CoxZn1−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.

The cesium halides (CsX) are ionic high-symmetry compounds, which at first would seem like well-understood systems. However, recent studies have shown that using the simple Perdew-Burke-Ernzerhof (PBE) functional in density-functional theory (DFT) calculations, CsX materials do not adopt their namesake structure. Furthermore, peculiar low thermal conductivities have been observed experimentally in both CsCl and CsI at room temperature, and the origin has been linked to low-temperature anharmonicity derived from different types of experiments. In the case of CsCl the anharmonicity was observed from x-ray diffraction as an octahedral nuclear probability density function (nPDF), which, in contrast to expectations, becomes spherical at elevated temperature. Here, we study the nPDF of CsBr and CsI from single-crystal x-ray diffraction to compare with the findings of CsCl. It is shown that the aspherical features become less pronounced when substituting for a heavier halide. From periodic DFT calculations on CsCl, CsBr, and CsI probing the potential-energy surfaces this can be explained by progressively more similar masses upon substitution linked with Pauli repulsion. The apparent disappearance of the anharmonic features in CsCl with increasing temperatures can be understood as relatively larger population of acoustic phonons compared to the optical phonons following the Bose-Einstein distribution function. Finally, it is shown that theory can reproduce the correct equilibrium structures as well as phonon dispersions comparable to experimental values when adding a functional form of van der Waals interactions to a PBE DFT calculation.

The limited stability of thermoelectric materials under operating conditions is a major inhibiting factor in the development of thermoelectric technology. With increasing awareness of the stability issue, many stability studies have been carried out during the past decade, and a wide range of characterization methods have been employed for evaluating stability. Here, we present a perspective on the most common characterization methods used in the thermoelectric literature for testing materials stability. Based on the parameter control and comparability to real-world conditions, we propose a division of stability tests into three overall levels to encourage more elaborate stability studies to be made with a high degree of real-world resemblance. For each level, we propose a reporting protocol to streamline the experimental information in stability studies, which will make comparison of results across studies much more reliable. We believe that these steps will help bridge the gap between fundamental materials science and actual application, which is necessary for better maturing thermoelectric technology towards broad commercialization.

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 Co‒N bond, which have distinct covalent features. Variable temperature Raman spectra reveal 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 nine solid solution structures of CoxZn1-x-ZIF-zni (x = 0.3 to 0.003) were synthesized, and a mere 4% Co-doping into Zn-ZIF-zni results in thermal decomposition instead of melting. These findings advocate significant chemical space for de-signing new MOF glasses either by engineering the chemical bonding with ligand functionalization or by avoiding defects at the metal site in the crystal lattice.

Chemical science 13(43), 12883 - 12891 (2022). doi:10.1039/D2SC04522B, The structures of metal ions in solution constitute essential information for obtaining chemical insight spanning from catalytic reaction mechanisms to formation of functional nanomaterials. Here, we explore Zr$^{4+}$ solution structures using X-ray pair distribution function (PDF) analysis across pH (0–14), concentrations (0.1–1.5 M), solvents (water, methanol, ethanol, acetonitrile) and metal sources (ZrCl$_4$, ZrOCl$_2$·8H$_2$O, ZrO(NO$_3$)$_2$·xH$_2$O). In water, [Zr$_4$(OH)$_8$(OH$_2$)$_{16}$]$^{8+}$-tetramers are predominant, while non-aqueous solvents contain monomeric complexes. The PDF analysis also reveals second sphere coordination of chloride counter ions to the aqueous tetramers. The results are reproducible across data measured at three different beamlines at the PETRA-III and MAX IV synchrotron light sources., Published by RSC, Cambridge

Journal of materials chemistry / A 10(3), 1473 - 1480 (2022). doi:10.1039/D1TA09489K, In recent years hierarchical thermoelectric materials have been engineered to reach record breaking thermoelectric figures-of-merit (zT) making them attractive in green transition energy conversion applications. PbTe constitutes an archetypical example, where PbS nanoinclusions presumably scatter phonons, while leaving the electrical properties intact. For widespread use of such materials, their structural integrity and chemical stability are equally important to the thermoelectric performance. Here, we show by operando X-ray diffraction and physical property measurements that PbS nanoinclusions in PbTe are not stable at operating conditions. The PbS nanoinclusions grow in size, but surprisingly this does not diminish the thermoelectric performance. Comparison with samples without nanostructures reveals that the PbS inclusions primarily affect the electrical properties at elevated temperatures, and to a lesser degree the lattice thermal conductivity. Nanostructured materials have significantly moved thermoelectric performance limits, but if we are to develop materials by design rather than trial-and-error, it is important that the nature, effect and stability of nanoinclusions are characterized under operating conditions., Published by RSC, London [u.a.]

Thermoelectric materials allow for conversion of waste heat into electrical energy, and they represent a green solution for improving our energy efficiency. Inclusion of 4f electrons near the Fermi level may boost the Seebeck coefficient, which is essential for high thermoelectric performance. In this study, Ce was successfully substituted for Ba on the guest atom sites in the type-I clathrate Ba

SrMn(2)P(2) and CaMn(2)P(2) are insulators that adopt the trigonal CaAl(2)Si(2)-type structure containing corrugated Mn honeycomb layers. Magnetic susceptibility χ and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Néel temperature [Formula: see text] K for SrMn(2)P(2) and a strong first-order AFM transition at [Formula: see text] K for CaMn(2)P(2). Both compounds exhibit isotropic and nearly T-independent [Formula: see text] , suggesting magnetic structures in which nearest-neighbor moments are aligned at [Formula: see text] to each other. The (31)P NMR measurements confirm the strong first-order transition in CaMn(2)P(2) but show critical slowing down above [Formula: see text] for SrMn(2)P(2), thus also evidencing second-order character. The (31)P NMR measurements indicate that the AFM structure of CaMn(2)P(2) is commensurate with the lattice whereas that of SrMn(2)P(2) is incommensurate. These first-order AFM transitions are unique among the class of (Ca, Sr, Ba)Mn(2) (P, As, Sb, Bi)(2) compounds that otherwise exhibit second-order AFM transitions. This result challenges our understanding of the circumstances under which first-order AFM transitions occur.

Brookite is the most difficult TiO2 polymorph to obtain in phase pure form. Here we report on a facile synthesis method for phase-pure brookite nanoparticles using a broad range of titanium precursors. General availability of phase pure brookite opens up multiple research activities to explore both fundamental properties as well as applications. The synthesis method is scalable and thus it potentially allows for large-scale industrial production.

We demonstrate a facile selective synthesis of phase-pure anatase, rutile, and brookite nanocrystal polymorphs of titania (TiO

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.

The recent discovery of n‐type Mg3Sb2 thermoelectrics has ignited intensive research activities on searching for potential n‐type dopants for this material. Using first‐principles defect calculations, here, a systematic computational screening of potential efficient n‐type lanthanide dopants is conducted for Mg3Sb2. In addition to La, Ce, Pr, and Tm, it is found that high electron concentration (≳1020 cm−3 at the growth temperature of 900 K) can be achieved by doping on the Mg sites with Nd, Gd, Ho, and Lu, which are generally more efficient than other lanthanide dopants and the anion‐site dopant Te. Experimentally, Nd and Tm are confirmed as effective n‐type dopants for Mg3Sb2 since doping with Nd and Tm shows higher electron concentration and thermoelectric figure of merit zT than doping with Te. Through codoping with Nd (Tm) and Te, simultaneous power factor improvement and thermal conductivity reduction are achieved. As a result, high zT values of ≈1.65 and ≈1.75 at 775 K are obtained in n‐type Mg3.5Nd0.04Sb1.97Te0.03 and Mg3.5Tm0.03Sb1.97Te0.03, respectively, which are among the highest values for n‐type Mg3Sb2 without alloying with Mg3Bi2. This work sheds light on exploring promising n‐type dopants for the design of Mg3Sb2 thermoelectrics., A systematic computational screening reveals that Nd, Gd, Ho, and Lu are strong candidates as efficient n‐type dopants for Mg3Sb2 thermoelectrics. Experimentally, Nd and Tm are confirmed as efficient n‐type dopants and high thermoelectric figure of merit zT values above 1.6 at 775 K are obtained in n‐type (Nd, Te)‐codoped and (Tm, Te)‐codoped Mg3Sb2 without Mg3Bi2 alloying.

FeSb

Titania nanocrystals are used in numerous applications but specific polymorphs (anatase, rutile, brookite) are typically required in specific applications making synthesis control over the crystal phase essential. Supercritical continuous flow reactors constitute fast, scalable alternatives to conventional autoclave hydrothermal synthesis. They provide outstanding control over nanoparticle characteristics such as size, crystallinity, and morphology but previous studies have always resulted in anatase products. Here we report, for the first time, a continuous hydrothermal flow method for obtaining phase pure rutile nanoparticles thereby significantly broadening the crystal design space for large scale titania applications. Through variation of the reactor temperature, the dimensions of the rod-like rutile crystallites are tunable in a range of 35 to 60 nm in length and 10 to 35 nm in width (maximum aspect ratio of ∼3.5) leading to a tunable band gap (3.2-3.5 eV) and high specific surface areas exceeding 200 m

n-type Mg

Nano-sized particles of rutile TiO