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Significance With rare exceptions, an antiferromagnetic (AFM) transition in zero magnetic field is thermodynamically of second order where the thermal-average magnetic moments of the magnetic atoms (ordered moments) vary continuously on cooling through the AFM ordering temperature T N with no latent heat at the transition. Such materials include the AFM pnictides CaMn 2 As 2 , SrMn 2 As 2 , CaMn 2 Sb 2 , SrMn 2 Sb 2 , and CaMn 2 Bi 2 . Here we demonstrate that the closely related SrMn 2 P 2 and CaMn 2 P 2 insulators instead exhibit first-order AFM transitions at T N = 53 and 70 K, respectively, where the heat capacity exhibits a latent heat at T N . The mechanism causing these first-order transitions remains to be explained, but its understanding may lead to the development of novel magnetic materials of technological interest.

Through meticulous care for detail, researchers have now shattered the ceiling on thermoelectric performance, achieving a figure of merit above 3 for bulk SnSe polycrystalline powder.

β-Zn

Hafnia, HfO

Developing alternatives to platinum-based electrocatalysts for the hydrogen evolution reaction (HER) is an important challenge for realizing the green transition. This is especially the case for alkaline conditions where Pt-based catalysts have very poor stability. Here, we demonstrate new solvothermal synthesis methods with facile allotropism control for selectively obtaining hexagonal-close-packed (hcp) and face-centered cubic (fcc) ruthenium nanoparticles. Both samples are highly active HER catalysts in alkaline conditions outperforming commercial Pt/C. However, the samples show markedly different stabilities. The hcp sample shows exceptional stability for 12 hours constant operation at 10 mA/cm2 with an overpotential that only increases 6 mV whereas the fcc sample increases 50 mV and the commercial Pt/C more than 350 mV. Thus, this study underlines the importance of controlling the crystal structure of nanoparticle electrocatalysts and shows the potential of using Ru as an alternative to Pt in alkaline conditions.

Operando characterization provides direct insight into material response under application conditions and it is essential to understand the stability limits of thermoelectric materials and their decomposition mechanisms. An operando setup capable of maintaining a thermal gradient while running DC current through a bar-shaped sample has been developed. Under operating conditions, X-ray scattering data can be measured along the sample to obtain spatially resolved structural knowledge in concert with measurement of electrical resistance and the Seebeck coefficient. Here thermoelectric β-Zn4Sb3, which is a mixed ionic–electronic conductor, is studied, and a significant temperature dependence of the Zn migration is directly observed. Measurements with the thermal gradient applied either along or opposite to the DC current establish that the ion migration is an electrochemical effect rather than a thermodiffusion. Consideration of only the applied critical voltage or current density is insufficient for deducing the stability limits and structural integrity of materials with temperature-dependent ion mobility. The present operando setup is not limited to studies of thermoelectric materials, and it also lends itself to studies of, for example, ion diffusion in solid-state electrolytes or structural transformations in solid-state reactions.

In this methodological brief, we demonstrate the usefulness of the restricted variance (RV) interaction to entrepreneurship research. RV reasoning can help scholars to specify precise roles for contextual moderators. This specificity allows for strengthening of arguments and for testing of one of the reasons for an interaction. In some cases, it points to otherwise unexpected interaction patterns. We illustrate the power of RV reasoning with a data set of 503 biotech firms to show how RV explains geographic differences in the relationship between number of alliances and initial public offering value. Finally, we show how these interactions can be tested given typical distributions.

Solid-state, natural-abundance (95)Mo NMR experiments of four different MoS(2) materials have been performed on a magnet B(0) = 19.6 T and on a new Series Connected Hybrid (SCH) magnet at 35.2 T. Employing two different 2H-MoS(2) (2H phase) materials, a “pseudo-amorphous” MoS(2) nano-material, and a MoS(2) layer on the Al(2)O(3) support of a hydrodesulphurization (HDS) catalyst have enabled introduction of solid-state (95)Mo NMR as an important analytical tool in studies of MoS(2) nano-materials. (95)Mo spin-lattice relaxation time (T(1)) studies of 160- and 4-layer 2H-MoS(2) samples at 19.6 and 35.2 T show their relaxation rates (1/T(1)) increase in proportion to B(0)(2). This is in accord with chemical shift anisotropy (CSA) relaxation being the dominant T(1)((95)Mo) mechanism, with a large (95)Mo CSA = 1025 ppm determined for all four MoS(2) nano-materials. The dominant CSA mechanism suggests the MoS(2) band-gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (ω(o)τ(c) << 1) for (95)Mo CSA in 2H-MoS(2). A decrease in T(1)((95)Mo) is observed for an increase in B(0) field and for a decrease in the number of 2H-MoS(2) layers. All four nano-materials exhibit identical (95)Mo electric field gradient (EFG) parameters. The T(1) results account for the several failures to retrieve (95)Mo spectral EFG and CSA parameters for multilayer 2H-MoS(2) samples in the pioneering solid-state (95)Mo NMR studies performed during the past two decades (1990–2010), because of the extremely long T(1)((95)Mo) = ~200–250 s observed at low B(0) (~9.4 T) used at that time. Much shorter T(1)((95)Mo) values are observed even at 19.6 T for the “pseudo-amorphous” and the HDS catalyst (MoS(2)-Al(2)O(3) support) MoS(2) nano-materials. These allowed useful solid-state (95)Mo NMR spectra for these two samples to be obtained at 19.6 T in a few to < 24 h. Most importantly, this research led to observation of an impressive (95)Mo MAS spectrum for an average of 1–4 thick MoS(2)-layers on a Al(2)O(3) support, i.e., the first MAS NMR spectrum of a low natural-abundance, low-γ quadrupole-nucleus species layered on a catalyst support. While a huge gain in NMR sensitivity, factor ~ 60, is observed for the (95)Mo MAS spectrum of the 160-layer sample at 35.2 T compared to 14.1 T, the MAS spectrum for the 4-layer sample is almost completely wiped out at 35.2 T. This unusual observation for the 4-layer sample (crumpled, rose-like and defective Mo-edge structures) is due to an increased distribution of the isotropic (95)Mo shifts in the (95)Mo MAS spectra at B(0) up to 35.2 T upon reduction of the number of sample layers.

The lead chalcogenides with rocksalt structure, $\mathrm{Pb}X$ ($X$ = S, Se, Te), show great promise for thermoelectric applications in various doped and alloyed forms. Part of the explanation for this is their favorable low lattice thermal conductivities whose origin has been heavily debated. Powder x-ray diffraction experiments suggested Pb to be off-centered particularly in PbS. However, as shown here, single crystal x-ray diffraction shows the anharmonicity to be insignificant in PbS from 20 to 300 K, while it is significant from 100 to 622 K in PbTe although without leading to any Pb off-centering. Inelastic neutron scattering and anharmonic ab initio studies previously suggested the soft transverse optical mode to play an important role with respect to lowering the thermal conductivities of the lead chalcogenides. Here, the diffuse x-ray scattering of single crystal PbS is modeled from 20 to 300 K by correlated atomic displacements. The results indicate softening of the zone center transverse optical mode with decreasing temperature. From a chemical bonding perspective, the soft transverse optical mode can be rationalized by invoking the concepts of stereoactive lone pair formation and metavalent/resonant bonding.

Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Rational material design requires a deep understanding about the relationship between the structure and properties of materials, which are both intimately related to their chemical bonding. Through the experimentally observable electron density, chemical bonding can be understood from experimental and theoretical points of view on an equal footing, and advances in accurate X-ray diffraction measurements and computational techniques over the past decades have provided access to electron density distributions in increasingly complex functional materials. In this Review, selected electron density studies from the literature on a wide range of materials classes are presented, including studies of thermoelectric materials, high pressure electrides, coordination polymers and non-linear optical materials. These studies demonstrate how detailed analysis of chemical bonding based on the electron density provides important understanding of materials beyond arguments based on structure and simple chemical concepts. In cases such as understanding the conducting properties of Zintl semiconductors or the effect of mutual electrical polarization in host–guest systems, it is clearly imperative to go beyond structure and examine the chemical bonding in detail. In the Review, the complementarity between theory and experiment is underlined, which allows for mutual validation of new chemical bonding concepts, and indeed experiment and theory may challenge each other based on the different strengths and weaknesses of each method.

Solar-driven production of renewable energy (e.g., H2) has been investigated for decades. To date, the applications are limited by low efficiency due to rapid charge recombination (both radiative and nonradiative modes) and slow reaction rates. Tremendous efforts have been focused on reducing the radiative recombination and enhancing the interfacial charge transfer by engineering the geometric and electronic structure of the photocatalysts. However, fine-tuning of nonradiative recombination processes and optimization of target reaction paths still lack effective control. Here we show that minimizing the nonradiative relaxation and the adsorption energy of photogenerated surface-adsorbed hydrogen atoms are essential to achieve a longer lifetime of the charge carriers and a faster reaction rate, respectively. Such control results in a 16-fold enhancement in photocatalytic H2 evolution and a 15-fold increase in photocurrent of the crystalline g-C3N4 compared to that of the amorphous g-C3N4.

The observation of an unusual crystal habit in the common diuretic drug hydrochlorothiazide (HCT), and identification of its subtle conformational chirality, has stimulated a detailed investigation of its crystalline forms. Enantiomeric conformers of HCT resolve into an unusual structure of conjoined enantiomorphic twin crystals comprising enantiopure domains of opposite chirality. The purity of the domains and the chiral molecular conformation are confirmed by spatially revolved synchrotron micro-XRD experiments and neutron diffraction, respectively. Macroscopic inversion twin symmetry observed between the crystal wings suggests a pseudoracemic structure that is not a solid solution or a layered crystal structure, but an unusual structural variant of conglomerates and racemic twins. Computed interaction energies for molecular pairs in the racemic and enantiopure polymorphs of HCT, and the observation of large opposing unit-cell dipole moments for the enantiopure domains in these twin crystals, suggest a plausible crystal nucleation mechanism for this unusual crystal habit.

High-performing thermoelectric materials such as Zn4Sb3and clathrates have atomic disorder as the root to their favorable properties. This makes it extremely difficult to understand and model their properties at a quantitative level, and thus effective structure–property relations are challenging to obtain. Cu2−xSe is an intensely studied, cheap and non-toxic high performance thermoelectric, which exhibits highly peculiar transport properties, especially near the β-to-α phase transition around 400 K, which must be related to the detailed nature of the crystal structure. Attempts to solve the crystal structure of the low-temperature phase, β-Cu2−xSe, have been unsuccessful since 1936. So far, all studies have assumed that β-Cu2−xSe has a three-dimensional periodic structure, but here we show that the structure is ordered only in two dimensions while it is disordered in the third dimension. Using the three-dimensional difference pair distribution function (3D-ΔPDF) analysis method for diffuse single-crystal X-ray scattering, the structure of the ordered layer is solved and it is shown that there are two modes of stacking disorder present which give rise to an average structure with higher symmetry. The present approach allows for a direct solution of structures with disorder in some dimensions and order in others, and can be thought of as a generalization of the crystallographic Patterson method. The local and extended structure of a solid determines its properties and Cu2−xSe represents an example of a high-performing thermoelectric material where the local atomic structure differs significantly from the average periodic structure observed from Bragg crystallography.

The covalent nature of the low-barrier N−H−N hydrogen bonds in the negative thermal expansion material H 3[Co(CN) 6] has been established by using a combination of X-ray and neutron diffraction electron density analysis and theoretical calculations. This finding explains why negative thermal expansion can occur in a material not commonly considered to be built from rigid linkers. The pertinent hydrogen atom is located symmetrically between two nitrogen atoms in a double-well potential with hydrogen above the barrier for proton transfer, thus forming a low-barrier hydrogen bond. Hydrogen is covalently bonded to the two nitrogen atoms, which is the first experimentally confirmed covalent hydrogen bond in a network structure. Source function calculations established that the present N−H−N hydrogen bond follows the trends observed for negatively charge-assisted hydrogen bonds and low-barrier hydrogen bonds previously established for O−H−O hydrogen bonds. The bonding between the cobalt and cyanide ligands was found to be a typical donor–acceptor bond involving a high-field ligand and a transition metal in a low-spin configuration.

Chemistry - a European journal 25(8), 2051 - 2058 (2019). doi:10.1002/chem.201805024, The concept of secondary building units (SBUs) is central to all science on metal‐organic frameworks (MOFs), and they are widely used to design new MOF materials. However, the presence of SBUs during MOF formation remains controversial, and the formation mechanism of MOFs remains unclear, due to limited information about the evolution of prenucleation cluster structures. Here in situ pair distribution function (PDF) analysis was used to probe UiO‐66 formation under solvothermal conditions. The expected SBU—a hexanuclear zirconium cluster—is present in the metal salt precursor solution. Addition of organic ligands results in a disordered structure with correlations up to 23 Å, resembling crystalline UiO‐66. Heating leads to fast cluster aggregation, and further growth and ordering results in the crystalline product. Thus, SBUs are present already at room temperature and act as building blocks for MOF formation. The proposed formation steps provide insight for further development of MOF synthesis., Published by Wiley-VCH, Weinheim

Solid-state, natural-abundance 95Mo NMR experiments of four different MoS2 materials have been performed on a magnet at B0 = 19.6 T and on a new series-connected hybrid magnet at 35.2 T. Employing two different 2H-MoS2 (2H phase) materials, a "pseudo-amorphous"MoS2 nanomaterial and a MoS2 layer on an Al2O3 support of a hydrodesulfurization (HDS) catalyst, has enabled the introduction of solid-state 95Mo NMR as an important analytical tool in the study of MoS2 nanomaterials. 95Mo spin-lattice relaxation time (T1) studies of 160- and 4-layer 2H-MoS2 samples at 19.6 and 35.2 T show their relaxation rate (1/T1) increase in proportion to B02. This is in accord with chemical shift anisotropy (CSA) relaxation, which is the dominant T1(95Mo) mechanism, with a large 95Mo CSA of 1025 ppm determined for all four MoS2 nanomaterials. The dominant CSA mechanism suggests that the MoS2 band gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (ωoτc ≈ 1) for 95Mo CSA in 2H-MoS2. A decrease in T1(95Mo) is observed for an increase in the B0 field and for a decrease in the number of 2H-MoS2 layers. All four nanomaterials exhibit identical 95Mo electric-field gradient (EFG) parameters. The T1 results account for the several failures in retrieving the 95Mo spectral EFG and CSA parameters for multilayer 2H-MoS2 samples in the pioneering solid-state 95Mo NMR studies performed during the past 2 decades (1990-2010) because of the extremely long T1(95Mo) = ∼200-250 s observed at a low B0 (∼9.4 T) used at that time. Much shorter T1(95Mo) values are observed even at 19.6 T for the "pseudo-amorphous"and the HDS catalyst (MoS2-Al2O3 support) MoS2 nanomaterials. These allowed obtaining useful solid-state 95Mo NMR spectra for these two samples at 19.6 T in a few to

We demonstrate systematic tuning in the optical bandgaps of molecular crystals achieved by the generation of molecular alloys/solid solutions of a series of diphenyl dichalcogenides - characterized by weak chalcogen bonding interactions involving S, Se, and Te atoms. Despite the variety in chalcogen bonding interactions found in this series of dichalcogenide crystals, they show isostructural interaction topologies, enabling the formation of solid solutions. The alloy crystals exhibit Vegard's law-like trends of variation in their unit cell dimensions and a nonlinear trend for the variation in optical bandgaps with respect to their compositions. Energy-dispersive X-ray and spatially resolved Raman spectroscopic studies indicate significant homogeneity in the domain structure of the solid solutions. Quantum periodic calculations of the projected density of states provide insights into the bandgap tuning in terms of the mixing of states in the alloy crystal phases.

Two-dimensional polymeric graphitic carbon nitride (g-C3N4) is a low-cost material with versatile properties that can be enhanced by the introduction of dopant atoms and by changing the degree of polymerization/stoichiometry, which offers significant benefits for numerous applications. Herein, we investigate the stability of g-C3N4under electron beam irradiation inside a transmission electron microscope operating at different electron acceleration voltages. Our findings indicate that the degradation of g-C3N4occurs with N species preferentially removed over C species. However, the precise nitrogen group from which N is removed from g-C3N4(C-N-C, NH or -NH2) is unclear. Moreover, the rate of degradation increases with decreasing electron acceleration voltage, suggesting that inelastic scattering events (radiolysis) dominate over elastic events (knock-on damage). The rate of degradation by removing N atoms is also sensitive to the current density. Hence, we demonstrate that both the electron acceleration voltage and the current density are parameters with which one can use to control the stoichiometry. Moreover, as N species were preferentially removed, thed-spacing of the carbon nitride structure increased. These findings provide a deeper understanding of g-C3N4

Two-dimensional polymeric graphitic carbon nitride (g-C3N4) is a low-cost material with versatile properties that can be enhanced by the introduction of dopant atoms and by changing the degree of polymerization/stoichiometry, which offers significant benefits for numerous applications. Herein, we investigate the stability of g-C3N4 under electron beam irradiation inside a transmission electron microscope operating at different electron acceleration voltages. Our findings indicate that the degradation of g-C3N4 occurs with N species preferentially removed over C species. However, the precise nitrogen group from which N is removed from g-C3N4 (C-N-C, [double bond, length as m-dash]NH or -NH2) is unclear. Moreover, the rate of degradation increases with decreasing electron acceleration voltage, suggesting that inelastic scattering events (radiolysis) dominate over elastic events (knock-on damage). The rate of degradation by removing N atoms is also sensitive to the current density. Hence, we demonstrate that both the electron acceleration voltage and the current density are parameters with which one can use to control the stoichiometry. Moreover, as N species were preferentially removed, the d-spacing of the carbon nitride structure increased. These findings provide a deeper understanding of g-C3N4.

The indium oxides, c-In2O3, h-In2O3, InOOH and In(OH)3, constitute an important class of wide band gap semiconductors. Synthesis of any indium oxide phase involves manoeuvring in a complex matrix of process parameters, and some phases are only obtained through controlled phase transformations. Considering the widespread use of indium oxide semiconductors it is restrictive that no coherent picture exists of the formation mechanisms of individual phases and phase transformations between them. Here we access the indium oxide system through solvothermal synthesis and/or powder calcinations, and we use in situ X-ray scattering in combination with thermal analysis to investigate the complex phase relations. This allows us to unravel synthesis pathways for the different indium oxide phases, and the insight is used to develop procedures for scalable continuous flow solvothermal synthesis. Direct formation of crystalline nanomaterials from precursor solutions was observed for In(OH)3, InOOH and cubic c-In2O3, while formation of hexagonal h-In2O3 requires thermal decomposition of InOOH. The in situ X-ray scattering data reveal new phase transformations from In(OH)3 to InOOH, and from InOOH to c-In2O3. Interestingly, solvothermal synthesis conditions facilitate different reactions mechanisms than dry powder calcinations, and both In(OH)3 and InOOH have different transformations under dry and wet conditions. This journal is.