The Ambiguous Origin of Thermochromism in Molecular Crystals of Dichalcogenides: Chalcogen Bonds versus Dynamic Se-Se/Te-Te Bonds

We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High-pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism-especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se-Se and Te-Te bonds. Further, a co-crystal of dpdSe with iodine (dpdSe-I2 ) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling.

Comparative study of conventional and synchrotron X-ray electron densities on molecular crystals

Five different electron density datasets obtained from conventional and synchrotron single crystal X-ray diffraction experiments are compared. The general aim of the study is to investigate the quality of data for electron density analysis from current state-of-the-art conventional sources, and to see how the data perform in comparison with high-quality synchrotron data. A molecular crystal of melamine was selected as the test compound due to its ability to form excellent single crystals, the light atom content, and an advantageous suitability factor of 3.6 for electron density modeling. These features make melamine an optimal system for conventional X-ray diffractometers since the inherent advantages of synchrotron sources such as short wavelength and high intensity are less critical in this case. Data were obtained at 100 K from new in-house diffractometers Rigaku Synergy-S (Mo and Ag source, HyPix100 detector) and Stoe Stadivari (Mo source, EIGER2 1M CdTe detector), and an older Oxford Diffraction Supernova (Mo source, Atlas CCD detector). The synchrotron data were obtained at 25 K from BL02B1 beamline at SPring-8 in Japan (λ = 0.2480 Å, Pilatus3 X 1M CdTe detector). The five datasets were compared on general quality parameters such as resolution, ⟨I/σ⟩, redundancy and R factors, as well as the more model specific fractal dimension plot and residual density maps. Comparison of the extracted electron densities reveals that all datasets can provide reliable multipole models, which overall convey similar chemical information. However, the new laboratory X-ray diffractometers with advanced pixel detector technology clearly measure data with significantly less noise and much higher reliability giving densities of higher quality, compared to the older instrument. The synchrotron data have higher resolution and lower measurement temperature, and they allow for finer details to be modeled (e.g. hydrogen κ parameters).

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.

A Machine-Learning-Based Approach for Solving Atomic Structures of Nanomaterials Combining Pair Distribution Functions with Density Functional Theory

Determination of crystal structures of nanocrystalline or amorphous compounds is a great challenge in solid-state chemistry and physics. Pair distribution function (PDF) analysis of X-ray or neutron total scattering data has proven to be a key element in tackling this challenge. However, in most cases, a reliable structural motif is needed as a starting configuration for structure refinements. Here, an algorithm that is able to determine the crystal structure of an unknown compound by means of an on-the-fly trained machine learning model, which combines density functional theory calculations with comparison of calculated and measured PDFs for global optimization in an artificial landscape, is presented. Due to the nature of this landscape, even metastable configurations and stacking disorders can be identified.

On single-crystal total scattering data reduction and correction protocols for analysis in direct space. Corrigendum

The name of the third author of the article by Koch et al. [Acta Cryst. (2021). A77, 611–636] is corrected.

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 Å similar 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.

A Novel Nanocomposite Material for Optically Stimulated Luminescence Dosimetry

Radiotherapy is a well-established and important treatment for cancer tumors, and advanced technologies can deliver doses in complex three-dimensional geometries tailored to each patient's specific anatomy. A 3D dosimeter, based on optically stimulated luminescence (OSL), could provide a high accuracy and reusable tool for verifying such dose delivery. Nanoparticles of an OSL material embedded in a transparent matrix have previously been proposed as an inexpensive dosimeter, which can be read out using laser-based methods. Here, we show that Cu-doped LiF nanocubes (nano-LiF:Cu) are excellent candidates for 3D OSL dosimetry owing to their high sensitivity, dose linearity, and stability at ambient conditions. We demonstrate a scalable synthesis technique producing a material with the attractive properties of a single dosimetric trap and a single near-ultraviolet emission line well separated from visible-light stimulation sources. The observed transparency and light yield of silicone sheets with embedded nanocubes hold promise for future 3D OSL-based dosimetry.

Highly efficient and stable Ru nanoparticle electrocatalyst for the hydrogen evolution reaction in alkaline conditions

Ru nanoparticles are prepared via solvothermal synthesis with allotropism control. Both fcc and hcp samples are active catalysts for the hydrogen evolution reaction, but the hcp sample is stable during 12 hour operation.

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.

On single-crystal total scattering data reduction and correction protocols for analysis in direct space

Data reduction and correction steps and processed data reproducibility in the emerging single-crystal total-scattering-based technique of three-dimensional differential atomic pair distribution function (3D-ΔPDF) analysis are explored. All steps from sample measurement to data processing are outlined using a crystal of CuIr₂S₄ as an example, studied in a setup equipped with a high-energy X-ray beam and a flat-panel area detector. Computational overhead as pertains to data sampling and the associated data-processing steps is also discussed. Various aspects of the final 3D-ΔPDF reproducibility are explicitly tested by varying the data-processing order and included steps, and by carrying out a crystal-to-crystal data comparison. Situations in which the 3D-ΔPDF is robust are identified, and caution against a few particular cases which can lead to inconsistent 3D-ΔPDFs is noted. Although not all the approaches applied herein will be valid across all systems, and a more in-depth analysis of some of the effects of the data-processing steps may still needed, the methods collected herein represent the start of a more systematic discussion about data processing and corrections in this field.

Tuning of bandgaps and emission properties of light-emitting diode materials through homogeneous alloying in molecular crystals

Alloy formation is ubiquitous in inorganic materials science, and it strongly depends on the similarity between the alloyed atoms. Since molecules have widely different shapes, sizes and bonding properties, it is highly challenging to make alloyed molecular crystals. Here we report the generation of homogenous molecular alloys of organic light emitting diode materials that leads to tuning in their bandgaps and fluorescence emission. Tris(8-hydroxyquinolinato)aluminium (Alq₃) and its Ga, In and Cr analogues (Gaq₃, Inq₃, and Crq₃) form homogeneous mixed crystal phases thereby resulting in binary, ternary and even quaternary molecular alloys. The M_xM′_(1−x)q₃ alloy crystals are investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy on single crystal samples, and photoluminescence properties are measured on the exact same single crystal specimens. The different series of alloys exhibit distinct trends in their optical bandgaps compared with their parent crystals. In the Al_xGa_(1−x)q₃ alloys the emission wavelengths lie in between those of the parent crystals, while the Al_xIn_(1−x)q₃ and Ga_xIn_(1−x)q₃ alloys have red shifts. Intriguingly, efficient fluorescence quenching is observed for the M_xCr_(1−x)q₃ alloys (M = Al, Ga) revealing the effect of paramagnetic molecular doping, and corroborating the molecular scale phase homogeneity.

Multicomponent molecular alloy crystals exhibit intriguing effects of tuning and quenching in their photoluminescence, suggesting ‘alloy-crystal engineering’ as a useful design strategy for molecular functional materials.

Stability and Thermoelectric Properties of Zn4Sb3 with TiO2 Nanoparticle Inclusions

β-Zn₄Sb₃ is a cheap nontoxic high-performance thermoelectric material, which unfortunately suffers from stability issues because of zinc migration in thermal and electrical gradients. Here, the thermoelectric properties and thermal stability of β-Zn₄Sb₃ mixed with varying sizes and weight percentages of TiO₂ nanoparticles are investigated. Furthermore, the stability of pressed β-Zn₄Sb₃-TiO₂ nanocomposite pellets is investigated by measuring high-energy synchrotron powder X-ray diffraction (PXRD) data during operating conditions using the Aarhus thermoelectric operando setup (ATOS). Through these studies, it is determined that TiO₂ nanoparticle addition in pressed pellets of β-Zn₄Sb₃ does not prevent Zn migration, and even though effects are seen in the thermal conductivity and electrical resistivity, the overall zT remains unchanged regardless of TiO₂ nanoinclusions. For the present samples, the Seebeck coefficients are unaffected by the addition of nanoparticles, and thus, there is no observed energy-filtering effect. The operando PXRD data reveal that the TiO₂ nanoinclusions lower the degradation rate by up to 75%, but all samples eventually decompose. This is corroborated by long-term stability tests performed using a thermal gradient. In conclusion, TiO₂ nanoinclusions do not degrade the excellent thermoelectric properties of β-Zn₄Sb₃, but the stabilizing effect is not sufficient for establishing long-term operating stability.

Unravelling the complex formation mechanism of HfO2 nanocrystals using in situ pair distribution function analysis

Hafnia, HfO₂, which is a wide band gap semiconducting oxide, is much less studied than the chemically similar zirconia (ZrO₂). Here, we study the formation of hafnia nanocrystals from hafnium tetrachloride in methanol under solvothermal conditions (248 bar, 225-450 °C) using complementary in situ powder X-ray diffraction (PXRD) and Pair Distribution Function (PDF) analysis. The main structural motif of the precursor solution (HfCl₄ dissolved in methanol) is a Hf oxide trimer with very similar local structure to that of m-HfO₂. Different measurements on precursor solutions show large intensity variation for the Hf-Cl correlations signifying different extents of HCl elimation. A few seconds of heating lead to a correlation appearing at 3.9 Å corresponding to corner-sharing Hf-polyhedra in a disordered solid matrix. During the next minutes (depending on temperature) the disordered structure rearranges and the nearest neighbour Hf-Hf distance contracts while the Hf-O coordination number increases. After approximately 90 seconds (at T = 250 °C) the structural rearrangement terminates and 1-2 nm nanocrystals of m-HfO₂ nucleate. Initially the m-HfO₂ nanocrystals have significant disorder as reflected in large Hf atomic displacement parameter (ADP) values, but as the nanocrystals grow to 5-6 nm in size during extended heating, the Hf ADPs decrease toward the values obtained for ordered bulk structures. The nanocrystal growth is not well modelled by the Johnson-Mehl-Avrami expression reflecting that multiple complex chemical processes occur during this highly nonclassical nanocrystal formation under solvothermal conditions.

Tuneable local order in thermoelectric crystals

Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Such disordered crystals with distinct local structures have unique properties arising from both collective and localized behaviour. Different local orderings can exist with identical average structures, making their differences hidden to Bragg diffraction methods. Using high-quality single-crystal X-ray diffuse scattering the local order in thermoelectric half-Heusler Nb1-x CoSb is investigated, for which different local orderings are observed. It is shown that the vacancy distribution follows a vacancy repulsion model and the crystal composition is found always to be close to x = 1/6 irrespective of nominal sample composition. However, the specific synthesis method controls the local order and thereby the thermoelectric properties thus providing a new frontier for tuning material properties.

Structural evolution in thermoelectric zinc antimonide thin films studied by in situ X-ray scattering techniques

Zinc antimonides have been widely studied owing to their outstanding thermoelectric properties. Unlike in the bulk state, where various structurally unknown phases have been identified through their specific physical properties, a number of intermediate phases in the thin-film state remain largely unexplored. Here, in situ X-ray diffraction and X-ray total scattering are combined with in situ measurement of electrical resistivity to monitor the crystallization process of as-deposited amorphous Zn-Sb films during post-deposition annealing. The as-deposited Zn-Sb films undergo a structural evolution from an amorphous phase to an intermediate crystalline phase and finally the ZnSb phase during heat treatment up to 573 K. An intermediate phase (phase B) is identified to be a modified β-Zn8Sb7 phase by refinement of the X-ray diffraction data. Within a certain range of Sb content (∼42-55 at%) in the films, phase B is accompanied by an emerging Sb impurity phase. Lower Sb content leads to smaller amounts of Sb impurity and the formation of phase B at lower temperatures, and phase B is stable at room temperature if the annealing temperature is controlled. Pair distribution function analysis of the amorphous phase shows local ordered units of distorted ZnSb4 tetrahedra, and annealing leads to long-range ordering of these units to form the intermediate phase. A higher formation energy is required when the intermediate phase evolves into the ZnSb phase with a significantly more regular arrangement of ZnSb4 tetrahedra.

Exciting Opportunities for Solid-State 95Mo NMR Studies of MoS2 Nano-structures in Materials Research from Low to Ultra-high Magnetic Field (35.2 T)

Solid-state, natural-abundance 95Mo NMR experiments of four different MoS2 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-MoS2 (2H phase) materials, a "pseudo-amorphous" MoS2 nano-material, and a MoS2 layer on the Al2O3 support of a hydrodesulphurization (HDS) catalyst have enabled introduction of solid-state 95Mo NMR as an important analytical tool in studies of MoS2 nano-materials. 95Mo spin-lattice relaxation time (T 1) studies of 160- and 4-layer 2H-MoS2 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(95Mo) mechanism, with a large 95Mo CSA = 1025 ppm determined for all four MoS2 nano-materials. The dominant CSA mechanism suggests the MoS2 band-gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (ω oτc << 1) for 95Mo CSA in 2H-MoS2. A decrease in T 1(95Mo) is observed for an increase in B 0 field and for a decrease in the number of 2H-MoS2 layers. All four nano-materials exhibit identical 95Mo electric field gradient (EFG) parameters. The T 1 results account for the several failures to retrieve 95Mo spectral EFG and CSA parameters for multilayer 2H-MoS2 samples in the pioneering solid-state 95Mo NMR studies performed during the past two decades (1990-2010), because of the extremely long T 1(95Mo) = ~200-250 s observed at low B 0 (~9.4 T) used at that time. Much shorter T 1(95Mo) values are observed even at 19.6 T for the "pseudo-amorphous" and the HDS catalyst (MoS2-Al2O3 support) MoS2 nano-materials. These allowed useful solid-state 95Mo 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 95Mo MAS spectrum for an average of 1-4 thick MoS2-layers on a Al2O3 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 95Mo 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 95Mo shifts in the 95Mo MAS spectra at B 0 up to 35.2 T upon reduction of the number of sample layers.

Bandgap Tuning in Molecular Alloy Crystals Formed by Weak Chalcogen Interactions

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.

Improved Thermoelectric Properties of N-Type Mg3Sb2 through Cation-Site Doping with Gd or Ho

The success of n-type doping has attracted strong research interest for exploring effective n-type dopants for Mg₃Sb₂ thermoelectrics. Herein, we experimentally study Gd and Ho as n-type dopants for Mg₃Sb₂ thermoelectrics. The synthesis, structural characterization, and thermoelectric properties of Gd-doped, Ho-doped, (Gd, Te)-codoped, and (Ho, Te)-codoped Mg₃Sb₂ samples are reported. It is found that Gd and Ho are effective n-type cation-site dopants showing a higher doping efficiency as well as a superior carrier concentration in comparison with anion-site doping with Te, consistent with the previous theoretical prediction. For n-type Mg₃Sb₂ samples doped with Gd or Ho, optimal thermoelectric figure of merit zT values of ∼1.26 and ∼0.94 at 725 K are obtained, respectively, in Mg_3.5Gd_0.04Sb₂ and Mg_3.5Ho_0.04Sb₂, which are superior to many reported Te-doped Mg₃Sb₂ without alloying with Mg₃Bi₂. By codoping with Gd (or Ho) and Te, reduced thermal conductivity and enhanced power factor values are achieved at high temperatures, which results in enhanced peak zT values well above unity at 725 K. This work reveals Gd and Ho as effective n-type dopants for Mg₃Sb₂ thermoelectric materials.

Tailoring the stoichiometry of C3N4 nanosheets under electron beam irradiation

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.

Locating Fe dopants in catalytic PtPd nanoparticles on γ-alumina using X-ray absorption spectroscopy

Adding Fe to the PtPd nanocatalyst increases propene activity by forming more metallic Pt and PdO during synthesis and increasing Pt–Pd bond formation upon aging.

Why Does Bi2WO6 Visible-Light Photocatalyst Always Form as Nanoplatelets?

Bi₂WO₆ nanocrystals exhibit excellent photocatalytic properties in the visible range of the solar spectrum, and intense efforts are directed at designing effective synthesis processes with control of size, morphology, and hierarchical structure. All known hydrothermal syntheses produce either nanoplatelet morphology or hierarchical structures based on such primary entities. Here we investigate the nucleation and growth of Bi₂WO₆ nanocrystals under hydrothermal conditions using in situ X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of Bi₂WO₆ nanoplates is due to the presence of disordered layers of Bi₂O₂²⁺ molecular complexes in the precursor solution with an approximate length of 13 Å. These layers interact with tetrahedral WO₄^2- molecular units and eventually form the disordered cubic (Bi_0.933W_0.067)O_1.6) crystalline phase. When enough tungsten units are intertwined between Bi₂O₂²⁺ layers formation of Bi₂WO₆ pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed, it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a (Bi_0.933W_0.067)O_1.6) impurity phase is always observed together with the main Bi₂WO₆ product. Excess tungsten is required in the precursor to form phase-pure Bi₂WO₆ material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.

A simple model for vacancy order and disorder in defective half-Heusler systems

Defective half-Heusler systems X 1-x YZ with large amounts of intrinsic vacancies, such as Nb1-x CoSb, Ti1-x NiSb and V1-x CoSb, are a group of promising thermoelectric materials. Even with high vacancy concentrations they maintain the average half-Heusler crystal structure. These systems show high electrical conductivity but low thermal conductivity arising from an ordered YZ substructure, which conducts electrons, while the large amounts of vacancies in the X substructure effectively scatters phonons. Using electron scattering, it was recently observed that, in addition to Bragg diffraction from the average cubic half-Heusler structure, some of these samples show broad diffuse scattering indicating short-range vacancy order, while other samples show sharp additional peaks indicating long-range vacancy ordering. Here it is shown that both the short- and long-range ordering can be explained using the same simple model, which assumes that vacancies in the X substructure avoid each other. The samples showing long-range vacancy order are in agreement with the predicted ground state of the model, while short-range order samples are quenched high-temperature states of the system. A previous study showed that changes in sample stoichiometry affect whether the short- or long-range vacancy structure is obtained, but the present model suggests that thermal treatment of samples should allow controlling the degree of vacancy order, and thereby the thermal conductivity, without changes in composition. This is important as the composition also dictates the amount of electrical carriers. Independent control of electrical carrier concentration and degree of vacancy order should allow further improvements in the thermoelectric properties of these systems.

Expression and interactions of stereochemically active lone pairs and their relation to structural distortions and thermal conductivity

In chemistry, stereochemically active lone pairs are typically described as an important non-bonding effect, and recent interest has centred on understanding the derived effect of lone pair expression on physical properties such as thermal conductivity. To manipulate such properties, it is essential to understand the conditions that lead to lone pair expression and provide a quantitative chemical description of their identity to allow comparison between systems. Here, density functional theory calculations are used first to establish the presence of stereochemically active lone pairs on antimony in the archetypical chalcogenide MnSb2O4. The lone pairs are formed through a similar mechanism to those in binary post-transition metal compounds in an oxidation state of two less than their main group number [e.g. Pb(II) and Sb(III)], where the degree of orbital interaction (covalency) determines the expression of the lone pair. In MnSb2O4 the Sb lone pairs interact through a void space in the crystal structure, and their their mutual repulsion is minimized by introducing a deflection angle. This angle increases significantly with decreasing Sb-Sb distance introduced by simulating high pressure, thus showing the highly destabilizing nature of the lone pair interactions. Analysis of the chemical bonding in MnSb2O4 shows that it is dominated by polar covalent interactions with significant contributions both from charge accumulation in the bonding regions and from charge transfer. A database search of related ternary chalcogenide structures shows that, for structures with a lone pair (SbX 3 units), the degree of lone pair expression is largely determined by whether the antimony-chalcogen units are connected or not, suggesting a cooperative effect. Isolated SbX 3 units have larger X-Sb-X bond angles and therefore weaker lone pair expression than connected units. Since increased lone pair expression is equivalent to an increased orbital interaction (covalent bonding), which typically leads to increased heat conduction, this can explain the previously established correlation between larger bond angles and lower thermal conductivity. Thus, it appears that for these chalcogenides, lone pair expression and thermal conductivity may be related through the degree of covalency of the system.

Direct Growth of Highly Strained Pt Islands on Branched Ni Nanoparticles for Improved Hydrogen Evolution Reaction Activity

The direct growth of Pt islands on lattice mismatched Ni nanoparticles is a major synthetic challenge and a promising strategy to create highly strained Pt atoms for electrocatalysis. By using very mild reaction conditions, Pt islands with tunable strain were formed directly on Ni branched particles. The highly strained 1.9 nm Pt-island on branched Ni nanoparticles exhibited high specific activity and the highest mass activity for hydrogen evolution (HER) in a pH 13 electrolyte. These results show the ability to synthetically tune the size of the Pt islands to control the strain to give higher HER activity.

Boosting Photocatalytic Hydrogen Production by Modulating Recombination Modes and Proton Adsorption Energy

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.

Investigation of an Unusual Crystal Habit of Hydrochlorothiazide Reveals Large Polar Enantiopure Domains and a Possible Crystal Nucleation Mechanism

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.

Solving the disordered structure of β-Cu2-xSe using the three-dimensional difference pair distribution function

High-performing thermoelectric materials such as Zn₄Sb₃ and 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. Cu_2-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-Cu_2-xSe, have been unsuccessful since 1936. So far, all studies have assumed that β-Cu_2-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 Cu_2-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.

Evolution of the Polymorph Selectivity of Titania Formation under Acidic and Low-Temperature Conditions

Evolution of the polymorph selectivity of titanium dioxide was studied under acidic and low-temperature synthesis conditions. Short synthesis times resulted in high relative amounts of the rutile phase, and long synthesis times resulted in high relative amounts of the brookite and anatase phases. The effect of titania precursor concentration was investigated and found to have a large impact on the polymorph selectivity. As the reaction proceeds with time, changes in the chemical environment, caused in particular by the gradually decreasing titania precursor concentration, are therefore likely the cause of the change in polymorph selectivity observed.

Measurement of Electric Fields Experienced by Urea Guest Molecules in the 18-Crown-6/Urea (1:5) Host-Guest Complex: An Experimental Reference Point for Electric-Field-Assisted Catalysis

High-resolution synchrotron and neutron single-crystal diffraction data of 18-crown-6/(pentakis)urea measured at 30 K are combined, with the aim of better appreciating the electrostatics associated with intermolecular interactions in condensed matter. With two 18-crown-6 molecules and five different urea molecules in the crystal, this represents the most ambitious combined X-ray/synchrotron and neutron experimental charge density analysis to date on a cocrystal or host-guest system incorporating such a large number of unique molecules. The dipole moments of the five urea guest molecules in the crystal are enhanced considerably compared to values determined for isolated molecules, and 2D maps of the electrostatic potential and electric field show clearly how the urea molecules are oriented with dipole moments aligned along the electric field exerted by their molecular neighbors. Experimental electric fields in the range of 10-19 GV m^-1, obtained for the five different urea environments, corroborate independent measurements of electric fields in the active sites of enzymes and provide an important experimental reference point for recent discussions focused on electric-field-assisted catalysis.

Nanoscale surface dynamics of Bi2Te3(111): observation of a prominent surface acoustic wave and the role of van der Waals interactions

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi2Te3. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi2Te3(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.

Solvothermal flow synthesis of zinc phosphate pigment

Synthesis of phase pure hopeite pigment through a solvothermal flow method is reported here for the first time. The products show two-step dehydration behaviour from thermogravimetric analysis (TGA), and a higher degree of purity and homogeniety than commercial zinc phosphate pigment. By increasing the reaction temperature stepwise from room temperature to 350 °C it was possible to decrease the size of the individual crystallite sheets and to tune their packing into larger assemblies. The conversion of reactants to product proved to be significantly higher at increased temperature with a measured yield of 98.7% at 250 °C versus 85.4% at room temperature. The synthesis route demonstrated here is environmentally sustainable, increases cost-efficiency through minimization of waste, and is compatible with a scale-up strategy.

Is RuAs2 a candidate for high temperature thermoelectric applications?

The mineral inspired material RuAs2 shows promise as a thermoelectric material with its high stability and attractive band structure. In order to validate these expectations phase-pure polycrystalline ruthenium arsenide was synthesized and densified using Spark Plasma Sintering. RuAs2 is an n-type semiconductor with an indirect band gap 0.69 eV as estimated from temperature dependent resistivity data, while the band gap calculated with DFT is 0.64 eV. The thermal conductivity and electrical resistivity are both high with room temperature values of 16 W m-1 K-1 and 170 mΩ cm respectively, leading to modest thermoelectric properties for the intrinsic system. Band structure calculations suggest that chemical modification should preferably be done at the As site to improve the intrinsic properties. Synchrotron powder X-ray diffraction and Rietveld structural refinements show RuAs2 to be a stable line phase up to 1000 K in both in air and in vacuum, and both as a powder and as a dense pellet. No indication of preferential orientation or material gradients are observed.

Pitfalls and reproducibility ofin situsynchrotron powder X-ray diffraction studies of solvothermal nanoparticle formation

In situpowder X-ray diffraction (PXRD) is a powerful characterization tool owing to its ability to provide time-resolved information about phase composition, crystal structure and microstructure. The application of high-flux synchrotron X-ray beams and the development of custom-built reactors have facilitated second-scale time-resolved studies of nanocrystallite formation and growth during solvothermal synthesis. The short exposure times required for good time resolution limit the data quality, while the employed high-temperature–high-pressure reactors further complicate data acquisition and treatment. Based on experience gathered during ten years of conductingin situstudies of solvothermal reactions at a number of different synchrotrons, a compilation of useful advice for conductingin situPXRD experiments and data treatment is presented here. In addition, the reproducibility of the employed portablein situPXRD setup, experimental procedure and data analysis is evaluated. This evaluation is based on repeated measurements of an LaB6line-profile standard throughout 5 d of beamtime and on the repetition of ten identicalin situsynchrotron PXRD experiments on the hydrothermal formation of γ-Fe2O3nanocrystallites. The study reveals inconsistencies in the absolute structural and microstructural values extracted by Rietveld refinement and whole powder pattern modelling of thein situPXRD data, but also illustrates the robustness of trends and relative changes in the extracted parameters. From the data, estimates of the effective errors and reproducibility ofin situPXRD studies of solvothermal nanocrystallite formation are provided.

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.

Probing the accuracy and precision of Hirshfeld atom refinement with HARt interfaced with Olex2

Hirshfeld atom refinement (HAR) is a novel X-ray structure refinement technique that employs aspherical atomic scattering factors obtained from stockholder partitioning of a theoretically determined tailor-made static electron density. HAR overcomes many of the known limitations of independent atom modelling (IAM), such as too short element-hydrogen distances, r(X-H), or too large atomic displacement parameters (ADPs). This study probes the accuracy and precision of anisotropic hydrogen and non-hydrogen ADPs and of r(X-H) values obtained from HAR. These quantities are compared and found to agree with those obtained from (i) accurate neutron diffraction data measured at the same temperatures as the X-ray data and (ii) multipole modelling (MM), an established alternative method for interpreting X-ray diffraction data with the help of aspherical atomic scattering factors. Results are presented for three chemically different systems: the aromatic hydro-carbon rubrene (orthorhombic 5,6,11,12-tetra-phenyl-tetracene), a co-crystal of zwitterionic betaine, imidazolium cations and picrate anions (BIPa), and the salt potassium hydrogen oxalate (KHOx). The non-hydrogen HAR-ADPs are as accurate and precise as the MM-ADPs. Both show excellent agreement with the neutron-based values and are superior to IAM-ADPs. The anisotropic hydrogen HAR-ADPs show a somewhat larger deviation from neutron-based values than the hydrogen SHADE-ADPs used in MM. Element-hydrogen bond lengths from HAR are in excellent agreement with those obtained from neutron diffraction experiments, although they are somewhat less precise. The residual density contour maps after HAR show fewer features than those after MM. Calculating the static electron density with the def2-TZVP basis set instead of the simpler def2-SVP one does not improve the refinement results significantly. All HARs were performed within the recently introduced HARt option implemented in the Olex2 program. They are easily launched inside its graphical user interface following a conventional IAM.

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.

Crystal structure across the β to α phase transition in thermoelectric Cu2-x Se

The crystal structure uniquely imparts the specific properties of a material, and thus provides the starting point for any quantitative understanding of thermoelectric properties. Cu2-x Se is an intensely studied high performing, non-toxic and cheap thermoelectric material, and here for the first time, the average structure of β-Cu2-x Se is reported based on analysis of multi-temperature single-crystal X-ray diffraction data. It consists of Se-Cu layers with additional copper between every alternate layer. The structural changes during the peculiar zT enhancing phase transition mainly consist of changes in the inter-layer distance coupled with subtle Cu migration. Just prior to the transition the structure exhibits strong negative thermal expansion due to the reordering of Cu atoms, when approached from low temperatures. The phase transition is fully reversible and group-subgroup symmetry relations are derived that relate the low-temperature β-phase to the high-temperature α-phase. Weak superstructure reflections are observed and a possible Cu ordering is proposed. The structural rearrangement may have a significant impact on the band structure and the Cu rearrangement may also be linked to an entropy increase. Both factors potentially contribute to the extraordinary zT enhancement across the phase transition.