Hexagonal Si−Ge Class of Semiconducting Alloys Prepared by Using Pressure and Temperature

Multi-anvil and laser-heated diamond anvil methods have been used to subject Ge and Si mixtures to pressures and temperatures of between 12 and 17 GPa and 1500–1800 K, respectively. Synchrotron angle dispersive X-ray diffraction, precession electron diffraction and chemical analysis using electron microscopy, reveal recovery at ambient pressure of hexagonal Ge−Si solid solutions (P6₃/mmc). Taken together, the multi-anvil and diamond anvil results reveal that hexagonal solid solutions can be prepared for all Ge−Si compositions. This hexagonal class of solid solutions constitutes a significant expansion of the bulk Ge−Si solid solution family, and is of interest for optoelectronic applications.     

A New Chain Polymer via Thermolysis: [Hg(μ‐Br)2(3,5‐Br2py)]∞

The new ligand‐deficient chain polymer [Hg(μ‐Br)₂(3,5‐Br₂py)]_∞ has been obtained in form of single crystals by thermolysis of the ligand‐rich [Hg(μ‐Br)₂(3,5‐Br₂py)₂]_∞ at 180 °C at ambient pressure. From this reaction, high quality crystals of the product are directly accessible. The title compound features Hg^II cations in a distorted square‐pyramidal coordination; their metal centers aggregate via edge‐sharing with asymmetric halide bridges to chains in which all apical N donor ligands are oriented to the same side of the [Hg(μ‐Br)₂]_∞ backbone. The new polymer cannot be prepared by stoichiometric reaction in solution.     

Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides

An hydrogenated nitrogen-rich graphitic carbon nitride, structurally related to the theoretical graphitic phase of C₃N₄, has been synthesized in a bulk well-crystallized form. This new material was prepared by thermal decomposition of thiosemicarbazide up to 600 °C at ambient pressure under nitrogen flow. Its composition was determined by elemental combustion analysis. Powder X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and C¹³ MAS NMR characterizations were performed. This material can be schematically described with a two-dimensional framework and a composition close to C₃N_4.17H_1.12. In this nitrogen-rich material, C₃N₃ voids are fully occupied by water molecules which are strongly trapped into the material. A loss of crystallinity associated with a modification of the thermal behavior is observed when the amount of trapped molecules decreases in the graphitic material, order being damaged both between and in the graphitic planes.     

Pentacoordinate Phosphorus in a High‐Pressure Polymorph of Phosphorus Nitride Imide P4N6(NH)

Coordination numbers higher than usual are often associated with superior mechanical properties. In this contribution we report on the synthesis of the high‐pressure polymorph of highly condensed phosphorus nitride imide P₄N₆(NH) representing a new framework topology. This is the first example of phosphorus in trigonal‐bipyramidal coordination being observed in an inorganic network structure. We were able to obtain single crystals and bulk samples of the compound employing the multi‐anvil technique. γ‐P₄N₆(NH) has been thoroughly characterized using X‐ray diffraction, solid‐state NMR and FTIR spectroscopy. The synthesis of γ‐P₄N₆(NH) gives new insights into the coordination chemistry of phosphorus at high pressures. The synthesis of further high‐pressure phases with higher coordination numbers exhibiting intriguing physical properties seems within reach.     

Ternary and quaternary layered nitride halides, Ca2N(X,X′) (X,X′=Cl, Br, I): Evolution of structure with composition

The quaternary systems Ca-N-Cl-Br and Ca-N-Br-I have been investigated resulting in the synthesis of a number of new layered nitride mixed halides. The evolution of structure with composition has been investigated by powder X-ray and powder neutron diffraction techniques. A continuous solid solution exists between Ca₂NCl and Ca₂NBr with intermediate compounds adopting the same anti- α-NaFeO₂ structure (rhombohedral space group ) as the ternary end members. A phase transition occurs in the Ca₂NBr_1−yI_y system between y=0.7 and y=0.8 corresponding to a switch from cubic close packing to hexagonal close packing of metal-nitrogen layers and corresponding adoption of the anti-β-RbScO₂ (filled anti-CdI₂) structure (hexagonal space group P6₃/mmc). While nitride and halide anions occupy distinct crystallographic sites, there is no ordering of halides in quaternary materials irrespective of stoichiometry or structure type. All the nitride halides show temperature independent paramagnetic behaviour between 2 and 300 K.     

Structural phase transitions in Zn(CN)2 under high pressures

High pressure behavior of zinc cyanide (Zn(CN)₂) has been investigated with the help of synchrotron-based X-ray diffraction measurements. Our studies reveal that under pressure this compound undergoes phase transformations and the structures of the new phases depend on whether the pressure is hydrostatic or not. Under hydrostatic conditions, Zn(CN)₂ transforms from cubic to orthorhombic to cubic-II to amorphous phases. In contrast, the non-hydrostatic pressure conditions drive the ambient cubic phase to a partially disordered crystalline phase, which eventually evolves to a substantially disordered phase. The final disordered phase in the latter case is distinct from the amorphous phase observed under the hydrostatic pressures.     

Synthesis and structure of the ternary and quaternary strontium nitride halides, Sr2N(X, X′) (X, X′=Cl, Br, I)

A number of new, layered nitride mixed halides have been synthesised in the quaternary phase systems Sr-N-Cl-Br and Sr-N-Br-I. The variation in structure with composition has been investigated by powder X-ray and powder neutron diffraction techniques and the structure of strontium nitride iodide, Sr₂NI, has been determined for the first time (rhombohedral space group R-3m, , , Z=3). A continuous solid solution exists between Sr₂NCl and Sr₂NBr with intermediate compounds adopting the same anti-α-NaFeO₂ structure (rhombohedral space group R-3m) as the ternary end members. A similar smooth and linear relationship between structure and composition is seen from Sr₂NBr to Sr₂NI and hence cubic close packing of metal-nitrogen layers is adopted regardless of halide, X (X′). While nitride and halide anions occupy distinct crystallographic sites, there is no ordering of the halides in the quaternary materials irrespective of stoichiometry or temperature (between 3 and 673 K).     

A High‐Pressure Polymorph of Phosphorus Nitride Imide

Phosphorus nitride imide, PN(NH), is of great scientific importance because it is isosteric with silica (SiO₂). Accordingly, a varied structural diversity could be expected. However, only one polymorph of PN(NH) has been reported thus far. Herein, we report on the synthesis and structural investigation of the first high‐pressure polymorph of phosphorus nitride imide, β‐PN(NH); the compound has been synthesized using the multianvil technique. By adding catalytic amounts of NH₄Cl as a mineralizer, it became possible to grow single crystals of β‐PN(NH), which allowed the first complete structural elucidation of a highly condensed phosphorus nitride from single‐crystal X‐ray diffraction data. The structure was confirmed by FTIR and ³¹P and ¹H solid‐state NMR spectroscopy. We are confident that high‐pressure/high‐temperature reactions could lead to new polymorphs of PN(NH) containing five‐fold‐ or even six‐fold‐coordinated phosphorus atoms and thus rivalling or even surpassing the structural variety of SiO₂.     

Compressibility of Gas Hydrates

Experimental data on the pressure dependence of unit cell parameters for the gas hydrates of ethane (cubic structure I, pressure range 0–2 GPa), xenon (cubic structure I, pressure range 0–1.5 GPa) and the double hydrate of tetrahydrofuran+xenon (cubic structure II, pressure range 0–3 GPa) are presented. Approximation of the data using the cubic Birch–Murnaghan equation, P=1.5B₀[(V₀/V)^7/3?(V₀/V)^5/3], gave the following results: for ethane hydrate V₀=1781 ų, B₀=11.2 GPa; for xenon hydrate V₀=1726 ų, B₀=9.3 GPa; for the double hydrate of tetrahydrofuran+xenon V₀=5323 ų, B₀=8.8 GPa. In the last case, the approximation was performed within the pressure range 0–1.5 GPa; it is impossible to describe the results within a broader pressure range using the cubic Birch–Murnaghan equation. At the maximum pressure of the existence of the double hydrate of tetrahydrofuran+xenon (3.1 GPa), the unit cell volume was 86 % of the unit cell volume at zero pressure. Analysis of the experimental data obtained by us and data available from the literature showed that 1) the bulk modulus of gas hydrates with classical polyhedral structures, in most cases, are close to each other and 2) the bulk modulus is mainly determined by the elasticity of the hydrogen‐bonded water framework. Variable filling of the cavities with guest molecules also has a substantial effect on the bulk modulus. On the basis of the obtained results, we concluded that the bulk modulus of gas hydrates with classical polyhedral structures and existing at pressures up to 1.5 GPa was equal to (9±2) GPa. In cases when data on the equations of state for the hydrates were unavailable, the indicated values may be recommended as the most probable ones.     

A Multifunctional Magnetic Material under Pressure

Fe^II(Metz)₆](Fe^IIIBr₄)₂ (Metz=1‐methyltetrazole) is one of the rare systems combining spin‐crossover and long‐range magnetic ordering. A joint neutron and X‐ray diffraction and magnetometry study allows determining its collinear antiferromagnetic structure, and shows an increase of the Néel temperature from 2.4 K at ambient pressure, to 3.9 K at 0.95 GPa. Applied pressure also enables a full high‐spin to low‐spin switch at ambient temperature.     

Lattice effects in cubic La2Mo2O9: Effect of vacuum and correlation with transport properties

This study aims to investigate correlations between lattice effects and transport properties in cubic La₂Mo₂O₉. High temperature neutron diffraction data, recorded in air and under vacuum, are used to follow the evolution with temperature of selected structural parameters, i.e. bond lengths and angles. Results suggest a possible correlation with the experimentally observed decrease of the activation energy for oxygen migration at high temperature. The effect on the structural properties of the low oxygen partial pressure used during the measurements in vacuum is negligible and this represents a valuable information in view of possible applications of the material in solid state devices.     

Boron nitride synthesized at ambient pressure and room temperature by plasma electrolysis

A mixture of cubic boron nitride (c-BN) and extra-diamond boron nitride (E-BN) has been synthesized at ambient pressure and room temperature by plasma electrolysis. The formation of c-BN was characterized by FTIR and TEM measurements. This method may not only offer a facile technique for c-BN production, but also provide a new research field in c-BN thermodynamics.     

Crystal Structure of the Ordered Double Perovskite,Sr2NiTeO6

The crystal structure of the ordered double perovskite Sr₂MnTeO₆ has been refined at ambient temperature from high resolution neutron and X‐ray powder diffraction data in the monoclinic space group I 1 2/m 1 with a = 5.6166(1) Å, b = 5.5807(1) Å, c = 7.8797(1) Å and β = 90.048(2)°. The structure is the result of out‐of‐phase (–) rotations of virtually undistorted NiO₆ and TeO₆ octahedra in the (0 – –) sense about two of the axes of the ideal cubic perovskite. Electron diffraction measurements have been used to confirm the proposed space group and structure.     

Synthesis and Crystal Structure of a Bixbyite‐Type Solid Solution Zr0.43Er0.57O1.07N0.43

A powder of the composition 40 mol‐% Er₂O₃ and 60 mol‐% ZrO₂ has been prepared. The anion deficient fluorite related compound Zr₃Er₄O₁₂ [R\bar{3} : a = 971.79(7) pm and c = 907.976(0) pm] was produced. This precursor was treated with gaseous ammonia at temperatures between 25 and 1200 °C. The reaction was followed in situ by X‐ray diffraction in a high temperature reaction chamber under a constant flow of ammonia. The nitridation of Zr₃Er₄O₁₂ led to a nitride oxide of the solid‐solution‐type with an apparent composition Zr_0.43Er_0.57O_1.07N_0.43 The compound crystallizes isostructural to bixbyite due to vacancy ordering [Ia\bar{3} : a = 1036.37(4) pm]. The reoxidation of the nitride oxide was monitored in DTA/TG experiments exhibiting an almost complete reoxidation to the starting composition except for some inclusions of dinitrogen.     

Order and disorder in Ca2ND0.90H0.10-A structural and thermal study

The structure of calcium nitride hydride and its deuterided form has been re-examined at room temperature and studied at high temperature using neutron powder diffraction and thermal analysis. When synthesised at 600 °C, a mixture of both ordered and disordered Ca₂ND_0.90H_0.10 phases results. The disordered phase is the minor component and has a primitive rocksalt structure (spacegroup Fm3m) with no ordering of D/N on the anion sites and the ordered phase is best described using the rhombohedral spacegroup R-3m with D and N arranged in alternate layers in (111) planes. This mixture of ordered and disordered phases exists up to 580 °C, at which the loss of deuterium yields Ca₂ND_0.85 with the disappearance of the disordered phase. In the new ordered phase there exists a similar content of vacancies on both anion sites; to achieve this balance, a little N transfers onto the D site, whereas there is no indication of D transferring onto the N-sites. These observations are thought to indicate that the D/N ordering is difficult to achieve with fully occupied anion sites. It has previously been reported that Ca₂ND has an ordered cubic cell with alternating D and N sites in the [100] directions [1]; however, for the samples studied herein, there were clearly two coexisting phases with apparent broadening/splitting of the primitive peaks but not for the ordered peaks. The rhombohedral phase was in fact metrically cubic; however, all the observed peaks were consistent with the rhombohedral unit cell with no peaks requiring the larger ordered cubic unit cell to be utilised. Furthermore this rhombohedral cell displays the same form of N-D ordering as the Sr and Ba analogues, which are metrically rhombohedral.     

Jahn–Teller distortion in perovskite KCuF3 under high pressure

Single crystals of KCuF₃ have been grown by the solution method. We report neutron diffraction under high pressure up to 8 GPa on a powder sample pulverized from KCuF₃ crystals. The type-A spin ordering structure found in KCuF₃ can be well-explained based on the orbital ordering resolved from this structural study. In comparison with perovskite oxides, the fluoride exhibits a much reduced bulk modulus. Our results also reveal the change of local structural distortion as a function of pressure; the magnitude of Jahn–Teller distortion at ambient condition is dramatically suppressed under high pressure. Extensive comparison of the pressure dependence of the Jahn–Teller perovskite fluoride with that of analogous perovskite oxides has also been made.     

Hydrogen-induced atomic rearrangement in MgPd3

The hydrogenation behavior of MgPd₃ has been studied by in situ X-ray powder diffraction and by neutron powder diffraction. At room temperature and p ≈500 kPa hydrogen pressure its structure is capable of incorporating up to one hydrogen atom per formula unit (α-MgPd₃H_≈1), thereby retaining a tetragonal ZrAl₃-type metal atom arrangement. Upon heating to 750 K in a hydrogen atmosphere of 610 kPa it transforms into a cubic modification with AuCu₃-type metal atom arrangement (β-MgPd₃H_≈0.7). Neutron diffraction on the deuteride reveals an anion deficient anti-perovskite-type structure (β-MgPd₃D_0.67, a=398.200(7) pm) in which octahedral sites surrounded exclusively by palladium atoms are occupied by deuterium. Complete removal of hydrogen (480 K, 1 Pa) stabilizes a new binary modification (β-MgPd₃, a=391.78(2) pm) crystallizing with a primitive cubic AuCu₃-type structure. Mechanical treatment (grinding) transforms both α and β modifications of MgPd₃ into a cubic face-centered solid solution Mg_0.25Pd_0.75 showing a random distribution of magnesium and palladium atoms.     

On the solid solution of the spinel phase in the system NiO-Al2O3

The solid solubility of Al₂O₃ in NiAl₂O₄ spinel has been investigated by powder X-ray diffraction of samples prepared by solid state synthesis. The solid solution region found was in agreement with a previous report. The cubic cell parameter of the spinel solid solution was observed to decrease with increasing alumina content. Spinel with high alumina content was shown to be close to an inverse spinel as previously reported for stoichiometric NiAl₂O₄ and the inversion parameter proved to be relatively independent of the overall composition.     

Twinned tetragonal structure and equation of state of NaTh2F9

The crystal structure and stability of NaTh₂F₉ have been studied using thermal analysis, powder X-ray diffraction at atmospheric conditions, and single-crystal X-ray diffraction at high pressure. Sodium dithorium fluoride is stable at least up to 5.0 GPa at room temperature and to 954 K at ambient pressure. In contrast to earlier investigations, which have reported the structure to be cubic (, Z=4), we observe a tetragonal distortion of the lattice. The actual crystal structure (, Z=4) is twinned and composed of corner-sharing distorted ThF₉ tricapped trigonal prisms and distorted NaF₆ octahedra. The twinning element is a three-fold axis from cubic symmetry. The ThF₉ polyhedra are rigid and it is the volume changes of the octahedra around the Na atoms that have the major contribution to the bulk compressibility. The zero-pressure bulk modulus B₀ and the unit-cell volume at ambient pressure V₀ are equal to 99(6) GPa and 663.1(1.0) Å³, respectively, with the fixed first pressure derivative of the bulk modulus B′=4.00. An inspection of the known crystalline phases in the system NaF-ThF₄ reveals that their bulk moduli increase with the increasing ThF₄ content.     

Supercharged CO2 Photothermal Catalytic Methanation: High Conversion,Rate, and Selectivity

To overcome the thermodynamic and kinetic impediments of the Sabatier CO₂ methanation reaction, the process must be operated under very high temperature and pressure conditions, to obtain an industrially viable conversion, rate, and selectivity. Herein, we report that these technologically relevant performance metrics have been achieved under much milder conditions using solar rather than thermal energy, where the methanation reaction is enabled by a novel nickel-boron nitride catalyst. In this regard, an in situ generated HOB⋅⋅⋅B surface frustrated Lewis's pair is considered responsible for the high Sabatier conversion 87.68 %, reaction rate 2.03 mol g_Ni⁻¹h⁻¹, and near 100 % selectivity, realized under ambient pressure conditions. This discovery bodes well for an opto-chemical engineering strategy aimed at the development and implementation of a sustainable ‘Solar Sabatier’ methanation process.