Higher Coordination Number Research Articles

Ta-Doped Sb2Te Allows Ultrafast Phase-Change Memory with Excellent High-Temperature Operation Characteristics

HighlightsPhase-change memory based on Ta-doped antimony telluride (Sb2Te) exhibits both high-speed characteristics and excellent high-temperature characteristics, allowing improved performance and new applications.The high coordination number of Ta and the strong bonds between Ta and Sb/Te atoms enhance the robustness of the amorphous structure, ensuring good thermal stability.Through the three-dimensional limit, the formation of small grains reduces the power consumption and improves the long-term endurance.

Slow magnetic relaxation in high-coordinate Co(II) and Fe(II) compounds bearing neutral tetradentate ligands.

The first-row transition metal compounds, [MII(L1)2](ClO4)2 (M = Ni (1); Co (2)), have been prepared by treatment of a neutral tetradentate ligand (L1 = N2,N9-dibutyl-1,10-phenanthroline-2,9-dicarboxamide) with metal perchlorate salts in MeOH. Both compounds have been structurally characterized by X-ray crystallography and it was found that the coordination numbers are 6 and 7, respectively. The reaction of 6,6'-bis(2-tbutyl-tetrazol-5-yl)-2,2'-bipyridine (L2) with hydrated FeII(ClO4)2 afforded a 8-coordinate Fe(II) compound, [FeII(L2)2](ClO4)2 (3); however its reaction with hydrated CoII(ClO4)2 resulted in 6-coordinate [CoII(L2)2](ClO4)2. It is interesting to observe field-induced slow magnetic relaxation in the 7-coordinate Co(II) compound 2 and 8-coordinate Fe(II) compound 3, which further supports the validity of designing high coordination number compounds as single-molecule magnets. Direct current magnetic studies demonstrate that 2 has a very large positive D value (56.2 cm-1) and a small E value (0.66 cm-1), indicating easy plane magnetic anisotropy. Consistent with the larger D value, an effective spin-reversal barrier of Ueff = 100 K (71.4 cm-1) is obtained, which is the highest value reported for 7-coordinate Co(II) complexes with a pentagonal bipyramidal geometry. In contrast, 8-coordinate Fe(II) compound 3 exhibits uniaxial magnetic anisotropy.

The magnetic properties of amorphous AlxFe100−x alloys investigated by the atomic structure, magnetoresistance and anomalous Hall effect

We report on experimental results of the specific resistivity ρ, Hall resistivity ρH and magnetoresistance MR of rapidly quenched AlxFe100−x binary amorphous alloys. Atomic structure investigation done by transmission electron microscopy at room temperature verifies that samples are in the pure amorphous state with a high coordination number 8<Nc<12. Systematic changes in the resistivity, magnetoresistance and Hall effect were observed with the varying Fe content. At room temperature, samples with Fe content ⩾40% show negative magnetoresistance, are p-type and show the presence of anomalous Hall effect, while samples with Fe content ⩽25% show positive magnetoresistance, are n-type and show no anomalous Hall effect contribution. Our experimental results are discussed in the concept of electronic stabilization Hume-Rothery amorphous alloys triggered by the hybridization effect of the Fe-d and the Al-s,p electrons. Previous literature results have predicted that when the Fe-d states are full, no ferromagnetic properties should be observed in the AlxFe100−x alloys, and this is for concentrations x⩾75, which is in agreement with our results, because the appearance of an anomalous contribution in the Hall effect would be a direct evidence of ferromagnetic order in the AlxFe100−x amorphous alloys.

High-Coordinate Mononuclear Ln(III) Complexes: Synthetic Strategies and Magnetic Properties

Single-molecule magnets involving monometallic 4f complexes have been investigated extensively in last two decades to understand the factors that govern the slow magnetization relaxation behavior in these complexes and to establish a magneto-structural correlation. The prime goal in this direction is to suppress the temperature independent quantum tunneling of magnetization (QTM) effect via fine-tuning the coordination geometry/microenvironment. Among the various coordination geometries that have been pursued, complexes containing high coordination number around Ln(III) are sparse. Herein, we present a summary of the various synthetic strategies that were used for the assembly of 10- and 12-coordinated Ln(III) complexes. The magnetic properties of such complexes are also described.

Correlation Between Structure and Mechanical Properties of Amorphous Cu–Ag Alloys

Using a molecular dynamics (MD) simulation and an embedded atom method (EAM), this article aims to study the structural properties of five Cu‐ and Ag‐based metallic glasses (MGs) with different compositions (Cu75Ag25, Cu50Ag50, Ag75Cu25, Cu‐monatomic, and Ag‐monatomic MGs) to shed light on the effect of composition on MGs’ properties. According to the radial distribution function (RDF) analysis, it is revealed that Ag atoms promote the glass forming ability (GFA) of the samples. Furthermore, increasing Ag content is found to favor the formation of face‐centered cubic (fcc) clusters and reduce the atomic fraction of icosahedral clusters, and this seems normal, because the distorted icosahedra are just an intermediate state between the icosahedra structure and the fcc structure as mentioned in the literature. Moreover, it is shown that Ag atoms tend to be at the center space of clusters with higher coordination numbers. It is also found that the obtained microstructure of the MGs, especially the full icosahedra, influences their mechanical response toward uniaxial tensile loading, as it enhances their strength by resisting to shear strain and reducing their plasticity, which encourages the accumulation of shear strain in shear bands.

Structural Evolution and Electronic Properties of TaSin-/0 (n = 2-15) Clusters: Size-Selected Anion Photoelectron Spectroscopy and Theoretical Calculations.

The structural evolution and electronic properties of TaSin-/0 (n = 2-15) clusters are explored using anion photoelectron spectroscopy accompanied by quantum chemical calculations. The Ta atom in TaSin-/0 is inclined to interact with more Si atoms and has high coordination numbers. The theoretical calculations show that TaSi2-/0 have trianglur structures and TaSi3-/0 adopt pyramid structures, while the geometries of TaSin-/0 (n = 4-7) are all exohedral structures dominated by bipyramid-based configurations with the Ta atom face-capping the Sin motifs. TaSi8-/0 and TaSi9-10- have boat-shaped geometries, whereas TaSi9-10 neutrals adopt bipyramid-based geometries instead of boat-shaped ones. TaSi11- and TaSi12 are confirmed as the critical size of transiting from exohedral to endohedral structures for anionic and neutral clusters, respectively. TaSi12-15-/0 have pentagonal or hexagonal prism-based geometries. Natural population analysis shows that the electron transfers from Sin skeletons to Ta atom. The second-order energy differences (Δ2E) and incremental binding energy (ΔEI) values exhibit strong odd-even alternations, suggesting that the TaSin-odd-/0 clusters are more stable than the adjacent TaSin-even-/0 ones, except that TaSi12-/0 are more stable than TaSi11-/0 and TaSi13-/0.

P-Sulfonatocalix[4]arene binding of monovalent, divalent and trivalent metal ions

Structural studies on p-sulfonatocalix[4]arene coordination compounds involving diverse metal ions with respect to their size and charge, caesium(I), copper(II) and neodymium(III) are reported. The metal coordination-driven self-assembly forms metallosupramolecules, either as a two-dimensional (2 D) or a three-dimensional (3D) network while retaining the commonly established bilayer array for p-sulfonatocalix[4]arene. The complex (Cs5-p-sulfonatocalix[4]arene-H+)·6H2O has five crystallographically independent caesium(I) ions with high coordination numbers. In the divalent complex {[Cu3(H2O)12-2(p-sulfonatocalix[4]arene)]2-}·2([Cu(H2O)5])2+·2NO3 -·7H2O, two different copper(II) coordination environments are present, one with octahedral stereochemistry while bridging two opposing calixarenes with the other having a square pyramidal geometry involving one sulfonate group. Eight-coordinate neodymium(III) metal ions prevail in Nd0.25(H2O)1.25(p-sulfonatocalix[4]arene + H+)0.25, involving four sulfonate groups from four different calixarenes.

Compaction characteristics of binary coal mixtures during binder less briquetting process

ABSTRACT Dried coal typically had certain Particle Size Distribution (PSD) resulting from the size reduction process. Effect of PSD to the compaction phenomenon was investigated on the binary mixtures of Indonesian middle-rank coal with different mass fraction of small and large particles (%L). Compacted density of each coal mixtures was measured during uniaxial briquetting process and their compaction characteristics were analyzed with Cooper-Eaton model. It was found that Cooper-Eaton model could fit excellently (R2 > 0.99) with the experimental data. Analysis of compaction characteristics of the model (Ai and ki ) and briquette density revealed that mixtures 0%L (PSD Small) and 100%L (PSD Large) were easier to compact compared to binary mixtures. Binary mixture 70%L was hardest to compact, evaluating from its lowest increase of density, from 893 to 1492 kg/m3. Relatively high coordination number at pressure 2 kPa on mixture 70%L (7.81 ± 0.20) might be the reason for high deformation difficulty. It was concluded that binary mixture of coal with the highest bulk density would results in briquette with the lowest density.

A rare octacoordinated mononuclear iron(III) spin-crossover compound: synthesis, crystal structure and magnetic properties.

The chemistry of transition-metal complexes with unusually high coordination numbers has been of interest because of their application in catalytic and biological systems. Deprotonation of the ionogenic tetradentate ligand 6,6'-bis(1H-tetrazol-5-yl)-2,2'-bipyridine [H2bipy(ttr)2] in the presence of iron(III) and tetra-n-butylammonium bromide, [n-Bu4N]Br, in solution resulted in the synthesis of a rare octacoordinated anionic mononuclear complex, tetra-n-butylammonium bis[6,6'-bis(tetrazol-1-id-5-yl)-2,2'-bipyridine]iron(III) methanol hemisolvate dihydrate, (C16H36N)[Fe(C12H6N10)2]·0.5CH3OH·2H2O or [n-Bu4N][Fe{bipy(ttr)2}2]·0.5CH3OH·2H2O (1), which has been structurally characterized by elemental analysis, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. In 1, the coordination sphere of the iron(III) ion is a distorted bis-disphenoid dodecahedron, in which the eight coordination positions are occupied by eight N atoms from two independent tetradentate [bipy(ttr)2]2- anionic ligands, therefore forming the anionic [Fe{bipy(ttr)2}2]- unit, with the negative charge balanced by a free [n-Bu4N]+ cation. An investigation of the magnetic properties of 1 revealed a gradual incomplete spin-crossover behaviour below 150 K.

Structural and mechanical behaviors of Mg-Al metallic glasses investigated by molecular dynamics simulations

Using molecular dynamics simulations with the embedded atomic method (EAM) to describe interatomic interactions, structural and mechanical properties of Mg-Al metallic glasses (MG) have been investigated for different compositions. The atomic structure is characterized using various techniques such as the Radial Distribution Function (RDF), the Voronoi Tessellation Analysis (VTA) and coordination number (CN) distribution. The results confirmed that the Mg-Al glass formation is accompanied by a splitting of the RDF second peak upon cooling process. The glass transition temperature is determined using different method involving a new suggested way consisting of the cross-over between low and high coordination numbers curves during cooling process. This last technique gives approximate results that converge to those given by classical methods. On the other hand, we applied a strain-rate of 1010 s−1 at 300 K and showed that with the increase of Al composition, the maximal stress increases as a function of strain. For a fixed composition of Mg-Al MG, the yield strength increases with the increase of strain-rate between 109 s−1 and 1010 s−1. The impact of temperature on the mechanical behaviour of Mg-Al under various strain-rates has been investigated and the result suggests that for the same applied strain-rate, the ultimate tensile strength decreases as a function of temperature. Finally, the annealing effect has led to a softening of elastic moduli especially the Young modulus (E) which has been determined through both elastic constants Cij and from the stress–strain curve.

CFD-DEM modeling of suffusion effect on undrained behavior of internally unstable soils

Abstract This paper investigates the effect of suffusion on the undrained behavior of an internally unstable soil using the coupled computational fluid dynamics and discrete element method (CFD-DEM). A series of eroded specimens are prepared by exerting an upward flow through the granular assembly under a range of hydraulic gradients and suffusion durations. Both the eroded and non-eroded specimens are then subjected to undrained triaxial compression (or equivalently constant volume tests). The results indicate that the erosion rate and volumetric response of the soil skeleton are closely related to the hydraulic conditions. The eroded specimens exhibit a non-uniform distribution of fine particles, higher coordination numbers, higher global void ratio, and lower equivalent intergranular void ratio. The undrained behavior of the specimen changes from a strain hardening behavior before suffusion to a softening behavior with limited flow deformation after suffusion. The undrained shear strength of eroded specimen is enhanced at small axial strain due to the enhanced connectivity of particles, but can increase or decrease at larger strain levels depending on the degree of erosion. It is also observed that both suffusion and shearing can induce some degree of fabric anisotropy by inspecting the directional distribution of contact unit normals and contact normal forces.

Effect of nano-TiO2 on C–S–H phase distribution within Portland cement paste

We investigate the mechanisms by which titanium dioxide nanoparticles (nano-TiO2) interact with cement hydration products. To this end, we synthesize nanomodified cement samples with 1 wt% and 5 wt% of TiO2. We investigate the physical properties using depth-sensing-based methods such as statistical nanoindentation and microscopic scratch testing. Fourier transform infrared spectroscopy yields the chemistry, whereas micromechanics modeling provides insights into the nanostructure. The macroscopic plane strain modulus increases by 16% and 83%, respectively, and the macroscopic indentation hardness increases by 37% and 40%, respectively. The fracture toughness rises by 3% and 11%, respectively. Environmental scanning electron microscopy reveals a 30% reduction in crack width for TiO2 cement nanocomposites compared to plain cement. Meanwhile, Fourier transform infrared spectroscopy and statistical deconvolution show an increase in the fraction of high-density calcium silicate hydrates (by 22% and 12%, respectively), and in the fraction of calcium hydroxide (by 101% and 251%, respectively). Within the framework of the colloidal and granular models of C–S–H, the increase in stiffness and strength after nano-TiO2 modification of cement paste is due to the closely packed structure and the high atomic coordination number of high-density C–S–H. Similarly, due to the high dimensional stability of high-density C–S–H and calcium hydroxide, our results explain the reported improvements in drying shrinkage and creep properties following cement modification with nano-TiO2.

Characteristics of high-rank coal structure parallel and perpendicular to the bedding plane via NMR and X-ray CT

Pores and fractures and their connectivity play a significant role in coalbed methane production. To investigate the growth characteristics and connectivity of pores and fractures in coal parallel and perpendicular to the bedding plane, the pores and fractures of high-rank coal samples collected from the southern Qinshui Basin were measured by low-field nuclear magnetic resonance, X-ray-computed tomography and field emission scanning electron microscopy. Then, the determinants of their connectivity were further discussed. The results show that the high-rank coal samples have similar pore size distributions both parallel and perpendicular to the bedding plane. They primarily contain mesopores (2–50 nm in width), followed by macrospores (> 50 nm in width). The research indicated that the high-rank coal connectivity parallel to the bedding plane is significantly better than that perpendicular to the bedding plane. The connectivity of high-rank coal is mainly determined by throats, and the orientation of the pores and fractures. The two connectivity modes in high-rank coal are “pore connectivity,” in which the throats are mainly pores with a low coordination number, and “microfissure connectivity”, in which the throats are mainly microfissures with a high coordination number.

Impacts of pore structure and wettability on distribution of residual fossil hydrogen energy after imbibition

Due to the heterogeneity of the pore structure, there is still much residual fossil hydrogen energy in the pore space. In this study, the distribution of residual fossil hydrogen energy after imbibition with liquid nanofluid (LNF) has been studied through the. Firstly, relevant properties of the LNF were tested, including particle size, wettability alteration, and interfacial tension (IFT). Then, one micromodel under both oil-wet and water-wet conditions was used to elucidate the difference of residual oil after imbibition. Moreover, the difference of imbibition processes with oil-wet and water-wet models was further studied. Finally, the influence of dimensions and coordination numbers of pores was investigated. Besides, the impact of fluid type on imbibition efficiency was also compared through microfluidic experiments. Results show that the residual oil saturation is significantly higher in the oil-wet micromodel than in the water-wet micromodel, even the wettability alteration additive was used. There are four kinds of locations for residual oil after imbibition in water-wet micromodel, while only three kinds of locations for residual oil in oil-wet micromodel. The imbibition rate and residual oil saturation in small pores are higher than the larger ones. Due to the unbalanced force at pore throats, a micromodel with a high coordination number has lower residual oil saturation. Besides, the lower residual oil saturation is obtained by adding the LNF. This paper provides a micro-visual method to understand the imbibition process under different wettability and pore structures in fossil hydrogen reservoirs.

Editorial: Biographical Memoirs , Volume 68

Editorial: <i>Biographical Memoirs</i> , Volume 68

Synthesis, Iodometric Analysis, and IR Spectroscopy of the Peroxide Double Salt [Zn(NH3)4][Mo(O2)4

This paper describes an upper-division undergraduate exercise involving the synthesis and iodometric and infrared spectroscopic analysis of the peroxide double salt [Zn(NH3)4][Mo(O2)4]. Here, iodometric methods are employed to determine the peroxide content of the compound, a procedure widely used for the industrial analysis of peroxides. The exercise increases student proficiency in synthesis, the handling of reactive chemicals, and titrimetry, while introducing aspects of metal peroxide chemistry and key chemical concepts related to high-coordination number complexes, kinetic versus thermodynamic stability, and group theoretical treatments of vibration spectroscopy.

Structural evolution and energy landscape of Zr55Cu35Al10 ternary alloy during glass transition

In this work, classic molecular dynamics simulations were employed to study the glass transition of Zr55Cu35Al10 ternary alloy. According to the structure and energy analysis, during quenching to the glass transition temperature (Tg) of the Zr55Cu35Al10 alloy, by quantitatively analyzing the cluster, i-zone and free volume atom pairs based on the tight-bond cluster model, it was found that the fraction of the cluster and i-zone atom pairs increased while the fraction of the free volume atom pairs decreased continuously. The Zr-centered first shell clusters tend to transform to the coordination polyhedra with high coordination number (CN) owning low cluster energy and the Cu- and Al-centered clusters tend to become the coordination polyhedra with CN = 12 presenting the lowest cluster energy in the short-range structure. The medium-range structure of the coordination polyhedra connection sharing three atoms with the lowest energy increased significantly. Furthermore, it was confirmed that the significance of Al addition on the increase in glass forming ability (GFA) in the ZrCuAl ternary alloys.

Robin Jon Hawes Clark. 16 February 1935—6 December 2018

Robin Clark was a distinguished physical/inorganic chemist who made major discoveries in the coordination chemistry of the early transition metals, especially of titanium and vanadium complexes with high coordination numbers (notably seven and eight) and of the structures and physical properties of mixed valence, linear chain and metal–metal bonded compounds. He applied far-infrared spectroscopy to study metal–ligand vibrations systematically and established the technique for structure elucidation of transition metal and main group compounds. He also developed Raman and resonance Raman spectroscopy applied to inorganic compounds and highly coloured solids including mineral samples. That work led to his seminal applications of microbeam Raman spectroscopy for the identification of pigments and other constituents of artworks and historical artefacts, thereby developing a basis for testing their provenance and the identification of forgeries.

Wheel-like{Ln6}luminescent lanthanide complexes covering the visible and near-infrared domains

Luminescence properties of wheel-like lanthanide complexes.

Main-group metal cyclophane complexes with high coordination numbers.

Density functional theory calculations using the PBE0-D3BJ hybrid functional have been employed to investigate the complexation of main-group metal-cations with [2.2.2]paracyclophane and deltaphane. Geometry optimization under symmetry constraints was performed to observe the mode of coordination that a metal-cation adopts when it resides inside the cyclophane cavity. Thermodynamic properties were investigated to note the trends of stability along a group of metals. To further investigate the bonding properties, Morokuma–Ziegler energy decomposition analysis, natural bond orbital analysis and Bader's analysis were employed. It was observed that most of the main-group metal complexes with cyclophanes prefer an η6η6η6 coordination mode where the metal-cation sits in the centre of the cyclophane cavity. There is an increased thermodynamic stability in [2.2.2]paracyclophane complexes compared to their deltaphane analogues while the reverse is true regarding the strength of coordination based on interaction energy.