Change In Coordination Number Research Articles

The difference in the coordination environment around Yb3+ ions between Yb–Al-doped and Yb–P-doped silica glasses

To examine the difference in coordination environment around Yb3+ ions between Yb–Al-doped (YAS) and Yb–P-doped (YPS) silica glasses, Al/Yb dependence of the local structure around Yb3+ ions was investigated by EXAFS and optical measurements in YAS glasses and compared with the previous results of the P/Yb dependence in YPS glasses. It was found that Al3+ ions gradually coordinate to the Yb3+ ions with increasing Al/Yb from 0 to 20, and Yb–O coordination number changes from 4 to 8 in YAS, while in YPS glasses P5+ ions rapidly coordinate to the Yb3+ ions with increasing P/Yb. Contrary to the slow change of the coordination structure in YAS glass, the clustering of Yb3+ ions in YPS is immediately suppressed even if Al/Yb is ∼0.4. The reason must be that Yb3+ is easy to take local charge compensation in YAS glasses, just as charge neutralization of Al3+ ions in Al-doped silica glasses. The local structure model, where the mixed existence of O(3) an Al(n) contributes to charge compensation around Yb3+ ions in YAS glasses, was proposed.

Potential Control of Oxygen Non-Stoichiometry in Cerium Oxide and Phase Transition Away from Equilibrium.

Cerium oxide (ceria, CeO2) is a technologically important material for energy conversion applications. Its activities strongly depend on redox states and oxygen vacancy concentration. Understanding the functionality of chemical active species and behavior of oxygen vacancy during operation, especially in high-temperature solid-state electrochemical cells, is the key to advance future material design. Herein, the structure evolution of ceria is spatially resolved using bulk-sensitive operando X-ray diffraction and spectroscopy techniques. During water electrolysis, ceria undergoes reduction, and its oxygen non-stoichiometry shows a dependence on the electrochemical current. Cerium local bonding environments vary concurrently to accommodate oxygen vacancy formation, resulting in changes in Ce-O coordination number and Ce3+/Ce4+ redox couple. When reduced enough, a crystallographic phase transition occurs from α to an α' phase with more oxygen vacancies. Nevertheless, the transition behavior is intriguingly different from the one predicted in the standard phase diagram of ceria. This paper demonstrates a feasible means to control oxygen non-stoichiometry in ceria via electrochemical potential. It also sheds light on the mechanism of phase transitions induced by electrochemical potential. For electrochemical systems, effects from a large-scale electrical environment should be taken into consideration, besides effective oxygen partial pressure and temperature.

Precise Characterization of the Rich Structural Landscape Induced by Pressure in Multifunctional FeVO4.

We have studied the high-pressure behavior of FeVO4 by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO4 is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO4-I'), which was not detected by previous powder XRD studies. We have determined that FeVO4, under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO4-I) to a second previously unknown triclinic structure (FeVO4-I'), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO4-II'), which was also previously detected in powder XRD experiments. Single-crystal XRD has enabled these novel findings as well as an accurate determination of the crystal structure of FeVO4 polymorphs under high-pressure conditions. The crystal structure of all polymorphs has been accurately solved at all measured pressures. The pressure dependence of the unit-cell parameters and polyhedral coordination have been obtained and are discussed. The room-temperature equation of state and the principal axes of the isothermal compressibility tensor of FeVO4-I and FeVO4-I' have also been determined. The structural phase transition observed here between these two triclinic structures at 2.11(4) GPa implies abrupt coordination polyhedra modifications, including coordination number changes. DFT calculations support the conclusions extracted from our experiments.

Quantitative analysis of flow dynamics of organic granular materials inside a versatile silo model during time-lapse X-ray tomography experiments

Gravitational flows, like silo discharge, vary in mode and are complex to identify since they depend on various parameters, ranging from silo structural properties to granular material properties. Moreover, the way cohesionless particles arrange themselves during the flow is still a matter of study and new experimental approaches are needed. Here, the paper introduces a new versatile silo model that was specially designed for in situ X-ray tomography studies of silo discharge for various flow conditions, namely concentric and eccentric. The presented work focuses on organic granular materials, i.e. sorghum and rice, which present relatively similar physico-mechanical properties but different elongation. The high-quality tomography images combined with adequate image processing strategy allow to unambiguously analyse individual grains dynamic behaviour during silo discharge for different hopper configurations. The paper focuses on the analysis of packing density, coordination number and grain rotation changes during funnel flow. The quantitative analysis essentially shows that hopper eccentricity and particle elongation influence flow dynamics of funnel flow. The proposed experimental methodology is proven to be solid enough to investigate in more detail dynamic phenomena during silo discharge. It can also serve as a strong baseline for numerical calibration and predictions.

Nearly Identical but Not Isotypic: Influence of Lanthanide Contraction on Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho).

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho) were grown as large single crystals (∼0.1-1 mm3) and characterized using single-crystal X-ray diffraction and magnetic susceptibility measurements. As we moved across the series, the structural studies revealed a change in the lanthanide coordination environment depending on the identity of the lanthanide. Although all compounds in the Cs2NaLn(PS4)2 series crystallize in the same space group and have the same Wyckoff atom positions, a slight change in size between Sm3+ and Gd3+ causes a subtle change in coordination number from 9 (for Ln = La-Sm) to 8 (for Ln = Gd-Ho), resulting in two distinct but virtually identical structure types. Ab initio calculations were performed, and the observed experimental trend was corroborated computationally. Magnetic measurements performed on the Cs2NaLn(PS4)2 (Ln = Ce, Pr, Nd, Gd, and Tb) compounds revealed paramagnetic behavior.

Melting behavior of SiO2 up to 120 GPa

The structure of liquid silicates is commonly described as a statistical mixture of various atomic entities with relative abundances that can vary with pressure, temperature and composition. Unfortunately, this view remains largely theoretical due to scarce experimental reports on the silicate melt structure, in particular under pressure. We performed X-ray diffraction of the SiO2 end-member to probe the melting curve up to ~120 GPa and 7000 K, and the melt structure up to ~80 GPa. We confirm the steep increase of the melting curve above ~14 GPa when stishovite becomes stable over coesite in subsolidus conditions, with a slope of about 80 K/GPa. Then, around 45 GPa and 5400 K, the melting curve flattens significantly, an effect most likely reflecting the densification of the SiO2 melt structure. The signal of diffuse X-ray scattering is compatible with a change of the Si coordination number from 4 to 6 along the melting curve, in agreement with previous works reporting a similar evolution during the cold compression of SiO2-glass. Because of the limited pressure range (within 10 to 20 GPa) in which the melting curve changes its slope, we speculate a difficult coexistence of tetrahedral SiO4 and octahedral SiO6 units in SiO2 melt at high pressures.

Ni(II)porphyrins as pH dependent light-driven coordination-induced spin-state switches (LD-CISSS) in aqueous solution

A water-soluble Ni(II)-porphyrin substituted with a covalently attached azopyridine ligand was synthesized. Upon irradiation with violet and green light, the azo unit performs a reversible cis–trans isomerization. This geometry change triggers a coordination/de-coordination of the pyridine nitrogen at the central nickel(II) ion. The concomitant change in coordination number at the Ni ion in turn switches the spin state between high and low spin, a process we coined a “light-driven coordination-induced spin state switch (LD-CISSS). To increase the coordination power of the pyridine, particularly in aqueous environments, we introduced an electron donating OH group in 4-position. With increasing pH, the hydroxyl group is deprotonated, further enforcing coordination. We report on the properties of this pH-dependent spin switch, particularly the magnetic properties.

Photochromism in Ruddlesden–Popper copper-based perovskites: a light-induced change of coordination number at the surface

Ultraviolet light causes bis(phenethylammonium) tetrachlorocuprate to change color from yellow to brown, which is reversible by exposure to air.

Network structure and thermal properties of bioactive (SiO2\u2013CaO\u2013Na2O\u2013P2O5) glasses

Ca- and P-based bioactive glasses are excellent candidates for design and manufacture of biomaterials. Understanding the structure and physico-chemical–thermal behaviour of bioactive glasses is a fundamental step towards the design of a new generation of biocompatible materials. In this study, the structure of SiO2–CaO–Na2O glasses and its derivatives, obtained by substituting Na2O with P2O5 and prepared by melt–quench technique, was studied with neutron and electron diffraction techniques combined with thermal analysis, high-resolution electron microscopy and X-ray photoelectron spectroscopy. Neutron and electron diffraction data were analysed with reverse Monte Carlo simulation and pair distribution function analysis, respectively. Bioactivity of P2O5 substituted glasses was also investigated and proven in vitro using simulated body fluid. Based on the structural analysis, it was found that Si and P atoms are in well-defined tetrahedral units with a bond distance of 1.60 Å for both Si–O and P–O bonds, although P exhibits a higher average coordination number than Si. With increasing phosphate content, tendentious changes in the glass behaviour were observed. Linear increase in Tg, supported by the changes in the average coordination numbers of Si and P, indicates strengthening of network structure with increasing P content and formation of P–O–Ca atomic linkages, which lead to Ca–P-rich atomic environments in the silicate network. These Ca–P-rich environments trap volatile elements and thus decrease the total weight loss during heating at higher P concentrations. In the case of the highest investigated P2O5 content (5 mol%), nanoscale structural inhomogeneity and the formation of Ca–P-rich clusters were also revealed by electron diffraction and atomic resolution imaging. This type of Ca–(Na)–P clustering has a key role in the behaviour of phosphate-substituted silicate glasses under physiological conditions.

Sulfated tin oxide with macro- and mesopores controlled using an integrated sol-gel and surfactant template route

Sulfated tin oxide was synthesized by a sol-gel process and pores were formed in the ceramic powders using Pluronic F-127 as a soft template. Sulfate ions linked to the tin oxide promoted the formation of acid sites on the particle surfaces. After calcination, the sulfate ions remained on the oxide particles as uni- and bidentate complexes, so changes in the tin atom coordination numbers at the second shells could be observed by X-ray absorption spectroscopy. The tin oxides showed a cassiterite nanocrystalline structure consisting of aggregates of spherical particles, with crystallite size of around 8 nm, radius of gyration ∼3.7 nm, and mean particle size between 7 and 10 nm, according to X-ray diffraction and small angle X-ray scattering measurements, together with the use of transmission electron microscopy images. The use of Pluronic F-127 as a template for the pores resulted in the formation of meso- and macropores sized around 7.5 nm and 60 μm, respectively, surface area of 128 m2 g−1 and porosities of 71%, after calcination at 500 ºC. The porous sulfated tin oxide was used to catalyze the isopropanol dehydration reaction, with production of propene. The catalytic activity and the temperature-programmed desorption of ammonia confirmed the presence of acid sites on the surface, suggesting that this material could be employed as a heterogeneous catalyst in acid-catalyzed reactions.

Numerical study of three-body diamond abrasive polishing single crystal Si under graphene lubrication by molecular dynamics simulation

In this study, molecular dynamics (MD) simulation was used to investigate a new mechanical polishing method for three-body polishing of single crystal Si under the lubrication of graphene. The polishing depth and polishing speed were adjusted under the same processing parameters to compare the results. The development of polishing force, atomic displacement, coordination number, temperature, potential energy, friction coefficient, and polishing surface morphology was studied during nano-polishing. The analysis shows that the polishing depth in silicon polishing plays a crucial role in the change of coordination number of silicon. The deeper the polishing depth, the higher the polishing force, the more obvious the subsurface damage layer, the higher the potential energy. High defect atomic number and high normal stress. Moreover, the material removal rate is proportional to the polishing depth; a larger polishing speed significantly increases temperatures and potential energy. However, a smaller polishing rate does not result in fewer defective atoms and Bct5-SI/SI-II type atoms, as well as lower material removal efficiency. Finally, graphene-lubricated three-body polished single crystal silicon can improve the surface quality and reduce material removal efficiency.

Optimising surface d charge of AuPd nanoalloy catalysts for enhanced catalytic activity

Understanding the catalytic mechanism of bimetallic nanocatalysts remains challenging. Here, we adopt an adsorbate mediated thermal reduction approach to yield monodispersed AuPd catalysts with continuous change of the Pd-Au coordination numbers embedded in a mesoporous carbonaceous matrix. The structure of nanoalloys is well-defined, allowing for a direct determination of the structure-property relationship. The results show that the Pd single atom and dimer are the active sites for the base-free oxidation of primary alcohols. Remarkably, the d-orbital charge on the surface of Pd serves as a descriptor to the adsorbate states and hence the catalytic performance. The maximum d-charge gain occurred in a composition with 33–50 at% Pd corresponds to up to 9 times enhancement in the reaction rate compared to the neat Pd. The findings not only open an avenue towards the rational design of catalysts but also enable the identification of key steps involved in the catalytic reactions.

The Structure of Amorphous and Deeply Supercooled Liquid Alumina

Liquid Al2O3 has been supercooled more than 500 K below its melting point (Tm = 2327 K) using aerodynamic levitation and laser heating techniques. High energy synchrotron x-ray measurements were performed over a temperature range of 1817 ≤ T (K) ≤ 2700 and stroboscopic neutron diffraction at 1984 K and 2587 K. The diffraction patterns have been fitted with Empirical Potential Structure Refinement (EPSR) models and compared to classical molecular dynamics (MD) simulation results. Both sets of models show similar trends, indicating the presence of high populations of AlO4 and AlO5 polyhedral units predominantly linked by triply shared oxygen atoms. EPSR reveals that the mean Al–O coordination number changes linearly with temperature with nAlO = 4.41 – [1.25 × 10-4] (T – Tm), with a 2.5 A cutoff. Both EPSR and MD simulations reveal a direction of the temperature dependence of the aluminate network structure which moves further away from the glass forming ideal i.e. towards nAlO = 3 during supercooling. Furthermore, we provide new experimental data and models for amorphous alumina grown by sequential infiltration synthesis of a polymer template. The amorphous solid form likely has a larger Al–O coordination number than the liquid, consistent with expectations for the hypothetical glass.

Structure of Glasses of the Li2O–K2O–GeO2 System: Raman Spectroscopic Data

Abstract—Glasses of the three-component Li2O–K2O–GeO2 system are studied by Raman spectroscopy. Spectra are curve-fitted and the main structural units formed in the system are established. Conditions of formation of highly coordinated germanium atoms are determined and non-statistic distribution of network-modifying cations is found. It is shown that lithium for potassium substitution is accompanied by a change of coordination number of germanium, which could cause the destruction of Ge–O–Ge bonds with formation of non-bridging oxygens.

A constitutive model for granular materials with evolving contact structure and contact forces\u2014part II: constitutive equations

This and the companion paper present a constitutive model for granular materials with evolving contact structure and contact forces, where the contact structure and contact forces are characterised by some statistics of grain-scale entities such as contact normals and contact forces. And these statistics are actually the “fabric” or “force” terms in the “stress–force–fabric” (SFF) equation. The stress–strain response is obtained by inserting the predicted “fabric” or “force” terms from evolution equations into the SFF equation. In the model, the critical state is characterised by two fitting equations and three critical state parameters. A semi-mechanistic analysis is conducted about the change of the contact number and the obtained results are combined with observed phenomena in DEM virtual experiments to give the constitutive equations for the “fabric” terms. The change of fabric anisotropy is related to the strain rate, current fabric anisotropy and also contact forces. The change of coordination number is induced by two terms related to volumetric and shear deformations, and also an additional term related to the change of fabric anisotropy. The constitutive equations regarding the “force” terms are also proposed. All the “fabric” or “force” terms are modelled to tend toward their critial state value, which agrees with Li and Dafalias’s (J Eng Mech 138(3):263–275, 2012. https://doi.org/10.1061/(ASCE)EM.1943-7889.0000324) basic philosophy in their evolution equation for the fabric tensor. These equations along with the SFF equation form a constitutive model.

Tetranuclear dysprosium single-molecule magnets: tunable magnetic interactions and magnetization dynamics through modifying coordination number.

The study of mononuclear lanthanide-based systems, where the observed Single Molecule Magnets (SMMs) properties originate from the local magnetic anisotropy of the single lanthanide ion, has been extensively investigated in the literature. The case for polynuclear lanthanide SMMs becomes more challenging both experimentally and theoretically due to the complexity of such architectures involving interactions between the magnetic centers. Much interest was devoted to the study of the structural effect on the magnetic interactions and relaxation dynamics. However, the understanding of the structural influence on those two factors remains a difficult task. To address this issue, a system containing two structurally related tetranuclear Dy(iii) SMMs, namely [Dy4(L)4(OH)2(DMF)4(NO3)2]·2(DMF)·(H2O) (1) and [Dy4(L)4(OH)2(DMF)2(NO3)2] (2) (H2L = 2-(2-hydroxy-3-methoxybenzylideneamino)phenol), is introduced and investigated. Through modifying the ligands on the changeable coordination sites, the intramolecular magnetic interactions and relaxation dynamics in these two Dy(iii)4 SMMs can be tuned. Both complexes exhibit slow relaxation of their magnetization with a relaxation barrier of 114 K for complex 2 while a blocking temperature below 2 K is observed for complex 1. Ab initio calculations reveal that changes in coordination numbers and geometries on the Dy(iii) sites can significantly affect the magnetic interactions as well as single-ion anisotropy. The combination of experimental work and ab initio calculations offers insight into the relationship between structures and magnetic properties and sheds light on the rational design of future polynuclear lanthanide SMMs with enhanced magnetic properties.

Iron oxidation state of impact glasses from the Zhamanshin crater studied by X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) has been used to analyze the change in iron oxidation state and coordination numbers of impact glasses from the Zhamanshin crater. Series of samples, specifically aerodynamically shaped small glassy bodies – irghizites, and silica-rich and silica-poor glasses – zhamanshinites, have been investigated. The Fe2.2+ oxidation state was determined for the irghizites and the highest value Fe2.8+ for the silica-poor zhamanshinite. These values agree with the trend of oxidation processes upon high speed interaction of meteorite parent body and target rocks. The obtained data can be used for future establishment of the formation parameters (P, T, fO2 conditions) of these impact glasses as well as relationships between the rocks.

Thermodynamic stability of nanoparticles based on coordination number

A new model based on calculation of the average coordination number has been developed to predict the size-dependent melting point of nanoparticles when the size and shape are also concerned in the model. As is clear, differences in the shape of nanoparticles will result in changes in coordination number in a system, due to differences in the surface structure. The established model can reflect the influences of both size and shape; moreover, this model is free of any adjustable parameters, and is utilized to predict the thermal stability for low-dimensional materials in this work. For simplicity, the nanoparticle shape is taken to be cuboctahedral, and this is proved to be valid. This is because the theoretical predictions are consistent with experimental evidences and the results of molecular dynamic simulation for many nanoparticles of different elements. Moreover, the results also show that nanoparticles' melting thermodynamic functions are dependent on their size and their atomic diameter for a certain shape.

Investigation of γ ray shielding, structural and dissolution rate studies of alkali based bismuth borate glass systems with MoO3 added

Abstract Bi2O3–B2O3–Li2O–K2O–MoO3 glass system has been synthesized using melt-quenching technique. γ ray shielding properties have been evaluated in terms of mass attenuation coefficient at photon energies 356, 662 and 1173 keV. These shielding parameters are also compared with standard nuclear radiation shielding material “barite concrete” at the same photon energies. Most of our prepared samples show better γ ray shielding properties as compared to barite concrete at the same photon energies. Density, molar volume, XRD, FTIR and Raman studies have been employed to study the structural properties of the prepared glass system. Different structural groups such as [BO3], [BO4], [MoO4] and [MoO6] have been detected in the amorphous network. The presence of non-bridging oxygens, bridging oxygens and change of coordination number has been discussed. The optical absorption spectra are recorded in the wavelength range 200–800 nm and optical band gap is calculated which has shown strong dependence on Bi2O3/B2O3 ratio. The samples have also been tested for their durability by measuring dissolution rate properties using distilled water at 90°C. It has been concluded that increase in the content of Bi2O3 in the composition improves durability and γ ray shielding properties of the samples.

Local shear dominance in equation of state of metallic glass under hydrostatic pressure

Amorphous solids are generally believed to best obey the equation of state derived for homogeneous and isotropic solids under hydrostatic pressure. However, departure from some of the widely accepted theories is observed recently in metallic glasses and no mechanistic explanation is given. Here, we reveal that the underlying cause is the presence of the local internal shear induced by hydrostatic pressure. This subtle phenomenon is difficult to acquire experimentally and is generally ignored in formulating the equation of state. We show that the applied hydrostatic pressure can indeed induce and simultaneously couple to the internal local shear stress that leads to local shear deformation. The internal shear deformation results in a topological transition characterized by a gradual change of the nearest coordination numbers manifested in the deviation in the equation of state. We argue that the reported pressure-shear interaction is a general feature for the class of disordered materials.