On the defect structure in nonstoichiometric metal oxides

A number of important physical and chemical properties of nonstoichiometric transition metal oxides are influenced by kind and concentration of defects in the crystalline lattice of these materials. Until quite recently it was thought that the relation between the nature of defects in the lattice of these oxides and their physical and chemical properties could be explained satisfactorily based on point defect theory. Today it is known that such an approach is correct only in the case of very small defect concentrations. At higher concentrations the mutual interaction between point defects results in formation of complexes and defect clusters called extended defects. These extended defects can become further ordered, which leads to superstructure ordering and to formation of intermediate phases. In some cases point defects can become eliminated in the process of crystallographic shear which is connected with formation of a whole series of intermediate phases.     

Defect clusters in wustite,Fe1−xO

A quantum chemical approach based on predominantly covalent “normalized ion energies” has been developed for estimating structures and energies for defect clusters in quenched nonstoichiometric wustite (Fe_1?xO). Small defect clusters of zinc blende structure show special stability over other clusters considered. Of these, either a 13:5 or a 16:7 defect cluster (13 or 16 Fe³⁺ vacancies and 5 or 7 tetrahedral Fe³⁺ interstitials) have the proper structure and composition to account for the observed P′ and P″ phases in wustite.     

Ni-B非晶态合金局域结构和电荷转移性质的理论研究

根据非晶态合金结构的短程有序、Ni和B间存在较强的化学作用、结构中存在B－B键直接相连的实验事实,选择Ni~mB~2(m=1,2,4)原子簇作为非晶态局域结构的计算模型。考虑原子簇间的相互作用,又对[Ni~4B~2]~n(n=1,2,5,7)簇团进行了系统计算。结果表明在所选Ni-Br的簇模型中,都是B原子提供电荷给Ni原子,这些均与非晶态合金实验结果和一些理论计算结果相符。簇团的计算结果还表明,小原子簇Ni~4B~2内原子间存在较强的化学键作用,而簇间的相互作用相对较弱,很容易造成对称性破缺而导致产生长程无序,因此Ni-B非晶态可以被看成是由大量i上原子簇无序堆砌成的,这一点也同实验事实相吻合。计算结果也表明我们前面工作中所取的和最常见的Ni-B非晶态合金Ni~6~4B~3~6有相似组成的Ni~4B~2原子簇模型能在一定程度上反映Ni-B非晶态合金的局域结构特征。     

Validation of interstitial iron and consequences of nonstoichiometry in mackinawite (Fe(1+x)S)

A theoretical investigation of the relationship between chemical composition and electronic structure was performed on the nonstoichiometric iron sulfide, mackinawite (Fe(1+x)S), which is isostructural and isoelectronic with the superconducting Fe(1+x)Se and Fe(1+x)(Te(1-y)Se(y)) phases. Even though Fe(1+x)S has not been measured for superconductivity, the effects of stoichiometry on transport properties and electronic structure in all of these iron-excess chalcogenide compounds has been largely overlooked. In mackinawite, the amount of Fe that has been reported ranges from a large excess, Fe(1.15)S, to nearly stoichiometric, Fe(1.00(7))S. Here, we analyze, for the first time, the electronic structure of Fe(1+x)S to justify these nonstoichiometric phases. First principles electronic structure calculations using supercells of Fe(1+x)S yield a wide range of energetically favorable compositions (0 < x < 0.30). The incorporation of interstitial Fe atoms originates from a delicate balance between the Madelung energy and the occupation of Fe-S and Fe-Fe antibonding orbitals. A theoretical assessment of various magnetic structures for "FeS" and Fe(1.06)S indicate that striped magnetic ordering along [110] is the lowest energy structure and the interstitial Fe affects the values of moments in the square planes as a function of distance. Moreover, the formation of the magnetic moment is dependent on the unit cell volume, thus relating it to composition. Finally, changes in the composition cause a modification of the Fermi surface and ultimately the loss of a nested vector.     

Forces between cylindrical nanoparticles in a liquid crystal

Using classical density functional theory, the forces between two cylindrical nanoparticles in a liquid crystal solvent are calculated. Both the nematic and isotropic phases of the solvent are considered. In the nematic phase, the interaction is highly anisotropic. At short range, changes in the defect structure around the cylinders leads to a complex interaction between them. In the isotropic phase, an attractive interaction arises due to overlap between halos of ordered fluid adsorbed on the surfaces of the cylinders.     

Exploring point defects in hexagonal boron-nitrogen monolayers

A comprehensive theoretical study of selected point defects for a monolayer of hexagonal boron nitride (h-BN) is presented. Two-dimensional structures were simulated through large h-BN molecular clusters and used to examine various defects, like: atom vacancies, atom substitutions, or distortions of the hexagonal lattice. Since carbon contaminations are very common in the h-BN technology, a particular attention has been paid to carbon impurities. The calculations of IR spectra for the doped molecular clusters reveal the presence of additional frequencies, which in many cases correspond to defect-bound modes. In particular, when two carbon atoms are close to each other, a localized stretching C C mode of a high intensity has been found, with a frequency value of about 100-200 cm^–1 higher than for collective BN stretch frequencies. Absorption UV-Vis spectra obtained from time-dependent density functional theory show that the inclusion of impurities results in an emergence of several low-energy electronic excitations, from which some are localized on a defect, while other are delocalized. Energies of these excitations are strongly dependent on the defect type, and they range from about 0.7 to 6.1 eV for the lowest excitations. Based on UV-Vis spectra we propose several candidates which could be responsible for the experimental 4 eV color band. These defects are built from two or four adjacent carbon atoms and have the lowest excitation ranging from 3.9 to 4.8 eV, which is strongly localized on the defect.     

Ni-P非晶态合金中电子转移问题的DFT研究

根据Ni-P非晶态合金结构的短程有序、Ni和P之间具较强化学作用和结构中不存在P-P直接相连的实验事实,选择了Ni~nP(n=1-6)原子簇模型,用密度泛函理论方法对其进行计算。结果表明,在模型体系中,随着P含量的减少,电子转移方向发生变化,P原子由得电子变为失电子。这与Ni-P非晶态合金的实验结果一致,说明Ni~nP(n=1-6)原子簇模型能反映Ni-P非晶态合金的结构特点。     

Ordered structures of defect clusters in gadolinium-doped ceria

The nano-domain, with short-range ordered structure, has been widely observed in rare-earth-doped ceria. Atomistic simulation has been employed to investigate the ordering structure of the nano-domain, as a result of aggregation and segregation of dopant cations and the associated oxygen vacancies in gadolinium-doped ceria. It is found that the binding energy of defect cluster increases as a function of cluster size, which provides the intrinsic driving force for the defect cluster growth. However, the ordered structures of the defect clusters are different from the chain model as previously reported. Adjacent oxygen vacancies prefer to locate along <110>/2 lattice vector, which results in a unique stable structure (isosceles triangle) formation. Such isosceles triangle structure can act as the smallest unit of cluster growth to form a symmetric dumbbell structure. This unique dumbbell structure is hence considered as a building block for the development of larger defect clusters, leading to nano-domain formation in rare-earth-doped ceria.     

Distinct Short‐Range Order Is Inherent to Small Amorphous Calcium Carbonate Clusters (<2 nm)

Amorphous intermediate phases are vital precursors in the crystallization of many biogenic minerals. While inherent short‐range orders have been found in amorphous calcium carbonates (ACCs) relating to different crystalline forms, it has never been clarified experimentally whether such orders already exist in very small clusters less than 2 nm in size. Here, we studied the stability and structure of 10,12‐pentacosadiynoic acid (PCDA) protected ACC clusters with a core size of ca. 1.4 nm consisting of only seven CaCO₃ units. Ligand concentration and structure are shown to be key factors in stabilizing the ACC clusters. More importantly, even in such small CaCO₃ entities, a proto‐calcite short‐range order can be identified but with a relatively high degree of disorder that arises from the very small size of the CaCO₃ core. Our findings support the notion of a structural link between prenucleation clusters, amorphous intermediates, and final crystalline polymorphs, which appears central to the understanding of polymorph selection.     

Size-dependent properties of Zn(m)S(n) clusters: a density-functional tight-binding study

We present the results of our theoretical calculations on structural and electronic properties of ligand-free Zn(n)S(n) [with n ranging from 4 to 104 (0.8-2.0-nm diameter)] clusters as a function of size of the clusters. We have optimized the structure whereby our initial structures are spherical parts of either zinc-blende or wurtzite structure. We have also considered some hollow bubblelike structures. The calculations are performed by using a parametrized linear combination of atomic orbitals-density-functional theory-local-density approximation-tight-binding method. We have focused on the variation of radial distribution function, Mulliken populations, electronic energy levels, band gap, and stability as a function of size for both zinc-blende and wurtzite-derived ZnS clusters. We have also reported the results of some nonstoichiometric Zn(m)S(n) (with m+n=47, 99, 177) clusters of zinc-blende modification.     

Defect‐Mediated Ordering of Condensed Water Structures in Microporous Zeolites

Ab‐initio molecular dynamics simulations and transmission infrared spectroscopy are employed to characterize the structure of water networks in defect‐functionalized microporous zeolites. Thermodynamically stable phases of clustered water molecules are localized at some of the defects in zeolite Beta, which include catalytic sites such as framework Lewis acidic Sn atoms in closed and hydrolyzed‐open forms, as well as silanol nests. These water clusters compete with ideal gas‐like structures at low water densities and pore‐filling phases at higher water densities, with the equilibrium phase determined by the water chemical potential. The physical characteristics of these phases are determined by the defect identity, with the local binding and orientation of hydroxyl moieties around the defects playing a central role. The results suggest general principles for how the structure of polar solvents in microporous solid acids is influenced by local defect functionalization, and the thermodynamic stability of the condensed phases surrounding such sites, in turn, implies that the catalysis of Lewis acids will be influenced by local water ordering.     

Kinetics of localized adsorption of colloid particles

Methods of analyzing localized adsorption of colloid particles at solid/liquid interfaces were extensively reviewed. First, the initial adsorption fluxes calculated using the Levich-Smoluchowski approximation were discussed. The uniformly, and nonuniformly accessible interfaces were distinguished and the superiority of the former in experimental studies was pointed out. A criterion was introduced for estimating the relative significance of the bulk transfer and surface adsorption steps. It was shown that for the majority of experimental and practical situations the surface mass balance equation can be decoupled from the bulk continuity equation. Thus, in due course attention was focused on theoretical and experimental methods of determining the surface blocking parameter B. It was shown that for low and moderate surface concentration range the statistical mechanic approach can be effectively used for predicting B. By introducing the equivalent hard sphere radius r¹ it became possible to analyze quantitatively blocking effects of interacting as well as nonspherical particles. The analytical solutions were compared with numerical simulation methods valid for the entire range of surface concentrations. The Monte-Carlo algorithm based on the random sequential adsorption (RSA) concept was compared with the sequential Brownian-Dynamics (SBD) method. Theoretical results obtained using these approaches were extensively discussed especially the role of repulsive electrostatic interaction among adsorbing particles. It was shown that these interactions diminish profoundly both the particle adsorption rate and the maximum surface concentration of particles forming “random” monolayers. When the electrostatic forces were operating (lower ionic strength) two distinctive adsorption regimes were predicted (i) fast Langmuir-type adsorption for short times and then (ii) very slow RSA-type approach to the maximum surface concentrations. As discussed such long lasting transient adsorption states could erroneously be interpreted as equilibrium adsorption isotherms. Then, the indirect and direct experimental methods aimed at a quantitative determination of particle adsorption kinetics were described. Illustrative experimental results performed for model latex suspensions were evoked. A satisfactory agreement with theoretical predictions was found for a variety of important physicochemical parameters studied. The RSA approach was found useful for describing particle adsorption kinetics for low and moderate surface concentrations in the case when the flow induced effects could be neglected. On the other hand, the SBD method was found of general validity especially in describing the hydrodynamic scattering effect observed experimentally for higher shear rates. Finally, the theoretical and experimental results concerning structure formation in adsorption processes were presented. The experimentally measured two-dimensional (2D) pair correlation function g12 of adsorbed particles suggested a liquid-like short range ordering occurring for larger surface concentrations. The extent of the 2D ordering was influenced by the adsorption mechanisms of particles, especially the presence of external field of forces.     

X-ray diffraction study of the structure of Al-Co melts

The structure of Al-Co melts with 0 at.%, 10 at.%, 18 at.%, 23.5 at.%, 26 at.%, 80.7 at.%, and 100 at.% Co has been studied by X-ray diffraction. Inverse Monte Carlo simulation using the experimental curves of the structural factor was employed to reconstruct the models of melts, analyzed by the Voronoi-Delaunay technique. The dependence of parameters that define short-and longer range ordering of atoms is nonmonotonous in the range 85–74 at.% Al and is extremal in the vicinity of a melt with 76.5 at.% Al. In the stated range of concentrations, the cobalt atoms basically lie at distances of ≈4.2 Å in the pentagonal rings of icosahedral type polytetrahedral clusters. Therefore one can speak about correlation in the character of atomic ordering in melts and corresponding periodic and quasiperiodic phases.     

Cubatic liquid-crystalline behavior in a system of hard cuboids

The lyotropic phase behavior of cuboidal particles was investigated via Monte Carlo simulations. Hard cubes were approximated by suitably shaped clusters of hard spheres. Changes in concentration and structure of the system were monitored as a function of osmotic pressure P* (imposed in an isobaric ensemble). As expected, an isotropic phase prevailed at low concentrations (low P*) and a crystalline phase formed at high concentrations (high P*). A third distinct phase was also observed for an intermediate range of concentrations (approximately marked by breaks in the P* versus concentration curve). The structure of this mesophase was characterized both visually and analytically by calculating radial distribution functions and order parameters. It was found that such a mesophase exhibits orientational ordering along three axes (cubatic order) but significant translational disorder, thus having a structure clearly distinct from both isotropic and crystalline phases.     

Percolation and jamming in structures built through sequential deposition of particles

The strength of attractive interaction among particles on a surface, which was studied in our previous work, leads to different degrees of clustering and ordering. A growing structure percolates when all clusters connect and become one and finally the structure is jammed when there is no space large enough to accommodate one more particle. The lowest jamming limit reported is for structures from the random sequential adsorption. We studied here, by means of Monte Carlo simulation, structures built through sequential deposition of particles, into which surface diffusion and various degrees of attractive forces are incorporated and reported jamming limits along with the percolation thresholds. The higher the strength of attractive interactions, the larger the percolation densities and jamming limits are. These results were shown in a diagram as a function of temperature (or equivalently the strength of attractive interaction), ranging from very low temperature to very high temperature (RSA limit).     

Ni-B非晶态合金中电子转移问题的DFT研究

根据Ni-B非晶态结构的短程有序、Ni和B之间是较强的化学作用和结构中存在B-B直接相连的实验事实,选择了Ni~mB~2^(^1^)(m=1-4)原子簇模型,用DFT方法对其进行高水平的量子化学计算,结果表明,模型体系中,B原子供给Ni原子电子,这与非晶态合金的实验结果一致,为了此较,同时选择了NiB^(^2^)(m=1-4)模型,计算结果与实验不符,这说明NiB~2^(^1^)(m=1-4)原子簇模型更能反映非晶态的结构特点。     

Local atomic structure of partially ordered NiMn in NiMn/NiFe exchange-coupled layers: 2. Electronic structure calculations

Local electronic and magnetic structure calculations for NiMn exchange bias alloys are reported for clusters containing NiMn in both the chemically disordered face-centered cubic and the chemically and magnetically ordered L1(0) phases. The results of these calculations are consistent with our local structure measurements that point toward the existence of nanometer-scale ordered clusters at the beginning stages of chemical ordering. The spatial dependence of both the local density of states and the magnetization is strongly influenced by the existence of magnetic order on short length scales, giving rise to an inhomogeneous profile for these quantities across the material with the greatest change at the interface that is still small enough within the domain to imply that the magnetization is still highly developed.     

Guidelines for tuning the macropore structure of monolithic columns for high‐performance liquid chromatography

The ability to control the external porosity and to tune the dimensions of the macropore size on multiple length scales provides the possibility of tailoring the monolithic support structure towards separation performance. This paper discusses the properties of conventional polymer–monolithic stationary phases and its limitations regarding the effects of morphology on kinetic performance. Furthermore, guidelines to improve the macropore structure are discussed. The optimal monolithic macropore structure is characterized by high external porosity (while maintaining ultra‐high‐pressure stability), high structure homogeneity, polymer globule clusters in the submicron range, and macropores with a diameter tuned toward speed (small diameter in the 100–500 nm range using short beds) or efficiency (larger macropores in the range of 500 nm–1 μm allowing the use of longer column formats). Finally, promising approaches to control the morphology are discussed.     

Etude par effet Mössbauer des composés de type “bronzes fluorés” MxFeF3 (M = K,Rb, Cs,NH4)

The hexagonal, tetragonal and pyrochlore-type nonstoichiometric iron fluorides M_xFeF₃ (M = K, Rb, Cs, NH₄) have been studied by Mössbauer spectroscopy over the temperature range 4.2 to 295 K. The magnetic transition temperatures have been determined. The ferrous and ferric ions remain in discrete oxidation states indicating the absence of charge hopping. The broadened lines of the spectra of the hexagonal and tetragonal phases are consistent with the disordering of Fe²⁺ and Fe³⁺ in the structure. By contrast, the narrow linewidths of the spectra of the pyrochlore-type phases characterize a structural ordering between the ferrous and ferric ions.