The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova     

Structure and Thermodynamic Characteristics of Impurity Centers in Lithium-Doped Cadmium Oxide: an <Emphasis Type="Italic">Ab Initio</Emphasis> Paw-Study

The ab initio projector augmented wave (PAW) method is used to calculate the electronic structure of Li-doped cadmium oxide with NaCl structure. The preference energy for Li atoms in interstitial sites and the energy of impurity oxidation are calculated. Interstitial positions for Li atoms are shown to be stable under thermodynamic equilibrium, but Li atoms can substitute Cd atoms in presence of vacancies in the oxygen sublattice. We consider the following complexes: one Li atom in the interstitial site and the other Li atom in Cd position; one Li atom in Cd position and one oxygen vacancy; a pair of oxygen vacancies; and show that these complexes are formed to have the shortest possible distance between their components. The band gap substantially decreases when Li atoms occupy interstitial sites to explain considerable increase of experimental conductivity.     

Some peculiarities of interactions in the Hf2Fe−H2 system at low temperatures and high pressures

Hydride formation was studied in the Hf₂Fe−H₂ system at hydrogen pressure of up to 2000 atm in a temperature range from 195 to 295 K. Hydride phases of different compositions were studied by the X-ray diffraction method. The hydrogenation reaction in the system can take two pathways to form two stable hydride phases depending on the conditions of initial hydrogenation. Absorption of hydrogens at a pressure of about 2000 atm yields a hydride which contains two H atoms per metal atom. Models of the arrangement of hydrogen atoms in the crystal lattice of hydride phases were suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 33–36, January, 1997.     

Hydrogen–hydrogen bonding in biphenyl revisited

The evidence for the stabilizing nature of the H–H bonding in planar biphenyl is succinctly reviewed. The stabilizing nature of the H–H bonding is revealed through a comparison of the atomic energy of every atom in planar biphenyl with the same atom in the twisted equilibrium structure. It is shown that the barrier to rotation via the planar transition state is the net resultant of a stabilisation of the four ortho-hydrogen atoms (by 8 kcal/mol each), a stabilisation of the two para-carbon atoms (by 3 kcal/mol each) and by the dominant destabilisation of the two carbon atoms joining the two rings—the two junction carbon atoms—(by 22 kcal/mol each). The energetic stabilisation of the four ortho-hydrogen atoms is further shown to be in large proportion due to the formation of the hydrogen–hydrogen interatomic surface. Furthermore, neither the “bond order” between the two junction carbon atoms nor the total electron delocalisation between the two rings exhibit a significant change in going from the planar to the twisted equilibrium geometry. These findings are in contrast with the classical view of a balance between “steric non-bonded repulsion” and better electron delocalisation as a function of the twist dihedral angle. Similar conclusions have been recently reached by Pacios and Gómez through a study of the electrostatic potential at the position of the hydrogen nuclei. We dedicate this article to Professor TM Krygowski on the occasion of his 70th birthday wishing him a long and productive life.     

Atomic correlations and atomic ordering in binary alloys with a B2 superstructure

The problem on atomic ordering in a binary alloy with pairwise interactions at the sites of a three-dimensional lattice is solved by the direct algebraic method applied in statistical quantum physics to correlation function calculations. A set of equations is derived for the one-particle and pair functions of probability distribution of atoms at lattice points. Plots of temperature vs the long-range order parameter and of concentration vs the critical temperature of order-disorder are given based on the results of the numerical solution of the corresponding equations obtained in a nearest neighbor approximation for beta-brass type alloys. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1141–1144, November–December, 1998.     

Construction of basis sets of hybrid atomic orbitals for describing the nonequivalent bonds of an atom with four neighboring atoms (sp 3 hybridization)

Methods for constructing the basis sets of sp³ hybrid atomic orbitals for atoms surrounded with four neighboring atoms are discussed in detail. The results obtained are generalized as a theorem of sp³ hybridization. A general method is proposed for constructing basis sets of orthogonal sp³ hybrid atomic orbitals for atoms with distorted tetrahedral environments. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp.969–975, November–December, 1995. Translated by I. Izvekova     

A new method for estimating the dispersity of deposited metallic nanoparticles and extent of their interaction with the support matrices

Using a fully relativistic DV cluster method, we study the electronic structure of a large fragment of the crystal lattice of zircon ZrSiO₄ with a plutonium dopant atom replacing a Zr⁴⁺ zirconium atom. Three possible states of the impurity center are considered: Pu⁴⁺ (isovalent substitution), Pu³⁺ (non-isovalent substitution), and Pu³⁺ with an oxygen vacancy in the nearest environment that provides charge compensation. Relaxation of the ZrSiO₄ crystal lattice near a defect is simulated using a semi-empirical method of atomic pair potentials (GULP program). An analysis of overlap populations and effective charges on atoms shows that the chemical bonding of plutonium with a matrix is covalent, while isovalent substitution yields a more stable system than a Pu³⁺ impurity. In the presence of vacancies the structure of chemical bonding is intermediate with respect to substitutions Pu⁴⁺ → Zr⁴⁺ and Pu³⁺ → Zr⁴⁺.     

Chain numbering and encoding of cyclic hydrocarbons

The numbering of carbon atoms in organic molecules is called chain numbering. It is suggested that bonds be classified as chain bonds (between atoms with consecutive indices) and nonchain bonds. Among atoms we distinguish the first atom (No. 1) and the last atom (having the largest index) as well as initial and final atoms (starting and finishing unbranched sections of atomic chains). Also, internal numbering of elements within these classes is proposed. For condensed cyclic hydrocarbons, formulas for deriving the number of cycles are given, and a linear-chain encoding system is worked out; in this system, nonchain bonds are assigned the indices of their atoms. Rules for unique numbering (canonical chain numbering) are developed; they involve maximization of the indices of the initial atoms and nonchain bonds. The linear-chain encoding of cyclic hydrocarbon structures ensures unification and systematization of their names. Tambov State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 731–736, July–August, 1995. Translated by L. Smolina     

氧空位和磷掺杂协同增强铁钴基材料电解水催化性能

为了研发高效、稳定的电解水催化剂,我们以氧空位和磷掺杂为基础,通过原位浸泡生长和两步热处理的方法,在泡沫铁上合成具有氧空位和磷掺杂的纳米花结构作为析氢反应(HER)和析氧反应(OER)双功能电催化剂。CoFe₂O₄已被报道为一种很有前途的OER和氧还原反应(ORR)电催化剂,然而CoFe₂O₄在HER中表现出电导率差、电催化反应慢的特性。CoFe₂O₄中氧空位(Ov)的形成可以有效调控催化剂表面的电子结构,有助于产生更多的缺陷和空位,从而提高OER的活性。随后,引入磷原子填充在空位中,制备的P-Ov-CoFe₂O₄/IF在碱性电催化测试中展现出优异的HER和OER性能,在10 mA·cm^-2电流密度下HER和OER过电位仅为54和191 mV,Tafel斜率分别为57和54 mV·dec^-1,并具有良好的循环稳定性。     

Quantum-chemical modeling of defects in PbTe crystals doped with VTe<Subscript>2</Subscript> and V<Subscript>3</Subscript>Te<Subscript>4</Subscript>

Quantum-chemical modeling of defects in PbTe crystals doped with VTe₂ and V₃Te₄ (PbTe(V)) has been performed. It has been shown that, at the B3LYP/LanL2DZP level, the effect of single defects (like vanadium atoms or lead vacancies) in the charges and electrostatic potentials is small and does not exceed 0.1e and 0.3 eV, respectively. Calculations predict that the location of a lead vacancy in the vicinity of a vanadium atom is energetically favorable. Fragments of chains of tellurium atoms formed upon association of vacancies can serve as active annihilation centers (AACs). In PbTe clusters, Te-Te chains are most abundant, Te-Te-Te-Te chains are less abundant, and Te-Te-Te chains are even less frequently encountered. AACs form beginning with association of three vacancies in PbTe clusters and beginning with association of two vacancies in PbTe(V) clusters. This fact that a smaller number of vacancies are required for formation of AACs in the PbTe(V) cluster can be responsible for that the number of annihilation centers experimentally found by the positron annihilation method in PbTe(V) is ∼1.2–1.4 times as large as the number of AACs in undoped PbTe.     

Lattice Relaxation Effect on the Electronic Structure of Helium Solutions in Calcium Fluoride Crystals

The discrete variation (DV) and molecular static methods are used to study the effects of atomic displacements near the anion vacancies and He impurity centers in the interstitial and vacancy positions on the electronic structure of CaF₂. Lattice ion displacements near the helium atom were estimated at 1% lattice constant in the former case and 4% in the latter. This leads to decreased splitting between the occupied and vacant impurity levels in the presence of isolated defects and to increased splitting for interacting defects. Effective charges are less sensitive to lattice relaxation than the electronic energy spectrum, except the case of high defect concentration.     

Theory of lineshape in photoelectron and Auger spectra

A theory of lineshape in photoelectron spectra is developed based on the Green’s function calculation of the atomic vacancy structure. It is shown that the broadening of photoelectron lines is always somewhat asymmetrical, and the broadening of satellite lines arising in atomic photoionization is generally strongly asymmetrical and inverse. The approach is generalized to low-energy Auger spectra; it is shown that Auger decay lines of excited atomic states may narrow with respect to the width of the initial level. The results are compared with experimental photoelectron spectra. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 985–991, November–December, 1998.     

Electronic structure and chemical bonding of δ-Bi2O3

The band structure of the fluorite-type δ-Bi₂O₃ was calculated by the linear LMTO methods in the approximation of overlapping atomic spheres using the basis set of orthogonal orbitals (LMTO-ASA) and by the full-potential LMTO method (LMTO-FP) for two vacancy orientations over a wide range of oxygen concentrations. The calculated parameters of chemical bonds—the binding energy E_bin and the pressure of the electron-nuclear system—show that the most stable compound is that with two vacancies per unit cell, oriented predominantly along the (111) direction. The hybrid Bi−O bonds are weak, and mostly the Bi−Bi bonds are responsible for the structural stabilization of δ-Bi₂O₃. The mechanism of the formation of a semiconductor gap in the band structure of δ-Bi₂O₃ is discussed. Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 48–58, January–February, 1996. Translated by I. Izvekova     

Direct Observation of Oxygen Vacancy Self‐Healing on TiO2 Photocatalysts for Solar Water Splitting

Oxygen vacancy (Vo) on transition metal oxides plays a crucial role in determining their chemical/physical properties. Conversely, the capability to directly detect the changing process of oxygen vacancies (Vos) will be important to realize their full potentials in the related fields. Herein, with a novel synchronous illumination X‐ray photoelectron spectroscopy (SI‐XPS) technique, we found that the surface Vos (surf‐Vos) exhibit a strong selectivity for binding with the water molecules, and sequentially capture an oxygen atom to achieve the anisotropic self‐healing of surface lattice oxygen. After this self‐healing process, the survived subsurface Vos (sub‐Vos) promote the charge excitation from Ti to O atoms due to the enriched electron located on low‐coordinated Ti sites. However, the excessive sub‐Vos would block the charge separation and transfer to TiO₂ surfaces resulted from the destroyed atomic structures. These findings open a new pathway to explore the dynamic changes of Vos and their roles on catalytic properties, not only in metal oxides, but in crystalline materials more generally.     

A modified MNDO procedure for calculating the heats of formation of organic anions

A modification of the MNDO method is suggested based on the relation ζ_A=ζ_A ⁰(1 + ϰ_AQ_A) for Slater parameters that takes into account the dependence of overlap integrals and the MNDO parameters (expressed via these integrals) on atomic charges Q_A. The core parameters U_μμ include corrections for nonorthogonality of basis functions of atoms in a molecule. For free atoms, the parameters were determined from the condition of reproducible electron affinities. The method is advantageous for calculating the heats of formation of anions, particularly those with the charge concentrated on one atom. Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 1, pp. 3–11, January-February, 1994. Translated by O. Kharlamova     

Synthesis and properties of crystalline fullerene hydrides

The interaction of crystalline fullerence C₆₀ with highly pure hydrogen, which was evolved from hydrides of intermetallic compounds of rear-earth metals and nickel, was studied. Crystalline fullerene hydrides containing from 10 to 30 hydrogen atoms per fullerene molecule were synthesized (1.0–2.5 MPa and 300–673 K). Crystalline hydrides release hydrogen at 800 K with retention of the structure of the fullerene molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2093–2096, October, 1998.     

From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2–BaF2 System

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent “dopants” can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state‐of‐the‐art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF₂ lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF₂ phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF₂–BaF₂ multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge‐transfer that occurred at the hetero‐interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher‐dimensional doping) to homogeneous doping (zero‐dimensional doping).     

From composites to solid solutions: modeling of ionic conductivity in the CaF2-BaF2 system

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent "dopants" can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).     

Crycom program for approximate symmetry recognition in crystal structures

The CRYCOM program was previously proposed for recognition of similarity between crystal structures. It compares unit cell parameters and atomic coordinates, taking into consideration all factors leading to various structure representations (the choice of basic lattice vectors, the origin of coordinates, segregation domain, and atomic displacements). The program can also be utilized for seeking approximate symmetry by comparing an original structure representation with its equivalent representations. Operation of the program is illustrated by an example of distorted hexagonal symmetry recognition in the monoclinic structure of strontium aluminate. L. Ya. Karpov Physicochemical Research Institute. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 5, pp. 149–152, September–October, 1994. Translated by O. Kharlamova     

Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations

Sulfur vacancy on an MoS₂ basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS₂ basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.