Anion-anion Bond Research Articles

Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr2MnO2Cu1.5Ch2 (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu1.5Ch2]2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr2MnO2Ch2 slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

The development of methods for the research and synthesis of solid phases by the scientific school of Ya. A Ugai. Review

The scientific school founded by Yakov A. Ugai has existed at Voronezh State University for over fifty years. One of its focus areas has been the development of physics and chemistry for obtaining solid phases in systems with volatile components. This determined the necessity to develop methods for the investigation of vapour pressure (tensimetric methods). This article only focuses on some of the works by the VSU staff dedicated to the study and construction of P-T-x diagrams. This review analyses phase equilibria and the nature of the intermediate phases in the AIV – BV, AIV – BV – СV, and AIII – BVI systems.Owing to the special nature of the cation-cation and anion-anion bonds, these compounds have highly specific properties that make them promising materials (2D materials in particular). The article presents an overview of works devoted to the construction of P-T-x diagrams and the investigation of defect formation processes in binary and ternary systems based on AIVBV compounds. It should be emphasised that the known techniques needed updating due to the high values of vapour pressure. This allowed conducting experiments at pressures of about 35-40 atmospheres. The study of the AIII - BVI systems,on the contrary, is complicated by low values of vapour pressure over indium and gallium chalcogenides and the complex composition of the vapour. For such systems the auxiliary component method was developed. The possibilities of its application are wide and are not limited to AIIIBVI compounds. A new method for nonstoichiometry regulation was developed and applied using non-destructive selective chemical transport reactions (i.e. with the participation of an auxiliary component). This method is based on the introduction or removal of one of the sample components by means of a selective chemical transport reaction. In conclusion, the development of methods for the research and synthesis of intermediate phases with variable compositions (properties) was analysed based on the example of the discussed systems.

Bonding mechanism of some simple ionic systems: Bader topological analysis of some alkali halides and hydrides revisited

The ability of Bader's charge density topology analysis to explain various material properties has been examined for simple ionic systems of alkali halides and alkali hydrides. It was established that despite the fact that most of them share the same rock salt crystal structure phase, some of them belong to different topological classes. This fact was used to explain various experimentally observed properties of these materials, and to discuss their deviations from expected trends. Some phase transitions observed in these systems, and their possible relation to changes in the charge density topology have been also investigated from the same point of view. Reasons for anion-anion bond formation in some of them and its features are discussed, as well.

Atomic and electronic structure of perfect dislocations in the solar absorber materials CuInSe2 and CuGaSe2 studied by first-principles calculations

Structural and electronic properties of screw and ${60}^{\ensuremath{\circ}}$-mixed glide and shuffle dislocations in the solar absorber materials ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ are investigated by means of electronic structure calculations within density functional theory (DFT). Screw dislocations present distorted bonds but remain fully coordinated after structural relaxation. Relaxed ${60}^{\ensuremath{\circ}}$-mixed dislocations, in contrast, exhibit dangling and ``wrong,'' cation-cation or anion-anion bonds, which induce deep charge transition levels and are electrically active. Analysis of Bader charges and local density of states (LDOS) reveals that acceptor and donor levels are induced by $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ cores, respectively. Moreover, there is local charge accumulation in the surrounding of those cores which contain dangling or ``wrong'' bonds. Thus the apparently harmless nature of dislocations is not because they are electrically inactive, but can only be a result of passivation by segregating defects.

Effect of ZnO twin grain boundary on p-type conductivity of VZn-NO-H complex:a GGA+U study

The origin of the p-type conductivity in N-doped ZnO has been a controversial issue for years, since isolated N substituted for O site (NO) was found to have high ionization energy. A recent experiment demonstrates that the p-type conductivity is attributed to the VZn-NO-H shallow acceptor complex. However, besides the complex, there are many other defects in ZnO, such as twin grain boundaries. They are commonly two-dimensional defects, and inevitably affect the p-type conductivity of the complex. By applying first principle calculations, we present the electronic structures and p-type conductivity of ZnO ∑7 (1230) twin grain boundaries containing VZn-NO-H complexes. Four types of ∑7 twin grain boundaries are investigated, and the VZn-NO-H complex is found to have a tendency to appearing in the stress raisers of the twin grain boundaries. The lowest formation energy under Zn-rich condition is only 0.52 eV for the complex in GB7a, a type of ∑7 twin grain boundary with anion-anion bonds, while the value is 3.25 eV for the complex in bulk ZnO. For the ionization energy, the complex in GB7a is more easily ionized, and has a value of 0.38 eV, compared with 0.67 eV in bulk ZnO. The result of density of states shows that the electron transition is dominated by the empty defect levels in forbidden band, which are occupied by O 2p and N 2p orbital. Further analysis indicates that the special structure of GB7a shortens the distances between NO and its neighbor O atoms, and the shortest N–O bond is only 2.38 Å, which also means a strong orbital hybridization between O and N. As a result, the energy level splitting is enhanced, and the empty energy level in the forbidden band is shifted down to valence band maximum. So, GB7a can favor the ionization in VZn-NO-H complex. Although GB7a is a special case of the twin grain boundaries, the result also gives us a new idea to understand the origin of p-type conductivity in N-doped ZnO.

The use of cation-cation and anion-anion bonds to augment the bond-valence model

The bond-valence model has, for several decades, been widely used for creating quantitative structure-activity relationships (QSARs), crystal structure modeling, and verification of proposed structures. Certain limitations of the model, such as the neglect of interactions between cations and between anions, have prevented it from being more broadly applied. In this work, we use cation-cation and anion-anion bonds to augment the existing bonding model in the systems H-Al-Si-O and K-Al-Si-O. The bond valence-length curves for these interactions employ the same mathematical form as ionic bonds, but make only a small contribution to the overall bonding in ionic materials. In the systems examined here, oxygen-oxygen interactions were much more important than those between cations for producing accurate bond-valence sums. Both anion-anion and cation-cation bonding could prove important, however, for our ultimate goal of producing valence-based force fields for use in molecular dynamics simulations. Rolling these interactions into the bond-valence framework would produce behavior similar to hard-sphere repulsive or van der Waals terms, but would more flexibly account for the complete bonding environment. The overall improvement in valence sums was robust, was maintained outside the calibration set, and was invariant to elemental substitution. We conclude that this minor alteration of the bond-valence approach will significantly improve bond-valence models in conjunction with other recent extensions of the approach.

Dependence of Optical Band Gap on Residual Stress in Group IIB Iodide (ZnI2, CdI2, HgI2) Films

Group IIB iodides are layered structured semiconductors and are chosen particularly for studying the residual stress dependence of band gap because of their anisotropy along and across the layers. The development of residual stress with film thickness, substrate temperature, annealing temperature etc. was studied using X-ray diffraction in these films grown by vacuum evaporation. The three compounds show completely different growth and residual stress behaviors. The room temperature optical band gaps of these films were determined by optical absorption measurements. Only ZnI2 exhibits excitonic absorption at room temperature. The optical band gaps of ZnI2 and HgI2 correlate linearly with residual stress while CdI2 shows nonlinear behavior. The magnitude of band gap variation with residual stress can be interpreted as due to the affected cation-cation, cation-anion and anion-anion bond lengths. The ionicity of these three compounds decreases linearly with their cation-anion radii ratio and the band gap increases with ionicity.

Synthesis and structural characterization of thin films

thin films in the range have been synthesized by sulphuration/seleniation of Ni thin films thermally evaporated on glass substrates. Growth conditions of temperature, time and pressures have been established to obtain samples with different selenium content and a theoretical model of the process has been proposed. Structural properties of the films are studied by Rietveld refinement of x-ray diffraction data. Values of lattice constant, a, sulphur/selenium positional parameter, u, and Se content in solid solution have been obtained. Moreover, Ni-anion and anion-anion bond distances have been obtained and analysed in relation to the selenium in the samples. It has been found that the lattice parameter follows a Vegard law with the selenium content. Resistivity measurements as a function of temperature from 10 to 200 K have been obtained for samples with x = 0.22 and x = 0.40 respectively. A change in the resistivity behaviour is evident in the sample with x = 0.40 at K, which can be associated with a phase transition observed in single crystals with the same Se content.

Neutron diffraction studies on expanded liquid CsTl

We report on neutron diffraction experiments of the equiatomic liquid alloy CsTl over a wide range of temperature from 730 to 1700 K. In the crystalline state, the compound contains Tl polyanions (Tl 6 6−). For liquid CsTl, the diffraction results indicate the existence of large Tl clusters even up to high temperatures, as can be inferred from the peak of S(Q) at Q ≈ 0.70 A ̊ −1 , which is indicative of intermediate-range order, and the similarity of the anion-anion bond lengths in the liquid and solid state. At the highest temperatures, the prepeak vanishes and a strong increase of small-angle scattering is observed, indicating a loss of coherence between Tl clusters and a tendency towards microphase separation.

Extended x-ray-absorption fine-structure study of GaAsxP1-x semiconducting random solid solutions.

The extended x-ray-absorption fine-structure (EXAFS) technique is employed to probe the local atomic structure of the III-V pseudobinary solid solutions GaAsxP1-x as a function of the composition. The first- and second-neighbor bond lengths as well as their variation with the composition are reported. The results imply that the local atomic structure is distorted. Both first-neighbor bond lengths and cation-cation bond lengths exhibit a bimodal structure, showing a tendency to retain the values of pure compounds. Anion-anion bond lengths are much closer to those predicted by Vegard's law; however they still show a significant deviation from it. On the basis of this result, the authors propose that the atomic-scale structure of these compounds can be described by a random distribution of five special coordination tetrahedra. The relation between local atomic structure and EXAFS as well as x-ray-diffraction results is discussed.

Structural features of compounds with tetrahedral structures or tetrahedral anion partial structures derived from two valence electron concentration rules

The total and the partial valence electron concentrations(VEC), two variables which can be easily calculated from the composition of the compound and the valence electron numbers of the participating atoms, serve as useful parameters to predict probable structural features of compounds with a tetrahedral structure or a tetrahedral anion partial structure such as an anionic tetrahedron complex. The structural features to be predicted are the average number of nonbonding orbitals, the number of atoms in molecular structures, the average number of anion-anion bonds per anion or the average number of electrons which remain with the cation to be used either for cation-cation bonds or nonbonding orbitals.

PbFCl-type pnictides of niobium with silicon or germanium

NbSiAs, NbGeAs, NbSiSb, and NbGeSb have been prepared with the tetragonal PbFCl structure. A powder refinement of the structure of NbSiAs shows Nb in 2c ( z = 0.256), Si in 2a, and As in 2c ( z = 0.612) of space group P4 nmm . If atoms in the 2a position are assumed to form anion-anion bonds, the compositions for which PbFCl-type pnictides and chalcogenides exist can be rationalized.