Stabilities of small Ben and Bn clusters (4 ≤ n ≤ 8) by vibrational analysis

The stabilities Be_n and B_n clusters (4 ≤ n ≤ 8) based on the vibrational analysis were investigated by ab initio MO calculations. The computations were performed by using a 3-21G basis set at the R(U)HF level and at the R(U)MP4 level with the HF optimized structures. Spin-multiplicities were also considered up to quintet states (n ≤ 7). Of the 120 species that were treated, half of them were considered stable and some of these stable species were obtained by the deformations of transition state and unstable species, following the imaginary normal modes. The transformation barrier between the transition state species and corresponding stable ones was presented. It was found that there were two types of stable clusters: (1) a low symmetry species with lower frequencies and lower geometrical change barriers and (2) a high symmetry one with higher frequencies. The former type was considered as a structural “soft” species and the latter as a “hard” species.     

Comparative theoretical study of small Rhn nanoparticles (2 ≤ n ≤ 8) using DFT methods

This work is aimed at identifying some key characteristics (energy, geometry, and spin) concerning Rh_n particles (2 = n ≤ 8) to perform further studies on adsorption and coadsorption sites of pollutants (CO and NO). The DFT methods of the Gaussian 03 program with the LANL2DZ basis set and the LANL2 potential are used. With the purpose to obtain a better nanoparticles definition, five different functionals were tested: B3LYP, O3LYP, BPW91, BP86, and HCTH; and the corresponding results are used to determine which of them best describes distances, spin, and gives acceptable highest vibration frequency and binding energy values, by comparing these results with values measured or calculated by many other authors. For the structure optimization process of the particles, the initial geometric shape was taken mainly from the literature, using the Rh–Rh distance: 2.67 Å, known for the bulk; and doing a complete optimization. We also considered flat nanoparticles structures, which most of them display three‐dimensional structures after the optimization process. The few flat shapes are mainly higher in energy than those of three‐dimensional structure. For some Rh_n particles for different n values, the spin of the ground state present degeneration. In some cases, the optimization process changes the initial geometry, but in most cases, there are only minor changes in bonds and geometry. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Preparation and electrical and magnetic properties of Sr2−xNaxCuO3 (0 ≤ x ≤ 1) solid solutions

Mixed-valence copper(II/III) oxide solid solutions Sr_2?xNa_xCuO₃ (0 ≤ x ≤ 1) have been prepared by solid-state reactions in oxygen atmosphere. All solid solutions exhibit the structure of Sr₂CuO₃ (S.G. Immm). Electrical conductivity and thermoelectric power measurements show a semiconducting behavior in the whole composition range. The electronic structure of Sr₂CuO₃ is compared with that of La₂CuO₄ on the basis of an iono-covalent model. Interpretation of transport properties suggests the formation of small polarons. Magnetic susceptibility and EPR measurements show that the antiferromagnetic ordering of Sr₂CuO₃ tends to vanish as x increases, however magnetic interactions are still strong for a concentration of Cu²⁺ ions corresponding to x = 0.8.     

Density functional study of structural and electronic properties of AlnN (1 ≤ n ≤ 12) clusters

Low‐lying equilibrium geometric structures of Al_nN (n = 1–12) clusters obtained by an all‐electron linear combination of atomic orbital approach, within spin‐polarized density functional theory, are reported. The binding energy, dissociation energy, and stability of these clusters are studied within the local spin density approximation (LSDA) and the three‐parameter hybrid generalized gradient approximation (GGA) due to Becke–Lee–Yang–Parr (B3LYP). Ionization potentials, electron affinities, hardness, and static dipole polarizabilities are calculated for the ground‐state structures within the GGA. It is observed that symmetric structures with the nitrogen atom occupying the internal position are lowest‐energy geometries. Generalized gradient approximation extends bond lengths as compared with the LSDA lengths. The odd–even oscillations in the dissociation energy, the second differences in energy, the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps, the ionization potential, the electron affinity, and the hardness are more pronounced within the GGA. The stability analysis based on the energies clearly shows the Al₇N cluster to be endowed with special stability. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Tantalum‐Niobium Mixing in Ta3–xNbxTeI7 (0 ≤ x ≤ 3)

A substitutional study of the layered, trinuclear metal cluster system, Ta_3–xNb_xTeI₇ (0 ≤ x ≤ 3), has been performed. Synthetic, crystallographic, and spectroscopic results are presented for starting compositions corresponding to the x values: 1, 1.5, and 2. For the entire composition range studied, Ta(Nb) could readily substitute into the Nb(Ta)₃TeI₇ structure, but with changes in the observed stacking arrangements of the layers as x varies. For tantalum‐rich (x ≤ 1.8) phases, the structure conformed to the Nb₃SeI₇ structure type, also adopted by Ta₃TeI₇ and one polytype of Nb₃TeI₇. Niobium‐rich (i. e. x ≥ 1.7) phases were observed to adopt two structure types according to X‐ray powder diffraction, but crystals could only be obtained for the Nb₃SBr₇ structure type, which is a second modification of Nb₃TeI₇. Extended Hückel calculations are used to discuss the distribution of metal clusters in this system.     

High Lithium Ionic Conductivity in the Lithium Halide Hydrates Li3‐n(OHn)Cl (0.83≤n≤2) and Li3‐n(OHn)Br (1≤n≤2) at Ambient Temperatures

Lithium ionic conductivity and phase transitions in a series of lithium halides hydrates and hydroxides with general formula Li_3‐n(OH_n)X (0.83≤n≤2; X=Cl,Br) were studied using impedance measurements and ¹H and ⁷Li NMR spectroscopy. All compounds studied in this work crystallize in the antiperovskite structure or are closely related to this structure type. With the exception of LiCl?H₂O, all compounds with integer lithium content exhibit good lithium ionic conductivity in their high temperature cubic phases above T=33 °C. Lithium doping of samples LiX?H₂O and Li₂(OH)X leads to a suppression of the phase transition into the noncubic phases and the good ionic conductivity is extended down to lower temperatures (T<0 °C). Thus, lithium doping of the lithium halide hydrates provides a promising tool for tailoring the ionic conductivity at ambient temperatures to its optimum value.     

New Phases in the Lithiumnitridovanadate System — The Solid Solution Li7‐2xMgx[VN4] with 0 ≤ x ≤ 1

Solid solution phases Li_7‐2xMg_x[VN₄] (0 < x ≤ 1) with varying Mg‐content are obtained as yellow microcrystalline powders from heat treatment of mixtures of VN, Li₃N and Mg₃N₂ or from mixtures of Li₇[VN₄] and Mg₃N₂ at 1370 K in N₂ atmosphere at ambient pressure. At substitution parameter values of x > 0.5 a subsequent distortion from the ideal cubic unit cell to an orthorhombic unit cell is observed. The crystal structure of Li_7‐2xMg_x[VN₄] with x ≈ 1 was refined from neutron and X‐ray powder diffraction data (space group Pbca, No. 61, a = 963.03(3) pm, b = 958.44(3) pm, c = 951.93(2) pm, neutron pattern 14° — 156° 2θ, step non‐linear ≈ 0.0782° 2θ, No. of measured points 1816, R_p = 0.089, R_wp = 0.115, R_Bragg = 0.155, R_F = 0.114; X‐ray pattern 10° — 98° 2θ, step 0.005° 2θ, No. of measured points 17600, R_p = 0.028, R_wp = 0.045, R_Bragg = 0.113, R_F = 0.133, structure variables: 45). The crystal structure resembles a Li₂O type superstructure with the atomic arrangement of β‐Li₇[VN₄] and with two crystallographic Li‐sites each substituted by Mg with statistical occupation factors of 0.5. Chemical analyses prove the composition and XAS spectroscopy at the V K‐edge support the +5 oxidation state assignment for vanadium. XAS data also support the tetrahedral coordination of vanadium by N as indicated by the structure refinements.     

ChemInform Abstract: Valence Changes of Manganese and Structural Phase Transitions in Sr1‐xPrxMnO3 (0.1 ≤ x ≤ 0.6).

Fifteen perovskite manganites Sr_1‐xPr_xMnO₃ with x = 0.1—0.6 in steps of 0.025 are prepared by solid state methods and their crystal structures are determined by synchrotron powder XRD.     

Structures and energies of CnS (1 ≤ n ≤ 20) sulphur carbide clusters

C_nS (1 ≤ n ≤ 20) clusters have been investigated by means of the density functional theory. As a general rule, when 1 ≤ n ≤ 17 the energetically most favorable isomers are found to be the linear arrangement of nuclei (C_∞v) with the sulphur atom at the very end of the carbon chain. The electronic ground state is alternately predicted to be ¹∑⁺ for odd n or ³∑⁻ for even n with a conspicuous odd–even effect in the stability of these clusters. The C₁₈S cluster is predicted to have a S-capped monocyclic structure (¹A₁), but with a low barrier to linearity. On the other hand, C₁₉S and C₂₀S are unambiguously linear in the ¹∑⁺ and ³∑⁻ electronic ground states, respectively.     

Site Preference of Cations and Structural Variation in MgAl2–xGaxO4 (0 ≤ x ≤ 2) Spinel Solid Solution

The crystal structures of MgAl_2–xGa_xO₄ (0 ≤ x ≤ 2) spinel solid solutions (x = 0.00, 0.38, 0.76, 0.96, 1.52, 2.00) were refined using ²⁷Al MAS NMR measurements and single crystal X‐ray diffraction technique. Site preferences of cations were investigated. The inversion parameter (i) of MgAl₂O₄ (i = 0.206) is slightly larger than given in previous studies. It is considered that the difference of inversion parameter is caused by not only the difference of heat treatment time but also some influence of melting with a flux. The distribution of Ga³⁺ is little affected by a change of the temperature from 1473 K to 973 K. The degree of order‐disorder of Mg²⁺ or Al³⁺ between the fourfold‐ and sixfold‐coordinated sites is almost constant against Ga³⁺ content (x) in the solid solution. A compositional variable of the Ga/(Mg + Ga) ratio in the sixfold‐coordinated site has a constant value through the whole compositional range: the ratio is not influenced by the occupancy of Al³⁺. The occupancy of Al³⁺ is independent of the occupancy of Ga³⁺, though it depends on the occupancy of Mg²⁺ according to thermal history. The local bond lengths were estimated from the refined data of solid solutions. The local bond length between specific cation and oxygen corresponds with that expected from the effective ionic radii except local Al–O bond length in the fourfold‐coordinated site and local Mg–O bond length in the sixfold‐coordinated site. The local Al–O bond length in the fourfold‐coordinated site (1.92 Å) is about 0.15 Å longer than the expected bond length. This difference is induced by a difference in site symmetry of the fourfold‐coordinated site. The nature that Al³⁺ in spinel structure occupies mainly the sixfold‐coordinated site arises from the character of Al³⁺ itself. The local Mg–O bond length in the sixfold‐coordinated site (2.03 Å) is about 0.07 Å shorter than the expected one. Difference Fourier synthesis for MgGa₂O₄ shows a residual electron density peak of about 0.17 e/ų in height on the center of (Ga_0.59 Mg_0.41)–O bond. This peak indicates the covalent bonding nature of Ga–O bond on the sixfold‐coordinated site in the spinel structure.     

Low Temperature Synthesis and Characterization of Li1.2-y NayV3O8(0≤y≤1.2) from V2O5 Gel

Without overnight heating and stirring,Li1.2V3O8 and its analogs Li1.2-y NayV3O8(0≤y≤1.2) were successfully synthesized by adding mixed solution of LiOH and NaVO3 to V2O5 gel and dehydrating the prepared gel in 150-350℃.The simplicity awards this synthesis process superiority over other low temperature synthesis routes when mass production is concerned.TG-DTA,XRD and TEM experiments were carried out for physical characterization.By galvanostatic charge-discharge and cyclic voltammetry tests,these products showed better electrochemical performance than high temperature products as cathode active materials in secondary lithium batteries.After treatment of Li1.2V3O8 at 250℃,it exhibited a capacity of 350mAh/g when cycled at current rate of about 60 mA/g over the voltage range of 3.8-1.7V vs,Li^ /Li.The influence of partial substitution of Li by Na was also extensively studied.     

Medium‐sized phosphorus cluster cations P+2m+1 (6 ≤ m ≤ 32) studied by collision‐induced dissociation mass spectrometry

Medium‐sized phosphorus cluster cations were generated by laser ablation of red phosphorus and investigated by the method of collision‐induced dissociation mass spectrometry. Experimental results show that the primary dissociation channels of phosphorus cluster cations of P ⁺ _2m+1 (6 ≤ m ≤ 11) are all characterized by the loss of P₄ unit. For larger cluster cations, their dissociation pathways were more complex. For those magic cations of P ⁺ _8k+1 observed previously, their dissociation pathways progressively change from the loss of P₄ unit (for k = 3) to the loss of P₈ unit (for k = 4, 5). A new dissociation pathway characterized by the loss of P₁₀ unit was also indentified for larger cations of P ⁺ _8k+1 (6 ≤ k ≤ 8). Theoretical calculation also shows that, for cations of P ⁺ _2m+1 (4 ≤ m ≤ 10), the dissociation channel characterized by the loss of P₄ unit is more energetically favorable than other dissociation channels, which is in good agreement with the experimental results. Copyright © 2015 John Wiley & Sons, Ltd.     

A theoretical study on the stability difference of the borane and carborane C2Bn−2Hn (5 ≤ n ≤ 7) clusters

In order to study the electronic structure and structural stability of borane and carborane C₂B_n?2H_n (5 ≤ n ≤ 7) clusters, especially the stability difference between the borane and carborane C₂B₃H₅. The frontier orbital energy levels of the borane and carborane C₂B_n?2H_n (5 ≤ n ≤ 7) clusters are calculated at CCSD(T)/aug‐cc‐pVXZ//B3LYP/def2‐TZVPP level. The results are further analyzed by qualitative frontier orbital method based on the cap–ring interaction. The results reveal that: (1) the larger E_gap(HOMO‐LUMO energy gap) of carborane C₂B_n?2H_n (5 ≤ n ≤ 7) clusters than borane (5 ≤ n ≤ 7) clusters originates from the more effective cap–ring orbital overlap of carborane C₂B_n?2H_n (5 ≤ n ≤ 7) clusters than that of borane (5 ≤ n ≤ 7) clusters; (2) the smallest E_gap of the borane results from the highest energy level of the ring symmetry‐adapted linear combination orbital of cluster; and (3) the largest E_gap of the carborane C₂B₃H₅ is induced by the most effective cap–ring orbital interaction of C₂B₃H₅ cluster. © 2014 Wiley Periodicals, Inc.     

Les Phases KxMnO2 (x ≤ 1)

The K_xMnO₂ Phases (x ≤ 1) The investigation of the manganese-oxygen-potassium system for K/Mn ratios ≤1 leads to new phases having vacancy layer structures with oxygen packing AABBCC and AABB; The distorsions vary with the content of manganese(III). The larger size of potassium atoms allows only the formation of a hollandite type tunnel phase.     

Novel Derivatives of the CaIn2 Type of Structure: Yb1+xMg1—xGa4 (0 ≤ x ≤ 0.058) and YLiGa4

The compounds Yb_1+xMg_1—xGa₄ (0 ≤ x ≤ 0.058) and YLiGa₄ were synthesized by direct reaction of the elements in sealed niobium crucibles. The atomic arrangement of Yb_1+xMg_1—xGa₄ (x = 0.058) represents a new structure type (space group P6¯m2, a = 4.3979(3)Å and c = 6.9671(7)Å) as evidenced by single crystal structure analysis and can be described as an ordered variant of CaIn₂. YLiGa₄ is isotypic to the ytterbium compound according to X‐ray Guinier powder data (a = 4.3168(1)Å and c = 6.8716(2)Å). Measurements of the magnetic susceptibility of both compounds reveal intrinsic diamagnetic behaviour, i.e., ytterbium in the 4f¹⁴ configuration for Yb_1+xMg_1—xGa₄ (x = 0). From electrical resistivity data both compounds can be classified as metals. The compressibility of Yb_1+xMg_1—xGa₄ (x = 0.058) as measured in diamond anvil cells by angle‐dispersive X‐ray diffraction is compatible with a valence change of the ytterbium atoms at high‐pressures and indicates a slight anisotropy which is in accordance with the structural organisation of the gallium network. X‐ray absorption spectra of the Yb L_III edge of Yb_1+xMg_1—xGa₄ (x = 0.058) at pressures up to 25.0 GPa show a two‐peak structure which reveals the presence of Yb in the 4f¹⁴ and 4f¹³ states. The amount of ytterbium in the 4f¹³ state increases in two steps with progressing compression. The bonding analysis by means of the electron localization function reveals the Zintl‐like character of both compounds and confirms the 4f¹⁴ state for the majority of ytterbium atoms.