First-principles calculations of the structural, electronic and magnetic properties of BnN20−n (n = 6−18) clusters

The structures of B _n N_20 ? n    (n = 6?18), the clusters of boron nitride, are investigated by the density functional theory calculations. The structures of the obtained low-lying isomers can be described by the following six prototypes: single ring, double ring, three-ring, graphitic-like sheet, fullerene and others. B₁₀N₁₀ is demonstrated to be the most stable cluster against the nonstoichiometric ones. Nonzero magnetic moments, 1.999, 1.998, 2.000, 3.999 and 1.999μ _B respectively, are found in five B _n N_20?n (n = 6, 7, 11, 12, 13) clusters. Further analysis indicates that the magnetic moment of the B₆N₁₄ cluster is mainly originated from the N atoms, while those of others are from the B atoms. The magnetic moment are finally attributed to the interesting issues of the 2p electrons due to the breaking of local symmetries, the change of coordination number, charge distribution and orbital hybridization.     

Ionization-excitation of helium into the np (n = 2−5) states

Cross sections for the simultaneous ionization and excitation to the np states (n = 2−5) of the helium atom by fast proton and antiproton impact have been calculated. We have applied the impact parameter method and have used second-order perturbation approximation. Electron correlation has been taken into account in the initial state but has been neglected in the final state. Our cross sections are compared to the experimental data.     

Geometric structure and electron spectrum of YSi n − clusters (n = 6–17)

Results of the optimization of the geometric structure of YSi _n ^? anion clusters (n = 6–17) have been presented and their electron spectra have been calculated. Calculations have been performed by the density functional theory method. Actual geometric structures of clusters have been established by comparing the calculated and known experimental data.     

Spatial and electronic structures of the germanium-tantalum clusters TaGe n − (n = 8–17)

The results of optimizing the spatial structure and calculated electronic spectra of the TaGe _n ^? anion clusters (n = 8–17) have been presented. The calculations have been performed in terms of the density functional theory. The most probable spatial structures of clusters detected in the experiment have been determined by comparing the calculated and available experimental data.     

X-Ray spectral and theoretical investigations of the electronic structure of phenylcyclosilanes (SiPh2) n (n = 4–6)

The X-ray emission SiKα_{1, 2} and SiKβ₁ spectra of a series of phenylcyclosilanes (SiPh₂) _n (n = 4–6) have been obtained. Using the results of quantum-chemical calculations in the density functional theory approximation, the fine structure of SiKβ₁ spectra has been interpreted. Distributions of densities of electron states of silicon atoms over the valence band have been constructed and the types of chemical bonds providing the Si-Si and Si-Ph interactions have been established. Based on the theory of natural bond orbitals, the chemical bonding in the studied series of phenylcyclosilanes has been analyzed.     

Structure and magnetism of the R2Fe14−xCoxB ferrimagnetic systems (R = Dy and Er)

R₂Fe_14−xCo_xB ferrimagnetic systems (R = Dy and Er) have been synthesized and studied by X-ray and magnetometry methods to determine the lattice parameters, Curie temperatures, saturation magnetizations, anisotropy fields and spin-reorientation temperatures. It has been established that the single phase materials, exhibiting a tetragonal crystal structure, can be formed in the Dy-based system only for x ⩽ 8, while in the Er-based system only for x ⩽ 5. An average increase of the Curie temperature of 58 K per one substituted Fe atom by Co is observed for both systems (x ⩽ 5). A characteristic maximum in composition dependence of the saturation magnetizations at 295 K is found for x ≃ 2, although this is less pronounced than in light rare earth-based Co-substituted systems. The composition dependencies of the anisotropy fields for the Dy-based system at 77 and 295 K show distinct maxima for low Co concentrations and a sharp decrease for higher Co content. The spin-reorientation temperature for the Er-based system is shifted towards higher temperatures by the Co substitution. A brief comparison between ferro- and ferrimagnetic R₂Fe_14−xCo_xB systems is included.     

Structures,thermochemistry, and electron affinities of the disilicon fluorides,Si2F n /Si2F− n (n = 1–6)

A systematic investigation of Si₂F_n/Si₂F^? _n systems is carried out with five density functional (DFT) methods in conjunction with DZP++ basis sets. For each system, various structures, including minima, transition states, and energetically low lying saddle points, are optimized. The geometries and the relative energies are discussed and compared. Three kinds of electron affinity and dissociation energy pertaining to the global minimum for each compound are reported. The theoretical predictions are in good agreement with the limited experimental results. The zero-point vibrational energy (ZPVE) corrected adiabatic electron affinities (EA_ad) are predicted as 1.97 (Si₂F), 1.92 (Si₂F₂), 2.39 (Si₂F₃), 2.02 (Si₂F₄), 2.68 (Si₂F₅), and 0.73 (Si₂F₆)eV by the BHLYP method, which is considered to be the most reliable method in the present study for predicting the EAs. These theoretical predictions are quite different from those for the analogous silicon hydrides and fluorocarbons. For example, both Si₂F₂ and its anions have vinylidene-like (Si-SiF₂) global minima. The anion SiSi bond distance is about 0.1 Å shorter than that for the Si—SiF₂ neutral. Both Si₂F₃ and its anion have carbyne-like (Si-SiF₃) global minima, with the anion SiSi distance about 0.05 Å shorter. Both Si₂F₄ and its anion have carbene-like (FSi-SiF₃) global minima, again with the negative ion SiSi distance ～0.05 Å shorter. Surprisingly, doubly bridged structures of Si₂F₄ are energetically competitive. For the ethyl-radical-like Si₂F₅, the expected longer SiSi distance (by 0.13 Å) for the anion is predicted. Whereas Si₂H₄, C₂F₄, Si₂H₆, and C₂F₆ do not have significant electron affinities, Si₂F₄ and Si₂F₆ do bind an electron. However, the unexpected Si₂F^? ₆ species has a significantly longer SiSi distance (by 0.15 Å) than that of neutral Si₂F₆.     

Stability and superconductivity of TiPHn (n = 1−8) under high pressure

Titanium exhibits high affinity to hydrogen and the Ti-H compounds are thermodynamically stable. We introduced the Ti elements in the P-H compounds to deal with the thermodynamic-instability issue of the P-H compounds. In this work, ternary hydrogen-rich compounds TiPH_n (n = 1-8) were studied and calculated using the density function theory (DFT) method. The results showed that the TiPH₄ and TiPH₈ had high superconducting critical temperatures (T_c) as well as better stability. Among TiPH_n (n = 1-8), the TiPH and TiPH₄ were most stable at 50-200 and 250-300 GPa, respectively. For the R-3m-TiPH₄ structure, the predicted T_c's are 52 and 62 K at 100 and 250 GPa, respectively, and its larger Young's modulus suggested that it might be potentially synthetized in experiment.     

Effects of Co substitution on structural and magnetic properties of R3(Fe1−xCox)29−yVy (R=Tb,Dy)

Synthesis of two novel series of intermetallic compounds Tb₃(Fe_1−xCo_x)_27.4V_1.6 (x=0,0.1, 0.2, 0.3, 0.4) and Dy₃(Fe_1−xCo_x)_27.8V_1.2 (x=0, 0.1, 0.2, 0.3) with the monoclinic Nd₃(Fe,Ti)₂₉-type structure (3:29) is presented. In the Dy series for x=0.4 a disordered variant of the hexagonal Th₂Ni₁₇-type structure is formed. The cell parameters decrease and the Curie temperature increases with increasing of the Co content. In the case of the Tb₃(Fe_1−xCo_x)_27.4V_1.6 series in the M(T) curve a magnetic transition is observed which is attributed to spin reorientation phenomena. This critical temperature decreases with increasing Co from 473 K for x=0.1 to 393 K for x=0.3, and was not observed in the case of 0.4. XRD patterns of magnetically aligned powder samples reveal the presence of a tilted magnetic structure.     

Superionic conductivity of the heterovalent solid solutions R 1−x M x F 3−x (R=REE, M=Ca, Ba) with tysonite-type structure

The fluorine-ion conductivity of anion-deficient solid solutions R _1−x Ca_xF_3−x and R _1−x Ba_xF_3−x having the tysonite (LaF₃) structure was investigated by the impedance spectroscopy method. R _1−x Ca_xF_3−x (R=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho) and R _1−x Ba_xF_3−x (R=La, Pr, Nd) single crystals were grown from the melt by the Bridgman-Stockbarger method. The electrophysical measurements were performed in the frequency range 5−5×10⁵ and temperature range 300–700 K. The temperature dependences of the electrical conductivity for the crystals studied is determined by the migration of fluorine anions along various structural positions. It is shown that, from the standpoint of increasing the conductivity of tysonite matrices RF₃ (R=La, Pr, Nd), doping by CaF₂ and BaF₂ is less promising than SrF₂. Fiz. Tverd. Tela (St. Petersburg) 41, 638–640 (April 1999)     

Nonlinear representations of the group: (R +×SO(n)) ∧ R n

Nonlinear representations of the group G=(R ⁺×SO(n)) R ⁿ , such that the free part is an infinite-dimensional unitary irreducible representation of G, are proved to be formally linearizable on the space of infinitely differentiable functions with compact support.     

Ab initio and anion photoelectron studies of Rhn (n = 1 − 9) clusters

We present an analysis of Na-He collisional profiles at high density and very low temperature in a unified line shape semi-classical theory which contains the impact as well the quasistatic limits. Clearly understanding the variation of shape of the two fine-structure components of the 3s?3p line with increasing helium density allows us to understand experimental spectra of a Na atom attached to He nanodroplets. We compare our collisional approach to path integral Monte Carlo calculations using the same ab initio Na-He molecular potentials.     

Frequency response of the low-temperature ionic conductivity of single crystals R 1 − y M y F3 − y (R = La-Er; M = Ca, Sr, Ba, Cd)

The frequency response (10^?1–10^?7 Hz) of the ionic conductivity σ of R _{1 ? y} M _y F_{3 ? y} single crystals (R is a rare-earth element, and M stands for an alkaline-earth element and Cd) with a tysonite structure is studied over a wide temperature range (114–410 K), which includes (for the first time for these phases) the interval below room temperature. The dependences σ(ν) obtained are discussed within the hopping relaxation model. The characteristics of the relaxation and migration processes and the carrier concentration and mobility are determined.     

Theoretical studies on the structures and absorption spectra of–C n R2n+1(R = H,F; n = 1, 2) substituted 5-(2-pyridyl) pyrazolate boron complexes

The molecular structures, electronic structures and absorption characters of–CH₃,–C₂H₅,–CF₃,–C₂F₅ substituted 5-(2-pyridyl) pyrazolate boron complexes were presented by density functional theory (DFT). The ground state structures of the title complexes were optimised at B3LYP/6-31G* level. In addition, a time dependent density functional theory (TD-DFT) method is applied to investigate the properties of absorption spectra and electronic transition mechanism which were based on the ground state geometries. The results show that the chemical bond formed between nitrogen in the pyridyl ring and boron can be attributed to coordination effect. The boron centre has a typical tetrahedral geometry with the adjacent atoms. The calculated absorption wavelengths for–CF₃,–C₂F₅ substituted 5-(2-pyridyl) pyrazolate boron complexes are in good agreement with the experimental data.     

Bonding nature of the actinide tetrafluorides AnF4 (An = Th−Cm)

The knowledge of chemical bonding for actinide fluoride compounds is essential to understand and predict the physical and chemical behaviour of actinide elements in fluoride molten salt. In this work, the bonding nature of actinide tetrafluorides AnF₄ (An = Th?Cm) is investigated by using scalar relativistic density functional theory. Bond order analyses show relatively stronger An–F bonds for An = U?Np and weaker ones for An = Th, Am, and Cm. Despite the dominant ionic character of An–F bonds, a considerable covalent interaction is indicated by the overlap integral value of F 2p and actinide 5f, 6d orbitals. Both natural population analyses and electron density analyses show that An–F covalency rises initially before reducing in the latter systems with the maximum at Np and Pu and the obviously strong ionic bonding character in An = Th, Am, and Cm. Compared to AnCp₄ (Cp = η⁵–C₅H₅) reported in the literature, our study on AnF₄ suggests a much more prominent actinide–ligand covalent interaction. And the roles of orbital overlap and near-degeneracy in driving covalency are discussed.     

In-plane London penetration depths near the critical temperature of Tl2Ba2Ca n−1Cu n O2n+4 and (Bi,Pb)2Sr2Ca n−1 Cu n O2n+4 (n=2,3)

From isothermalM(H) curves nearT _c , measured on polycrystalline Tl₂Ba₂Ca _n–1Cu _n O_2n +4 and (Bi, Pb)₂Sr₂Ca _n–1Cu _n O_2n+4 (n=2,3) samples, we deduce the in-plane penetration depths _ab as functions of temperature. An estimate according to the BCS weak-coupling clean-limit fit, which produces the data nearT _c very well, yields _ab(0)=3100 Å, 2320 Å 2210 Å, and 1960 Å for Bi₂Sr₂CaCu₂O₈, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀, Tl₂Ba₂CaCu₂O₈, and Tl₂Ba₂Ca₂Cu₃O₁₀, respectively. A comparison between strong-coupling and weak coupling fitting curves clearly favours the weak-coupling temperature dependence of _ab nearT _c .     

Bottom-up substitution assembly of AuF4−n0,−+nPO3 (n = 1–4): a theoretical study of novel oxyfluoride hyperhalogen molecules and anions AuF4−n(PO3)n0,−

Compounds with high electron affinity, i.e. superhalogens, have continued to attract chemists’ attention, due to their potential importance in fundamental chemistry and materials science. It has now proven very effective to build up novel superhalogens with multi-positively charged centres, which are usually called ‘hyperhalogens’. Herein, using AuF₄^? and PO₃ as the model building blocks, we made the first attempt to design the Au,P-based hyperhalogen anions AuF_4?n(PO₃)_n^? (n = 1–4) at the B3LYP/6-311+G(d)&;SDD and CCSD(T)/6-311+G(d)&;SDD (single-point) levels (6-311+G(d) for O, F, P and SDD for Au). Notably, for all the considered Au,P systems, the ground state bears a dioxo-bonded structure with n ≤ 3, which is significantly more stable than the usually presumed mono-oxo-bonded one. Moreover, the clustering of the –PO₃ moieties becomes energetically favoured for n ≥ 3. The ground states of AuP₄O₁₂^0,? are the first reported cage-like oxide hyperhalogens. Thus, the ?PO₃ moiety cannot be retained during the ‘bottom-up’ assembly. The vertical detachment energy (VDE) value of the most stable AuF_4?n(PO₃)_n^? (n = 1–4) ranges from 7.16 to 8.20 eV, higher than the VDE values of the corresponding building blocks AuF₄^? (7.08 eV) and PO₃^? (4.69 eV). The adiabatic detachment energy values of these four hyperhalogens exceed 6.00 eV. Possible generation routes for AuF_4?n(PO₃)_n^? (n = 1–4) were discussed. The presently designed oxyfluorides not only enriches the family of hyperhalogens, but also demonstrates the great importance of considering the structural transformation during the superhalogen → hyperhalogen design such as for the present Au–P based systems.     

Observations of multi-phase microstructures in R2(Fe1−xCox)14B where R=Nd or Dy

Multi-phase microstructures were observed in the psuedo-quaternary phase field of the 2–14–1 magnet materials Nd₂Fe₁₄B, Nd₂Co₁₄B, Dy₂Fe₁₄B, and Dy₂Co₁₄B. At equilibrium, (Nd_1−yDy_y)₂(Fe_1−xCo_x)₁₄B had heretofore been widely assumed to be single phase where 1>x>0 and 1>y>0. In this study, three-phase microstructures were observed in (Nd_1−yDy_y)₂(Fe_1−xCo_x)₁₄B when x>0.3 and y>0.5. The Curie temperatures and peritectic decomposition temperatures for Nd₂(Fe_1−xCo_x)₁₄B are reported for several values of x in the range 1>x>0.