Electronic structure,magnetic properties,and mixed valence character of Ce2Ni3Si5 from first principles calculations

Cerium intermetallic compounds exhibit anomalous physical properties such as heavy fermion and Kondo behaviors. Here, an ab initio study of the electronic structure, magnetic properties, and mixed valence character of Ce₂Ni₃Si₅ using density functional theory (DFT) is presented. Two theoretical methods, including pure Perdew–Burke–Ernzerhof (PBE) and PBE + U , are used. In this study, Ce³⁺ and Ce⁴⁺ are considered as two different constituents in the unit cell. The formation energy calculations on the DFT level propose that Ce is in a stable mixed valence of 3.379 at 0 K. The calculated electronic structure shows that Ce₂Ni₃Si₅ is a metallic compound with a contribution at the Fermi level from Ce 4f and Ni 3d states. With the inclusion of the effective Hubbard parameter (U _eff), the five valence electrons of 5 Ce³⁺ ions are distributed only on Ce³⁺ 4f orbitals. Therefore, the occupied Ce³⁺ 4f band is located in the valence band (VB) while Ce⁴⁺ 4f orbitals are empty and Located at the Fermi level. The calculated magnetic moment in Ce₂Ni₃Si₅ is only due to cerium (Ce³⁺) in good agreement with the experimental results. The U _eff value of 5.4 eV provides a reasonable magnetic moment of 0.981 for the unpaired electron per Ce³⁺ ion. These results may serve as a guide for studying present mixed valence cerium‐based compounds. © 2017 Wiley Periodicals, Inc.     

Magnetic Investigations and 151Eu Mössbauer Spectroscopy of MYbSi4N7 with M = Sr,Ba, Eu

The isotypic nitridosilicates MYb[Si₄N₇] (M = Sr, Ba, Eu) were obtained by the reaction of the respective metals with Si(NH)₂ in a radiofrequency furnace below 1600 °C. On the basis of powder diffraction data of MYb[Si₄N₇] Rietveld refinements of the lattice constants were performed; these confirmed the previously published single‐crystal data. The compounds contain a condensed network of corner‐sharing [N(SiN₃)₄] units. The central nitrogen thus exhibits ammonium character. Magnetic susceptibility measurements of MYb[Si₄N₇] (M = Sr, Ba, Eu) show paramagnetic behavior with experimental magnetic moments of 3.03(2), (Sr), 2.73(2) (Ba), and 9.17(2) (Eu) μ_B per formula unit. In EuYbSi₄N₇ the europium and ytterbium atoms are in stable divalent and trivalent states, respectively. According to the non‐magnetic character of the alkaline earth cations, ytterbium has to be in an intermediate valence state Yb^III‐x in the strontium and barium compound. Consequently, either a partial exchange N^3—/O^2— resulting in compositions MYb^III‐x[Si₄N_7—xO_x] or an introduction of anion defects according to MYb^III‐x[Si₄N_7—x/3□_x/3] has to be assumed. The phase width 0 ≤ x ≤ 0.4 was estimated according to the magnetic measurements. ¹⁵¹Eu Mössbauer spectra of EuYb[Si₄N₇] at 78 K show a single signal at an isomer shift of δ = —12.83(3) mm s^—1 subject to quadrupole splitting of ΔE_Q = 5.7(8) mm s^—1, compatible with purely divalent europium.     

X-ray photoelectron spectra and electronic structure of rare-earth orthovanadates

Photoelectron spectra of 4d and valence states in RVO₄ (R = Y, Nd, Eu, Gd, Tb, Dy, Yb) have been investigated. The experimental spectra are interpreted using the results of the Xα discrete variational method calculations for orthovanadates. Transformations of electronic structure and covalency in the RVO₄ series are discussed. It is shown that lanthanide 4f orbitals significantly mix with the O 2pAO's and hybridize with the rare-earths 5pAO's. The 5p levels spin-orbital splitting in orthovanadates has been evaluated.     

The new binary intermetallic YbGe2.83

The new compound YbGe_2.83 was obtained from the reaction of Yb and Ge in liquid indium. The crystal structure of YbGe_2.83 adopts the trigonal, P3?m1 space group with a=b=8.3657(12) Å and c=7.0469(14) Å. The structure of YbGe_2.83 is a variant of the CaAl₂Si₂ structure type with ordered vacancies. Germanium atoms form double layers of puckered hexagons creating slabs that sandwich the Yb atoms. YbGe_2.83 can be classified as a Zintl compound with the formula Yb^(2+x)+(Ge_2.83)^(2+x)−. The deficiencies at the Ge sites cause a mixed/intermediate valent state of ytterbium (Yb^2.35+). Valence bond sum calculations suggest an average valence of Yb ions in YbGe_2.83 of 2.51 consistent with an intermediate valence compound.     

Analysis of the electronic structure of Hf(Si0.5As0.5)As by X-ray photoelectron and photoemission spectroscopy

The ternary hafnium silicon arsenide, Hf(Si_xAs_1−x)As, has been synthesized with a phase width of 0.5?x?0.7. Single-crystal X-ray diffraction studies on Hf(Si_0.5As_0.5)As showed that it adopts the ZrSiS-type structure (Pearson symbol tP6, space group P4/nmm, Z=2, a=3.6410(5) Å, c=8.155(1) Å). Physical property measurements indicated that it is metallic and Pauli paramagnetic. The electronic structure of Hf(Si_0.5As_0.5)As was investigated by examining plate-shaped crystals with laboratory-based X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoemission spectroscopy (PES). The Si 2p and As 3d XPS binding energies were consistent with assignments of anionic Si¹⁻ and As^1-. However, the Hf charge could not be determined by analysis of the Hf 4f binding energy because of electron delocalization in the 5d band. To examine these charge assignments further, the valence band spectrum obtained by XPS and PES was interpreted with the aid of TB-LMTO band structure calculations. By collecting the PES spectra at different excitation energies to vary the photoionization cross-sections, the contributions from different elements to the valence band spectrum could be isolated. Fitting the XPS valence band spectrum to these elemental components resulted in charges that confirm that the formulation of the product is Hf²⁺[(Si_0.5As_0.5)As]²⁻.     

X-ray and UV photoemission studies of valence electronic structure of NiO

The X-ray and UV photoemission valence band spectra of NiO are interpreted using the molecular orbital theory for the NiO^10?₆ cluster and the sudden approximation (monopole selection rules). They exhibit the effects of crystal field splitting, multiplet splitting, electron shake-up (O 2pe^b_g→ Ni 3de^a_g). relaxation and Ni 3dO 2p hybridization. Shake-up satellite data indicate that the NiO optical absorption edge near 4 eV is associated with an O 2p → Ni 3d transition. The NiO valence electronic structure obtained in this work is compared with band structure models of Wilson and Mattheiss.     

用密度泛函理论研究一种新型有机硅化合物异构体的振动光谱

在B3LYP/6-31＋G(d)的水平上, 对两种含有手性Si原子的新型有机硅单体Si₂(CH₃)₂H₂N₂(C₂H₅)₄和Si₄(CH₃)₄H₄N₂(C₆H₅)₂的几种异构体进行了研究, 在全参量几何构型优化的基础上, 进行了简谐振动频率计算, 同时对所研究的体系进行了热力学性质和低能激发态的含时密度泛函理论(TDDFT)计算. 理论计算表明, 构象异构体之间的红外光谱差异不大, 热力学和低能激发态性质也相似; 顺/反结构相似的异构体之间红外光谱差异不大, 但热力学和低能激发态性质却呈现差异; 旋光异构体或顺/反结构相差较大的异构体之间, 红外光谱和热力学及低能激发态性质有明显的差异. 从理论上解释了实验红外光谱中Si—H振动峰的裂分是由异构体的存在所致, 并找到裂分峰所对应的异构结构. Si—H键振动频率与其键长相关.     

A study of the electronic structure of phenylsilanes by X-ray emission spectroscopy and quantum chemical calculation methods

The electronic structure of a series of phenylsilanes Ph_4?n SiH _n (n = 0?C3) is studied by X-ray emission spectroscopy and quantum chemical calculations by the density functional theory method. Based on the calculations theoretical X-ray emission SiK??₁ spectra of phenylsilanes Ph_4?n SiH _n (n = 0?C4) are constructed and their energy structure and shape turn out to be well consistent with experiment. The distribution of the electron density of states with different symmetry of Si, C, H atoms are also constructed. An analysis of the obtained X-ray fluorescent SiK??₁ spectra and the distribution of the electron density of states in Ph₄Si and Ph₃SiH compounds shows that their energy structure is mainly determined by a system of the energy levels of phenyl ligands weakly perturbed by interactions with valence AOs of silicon. In the energy structure of MOs of the PhSiH₃ compound, energy orbitals related to t ₂ and a ₁ levels of tetrahedral SiH₄ are mainly presented.     

An X-ray photoelectron spectroscopic study of novel SiON glasses

Novel SiON glasses obtained by melting mixtures of crystalline α-SiO₂ and α-Si₃N₄ were investigated by means of X-ray photoelectron spectroscopy (XPS). The incorporation of nitrogen into the SiO₂ network was recently proved by ²⁹Si-MAS-NMR (magic-angle spinning nuclear magnetic resonance) and Si K-XANES (X-ray absorption near edge structure). The Si 2p XPS and the Si KLL XAES (X-ray excited Auger electron spectroscopy) studies of the SiON glasses confirm the formation of mixed structural units (SiO_xN_4-x) by the presence of an additional spectral component energetically located between SiO₂- and Si₃N₄-like signals. The N 1s and O 1s XPS spectra support the conclusion about the incorporation of nitrogen into the SiO₂ network.     

Two new phosphate langbeinites,Rb2YbTi(PO4)3 and Rb2Yb0.32Ti1.68(PO4)3, investigated at 293 and 150 K

The rubidium ytterbium titanium phosphates Rb₂YbTi(PO₄)₃, (I), and Rb₂Yb_0.32Ti_1.68(PO₄)₃, (II), have been structurally characterized from X‐ray data collected at both 293 and 150 K. Compound (II) is blue owing to the presence of mixed‐valence titanium (41% Ti³⁺ + 59% Ti⁴⁺). Both (I) and (II) belong to the langbeinite structure type, with mixed Yb/Ti populations in the two crystallographically independent octahedral sites (of symmetry 3). Ytterbium favours one of these sites, where about two‐thirds of the Yb atoms are found. The O‐atom displacement parameters are large in both compounds at both temperatures.     

Electronic structure of silanes in the semi-empirical equivalent orbital method

The problem of the prediction of the valence IPs for silanes is considered. It is shown that the data on the silicon band structure combined with the photoelectron spectra of SiH₄, and Si₂H₆ permit to obtain the parameter scale, which includes all the nearest neighbour, second neighbour and the main third neighbour interaction parameters. Using the derived parameter scale the vertical ionization potentials of Si₃H₈, SiH(SiH₃)₃, Si(SiH₃)₄, the infinite polysilane valence band structure and the inner a _1g level for disilane are calculated. All the calculated levels are located above ? 20 eV and are expected to be measurable by the He (I) photoelectron spectroscopy.     

The structures,thermochemistry, and electron affinities of hydrogenated silicon clusters Si6Hn/Si6H (n = 3–14)

The hydrogenated silicon clusters structures, electron affinities, and dissociation energies of the Si₆H_n/Si₆H (n = 3?14) species have been systematically investigated by means of three density functional theory (DFT) methods. The basis set used in this work is of double‐ζ plus polarization quality with additional diffuse s‐ and p‐type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Three different types of energy separations presented in this work are the adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vert), and the vertical detachment energy (VDE). The first Si? H dissociation energies D_e (Si₆H_n→ Si₆H_n?1+H) for the neutral Si₆H_n and D_e (Si₆H→Si₆H+H) for the anionic Si₆H species have also been reported. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Si X-ray absorption near edge structure (XANES) of Si,SiC, SiO2, and Si3N4 measured by an electron probe X-ray microanalyzer (EPMA)

Silicon K X-ray emission spectra of Si, SiC, Si₃N₄, and SiO₂ are measured using a wavelength dispersive electron probe X-ray microanalyzer. It is shown that the fine structures in the line shape of the low energy tail of the Kα characteristic X-ray emission spectra resemble those of the K X-ray absorption near edge structure (XANES). XANES spectra of 1 μm² area can be obtained by this method.     

X-ray photoelectron spectroscopic investigation on the elemental chemical shifts in multiferroic BiFeO3 and its valence band structure

A phenomenological rule based on the charge transfer, exists to predict the ionic/covalent character of the bonds in mixed oxides and is widely used to explain the binding-energy shifts of cations in mixed oxides compared to their simple oxides. Here, we have verified the above rule in the multiferroic BiFeO₃ and have applied the same to explain the X-ray photoelectron spectra of BiFeO₃ and its parent oxides Fe₂O₃ and Bi₂O₃. Ionic charges on Fe, Bi and O were calculated from density functional theory (DFT) within the local density approximation. Measured chemical shifts of O 1s, Fe 2p_3/2 and Bi 4f_5/2 were compared with the chemical shifts evaluated theoretically considering different contributions such as charge transfer, Madelung potential (initial state effect) and extra-atomic relaxation (final state effect). The chemical shift in the binding energy of O 1s photoelectron was used to build a covalence scale among Fe₂O₃, Bi₂O₃ and BiFeO₃. The effect of charge transfer on the valence band spectra of BiFeO₃ was also investigated.     

Mixed and partial oxidation states. Photoelectron spectroscopic evidence

X-ray photoelectron spectra of the single valence platinum complexes K₂[Pt(CN)₄] · 3H₂O(1),K₂[Pt(CN)₄]Cl_0.3 · n H₂O(2) and K₂[Pt(CN)₄]Cl₂ · 3H₂O(3) and the mixed valence compound [Pt^II(C₂H₅NH₂)₄]Cl₄ · [Pt^IV (C₂H₅NH₂)₄Cl₂] · 4H₂O(4) have been measured. It is found that one can distinguish clearly between mixed and single valence compounds by electron spectroscopy. The Pt spectrum of (4) is a superposition of a Pt^II and Pt_IV spectrum. The chemical shift between (1) and (3) is normal, however (2) shows an anomalous low binding energy for the Pt 4f electrons. The importance of using reliable reference peaks for obtaining absolute binding energies is emphasized.     

Study on the Geometric and Electronic Structures of Al<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript> (n = 3, 4, 5; m = 1, 2, 3, 4) Clusters

Genetic algorithm combined with the semi-empirical Hamitonian AM1/PM3 is used to search the low energy isomers of Al _n Si _m (n = 3, 5, m ≤ 3 and n = 4, m ≤ 4) and the charged clusters with 20 and 28 valence electrons. The candidate structures were optimized by the density functional theory PBE0 and B3LYP models with the triply split basis sets including polarization functions. The electronic structures show that Al–Si binary clusters behave like metal clusters. The molecular orbitals accord with that predicted by the jellium model, and the electron localization function shows the valence electrons are delocalized over the entire clusters. The clusters having 20 and 28 valence electrons exhibit pronounced stabilities and large energy gaps. The 20 valence electrons of Al₄Si₂ and Al₃Si₃ ⁺, Al₅Si^? form closed shells 1S ²1P ⁶2S ²1D ¹⁰. Al₄Si₄ and Al₅Si₃ ^? have oblate structures and the P, D, F levels spilt considerably in these clusters. The electron density distributions suggest that doping silicon in the aluminum clusters enhances the stability considerably.     

Valence States and Luminescence Properties of Ytterbium Ions in Strontium Haloborates

The luminescence properties of ytterbium ions in strontium haloborates were studied under optical, X-ray and synchrotron excitation. The coexistence of Yb ions in two valence states (divalent and trivalent) was detected in the Sr₂B₅O₉X:Yb (X=Cl, Br) powder materials prepared in a slightly reducing (H₂/N₂) or an oxidizing atmosphere. Under optical excitation, the 5d→4f Yb²⁺ luminescence at 420 nm is observed. Even under X-ray excitation, an emission band with a maximum of about 340nm appears in the spectra. This broadband emission is attributed to charge transfer luminescence of Yb³⁺. The influence of the structural features of Sr₂B₅O₉X and some preparative conditions of the samples on the luminescent behavior of Yb are discussed.     

Order-disorder transition and valence state of ytterbium in YbAuxGa2−x (0.26≤x≤1.31)

The intermetallic compounds YbAu_xGa_2−x (0.26≤x≤1.31) were synthesized by melting of elemental components and subsequent annealing. The crystal structure of YbAu_1.04Ga_0.96 was investigated using single-crystal X-ray diffraction data: structure type TiNiSi, space group Pnma, a=7.1167(3) Å, b=4.5019(3) Å, c=7.7083(3) Å, R_F=0.028 for 27 variables and 441 reflections. At 600 °C this compound is described as partially substituted TiNiSi type and shows a homogeneity range around the equiatomic composition YbAu_xGa_2−x (0.94≤x≤1.19). For the gallium- (0.26≤x≤0.83) and gold-rich (1.21≤x≤1.31) regions, the KHg₂ type of crystal structure (space group Imma) with mixed Au/Ga occupation is found. A temperature-driven phase transition for the composition YbAuGa from ordered TiNiSi to disordered KHg₂ structure type is observed at 629 °C. Yb L_III X-ray absorption spectra indicate an intermediate valence of +2.5 for Yb atoms in YbAuGa. For samples deviating from this composition a further reduced valence of Yb is observed. Magnetic susceptibility studies show a non-magnetic 4f¹⁴ ground state of Yb atoms with thermal fluctuations towards the 4f¹³ state.     

X-ray photoelectron study of mixed valence metatungstate anions

X-ray photoelectron spectra of metatungstate H₂W₁₂O^6?₄₀ and of reduced derivatives with 5, 12, and 24 electrons have been recorded. W(4f) signals are consistent with the presence of tetravalent tungsten in the reduced species. In particular the 12e^? derivative does not contain W^V but W^IV and W^VI in equal amounts. Valence band spectra show the W(5d) levels near 2 eV in reduced forms, this energy being appreciably lower than in W bronzes and WO₂. This can be correlated with the relative inertness of reduced metatungstates towards oxidation.