Variations of the FeGa3 Structure Type in the Systems CoIn3−xZnx and CoGa3−xZnx

We present an investigation of the quasibinary systems CoIn_3−xZn_x and CoGa_3−xZn_x which were structurally characterized by X-ray diffraction experiments and, in the case of CoGa_3−xZn_x, additionally by neutron powder diffraction experiments. The limiting compositions were found to be x=0.81(2) and x=0.73(2) for CoIn_3−xZn_x and CoGa_3−xZn_x, respectively. The isotypic binary compounds CoIn₃ and CoGa₃ crystallize with the FeGa₃ structure type (tetragonal, space group P4₂/mnm, Z=4) in which the p-block atoms form an array of columns of centered cubes defined by two different crystallographic sites. The substitution of In or Ga by Zn takes place in an ordered fashion and produces “colored” variants of the FeGa₃ parent structure: In both systems Zn enters exclusively the position corresponding to the cube centers. Additionally, in CoIn_3−xZn_x this position is substituted in such a way that for a composition CoIn_2.5Zn_0.5, columns of Zn- and In-filled In₈ cubes along the c axis alternate. The latter substitution pattern is accompanied by a symmetry lowering of the parent FeGa₃ structure: The structure of CoIn_3−xZn_x is described by the space group P4₂/m in which the cube center position is split into two separate sites. By performing first-principles electronic structure calculations we investigated the general bonding situation of the compounds CoIn₃ and CoGa₃ and the particular electronic effect when incorporating Zn. With respect to the density of states of the binary compounds the exchange of Ga or In by Zn virtually affects only the electronic states just below the Fermi level. On increasing Zn concentration a dip is created in the density of states which approximately coincides with the location of the Fermi level for an electron count corresponding to limiting composition of the two systems.     

Electronic Structure and Formation Heat of Pu3M and PuM3 (M=Ga, In, Sn and Ge) Compounds

The equilibrium structure, electronic structure, and formation heat of Pu₃M (PuM₃) (M=Ga, In, Sn, and Ge) compounds with AuCu₃ structure have been calculated using full potential linear augmented plane wave (FPLAPW) method within generalized-gradient approximation (GGA) including spin-orbit coupling (SOC) and spin polarization (SP). The calculated lattice parameters are in good agreement with the experimental values. Density of state analysis shows hybridization effects between Pu and M are governed by the competitions depending on the M amount: Pu 6d-Pu 5f, M p-Pu 6d, and M sp-M sp interactions. Electronegativity difference and electronic hybridization effect are two important factors to influence the formation heat and stability of Pu₃M (PuM₃) compounds. The larger is the electronegativity difference and the lower is M s-band or p-band center relative to the Fermi energy, the more negative is the formation heat and the more stable are Pu₃M (PuM₃) compounds.     

Subnitride chemistry: A first-principles study of the NaBa3N, Na5Ba3N, and Na16Ba6N phases

An ab initio study on the electronic structure of the subnitrides NaBa₃N, Na₅Ba₃N, and Na₁₆Ba₆N is performed for the first time. The NaBa₃N and Na₅Ba₃N phases consist of infinite ¹_∞[NBa_6/2] strands composed of face-sharing NBa₆ octahedra surrounded by a “sea” of sodium atoms. The Na₁₆Ba₆N phase consist of discrete [NBa₆] octahedra arranged in a body-cubic fashion, surrounded by a “sea” of sodium atoms. Our calculations suggest that the title subnitrides are metals. Analysis of the electronic structure shows partial interaction of N(2s) with Ba(5p) electrons in the lower energy region for NaBa₃N and Na₅Ba₃N. However, no dispersion is observed for the N(2s) and Ba(5p) bands in the cubic phase Na₁₆Ba₆N. The metallic band below the Fermi level shows a strong mixing of N(2p), Ba(6s), Ba(5d), Ba(6p), Na(3s) and Na(3p) orbitals. The metallic character in these nitrides stems from delocalized electrons corresponding to hybridized 5d^l6s^m6pⁿ barium orbitals which interact with hybridized 3sⁿ3p^m sodium orbitals. Analysis of the electron density and electronic structure in these nitrides shows two different regions: a metallic matrix corresponding to the sodium atoms and the regions around them and heteropolar bonding between nitrogen and barium within the infinite ¹_∞[NBa_6/2] strands of the NaBa₃N and Na₅Ba₃N phases, and within the isolated [NBa₆] octahedra of the Na₁₆Ba₆N phase. The nitrogen atoms inside the strands and octahedra are negatively charged, the anionic character of nitrogens being larger in the isolated octahedra of the cubic phase Na₁₆Ba₆N, due to the lack of electron delocalization along one direction as opposed to the other phases. The sodium and barium atoms appear to be slightly negatively and positively charged, the latter to a larger extent. From the computed Ba-N overlap populations as well as the analysis of the contour maps of differences between total density and superposition of atomic densities, we suggest partial covalent bonding between nitrogen and barium atoms along the infinite ¹_∞[NBa_6/2] strands and within isolated [NBa₆] octahedra.     

Coordination and bonding in Fe3PTi3PV3STa3As-type compounds: The crystal structures of Hf3Sb and h-Ta3Ge

Interatomic distances in Hf₃Sb and h-Ta₃Ge (Fe₃P-type structure, space group $\text{I}\text{4}\text{, Z = 8}$) have been determined by crystal structure refinements based on Rietveld-type full-profile analyses of Guinier-H:agg X-ray powder film intensity data. The results, together with data from the recently refined Hf₃As and Ta₃As structures, are included in a survey of coordination and bonding in Fe₃PTi₃PV₃STa₃As-type compounds. It is shown that the trends in atomic coordination observed can be explained in terms of an interplay of d-d and d-p electronic interactions.     

The novel silicide Eu3Si4: structure, chemical bonding, magnetic behavior and electrical resistivity

The novel binary europium silicide Eu₃Si₄ was synthesized from the elements. Its crystal structure is a derivative of the Ta₃B₄ type: space group Immm, a=4.6164(4) Å, b=3.9583(3) Å, c=18.229(1) Å, Z=2. In the structure, the silicon atoms form one-dimensional bands of condensed hexagons. Deviating from the prototype structure, a partial corrugation of the initially planar bands may be concluded from the analysis of the experimental electron density in the vicinity of the Si1 atoms. In the paramagnetic region, Eu₃Si₄ shows a 4f⁷ electronic configuration for the europium atoms. Two consecutive magnetic ordering transitions were found at 117 and 40 K. The first one is attributed to a ferromagnetic ordering of the Eu2 atoms; the second one is caused by a ferromagnetic ordering of the Eu1 atoms resulting in a ferrimagnetic ground state with a net magnetization of 7 μ_B at 1.8 K. The temperature dependence of the electrical resistivity reflects the metallic character of the investigated compound. Furthermore, the pronounced changes of the dρ/dT slope confirm the magnetic transitions. From bonding analysis with the electron localization function, Eu₃Si₄ shows a Zintl-like character and its electronic count balance can be written as (Eu^1.83+)₃(Si1^0.95−)₂(Si2^1.8−)₂, in good agreement with its magnetic behavior in the paramagnetic region.     

Crystal structure and luminescence property of novel three-dimensional network of {Eu2(p-BDC)3(Phen)2(H2O)2}n

The single crystal of a supramolecule, {Eu₂(p-BDC)₃(Phen)₂(H₂O)₂}_n (p-BDC=1,4-benzenedicarboxylate), with characteristic luminescence of Eu³⁺ was obtained by means of soft chemistry. The crystal structure determination reveals that each Eu³⁺ ion is coordinated by five oxygen atoms of p-BDC anions, one oxygen atom from water molecule, and two nitrogen atoms of Phen, respectively, resulting in an eight-coordinated Eu³⁺ center and a distorted square antiprism coordination polyhedron. Four bridges, two carboxylates of μ₄-p-BDC and two of μ₃-p-BDC, connect two Eu atoms into a binuclear unit. Moreover, the μ₃-p-BDC integrates the binuclear building blocks at the direction of b axis and the μ₄-p-BDC polymerizes the structure roughly along the direction of the sum vector of axis b and c, respectively, forming two-dimensional layers. Hydrogen bonds between layers make the structure a three-dimensional network. The luminescence spectra measured under 77 K demonstrate the antenna effect of Phen and the ⁵D₁→⁵D₀ energy transfer path within Eu³⁺ ion. Both luminescence spectra and crystal structure lead to the conclusion that the local symmetry around the Eu³⁺ ion is C₁ and that more than one Eu³⁺ ion sites having slight environmental difference are present.     

UTa2O(S2)3Cl6: A ribbon structure containing a heterobimetallic 5d-5f M3 cluster

A new solid-state compound containing a heterobimetallic cluster of U and Ta, UTa₂O(S₂)₃Cl₆, has been synthesized and its structure has been characterized by single-crystal X-ray diffraction methods. UTa₂O(S₂)₃Cl₆ was synthesized from UCl₄ and Ta_1.2S₂ at 883 K. The O is believed to have originated in the Ta_1.2S₂ reactant. The compound crystallizes in the space group P1¯ of the triclinic system. The structure comprises a UTa₂ unit bridged by μ₂-S₂ and μ₃-O groups. Each Ta atom bonds to two μ₂-S₂, the μ₃-O, and two terminal Cl atoms. Each U atom bonds to two μ₂-S₂, the μ₃-O, and four Cl atoms. The Cl atoms bridge in pairs to neighboring U atoms to form a ribbon structure. The bond distances are normal and are consistent with formal oxidation states of +IV/+V/-II/-I/-I for U/Ta/O/S/Cl, respectively. The optical absorbance spectrum displays characteristic transition peaks near the absorption edge. Density functional theory was used to assign these peaks to transitions between S^1- valence-band states and empty U 5f-6d hybrid bands. Density-of-states analysis shows overlap between Ta 5d and U bands, consistent with metal-metal interactions.     

D-isotope-labelled n-butane cations in CF3CCl3 and in CF2ClCFCl2: Structure and reactions at low temperature

Cations of n-butane, n-butane-1,4-d₆, -d₄, -d₂ and n-butane-2,3-d4 produced by X-irradiation of CF₂ClCFCl₂ and CF₃CCl₃ 3 matrices containing 1–2 mole% of solute have been investigated by ESR spectroscopy. The measurements have been performed at 77–130 K. The results confirm that the two largest couplings are assigned to two protons on the methyl groups. Smaller couplings have been resolved in CF₃CCl₃ and assigned to specific H atoms. A geometry with a non-planar carbon skeleton is proposed. Partly deuterated methyl groups give rise to rotational isomers. Thermal and photoinduced decompositions producing 2-butyl radicals and 2-butane cations have been further studied.     

Dependence of the lone pair of bismuth on coordination environment and pressure: An ab initio study on Cu4Bi5S10 and Bi2S3

DFT calculations have been carried out for Cu₄Bi₅S₁₀ and Bi₂S₃ to provide an analysis of the relation between electronic structure, lone electron pairs and the local geometry. The effect of pressure is considered in Bi₂S₃ and the results are compared to published experimental data. Bi³⁺ in Cu₄Bi₅S₁₀ is found at both symmetrically and asymmetrically coordinated sites, whereas the coordination environments of Bi in Bi₂S₃ are asymmetric at room conditions and get more regular with increasing pressure. The charge density maps of the asymmetric sites show the lone pairs as lobes of non-shared charge. These lobes are related to an effective Bi s-Bi p hybridization resulting from coupling to S p orbitals, supporting the modern view of the origin of the stereochemically active lone pair. No effective Bi s-p hybridization is seen for the symmetric site in Cu₄Bi₅S₁₀, whereas Bi s-p hybridization coexists with a much reduced lone pair in Bi₂S₃ at high pressure.     

Étude lcao-mo-cndo/2 des liaisons po et no dans des composés X3PO ET X3NO

The authors have studied the electronic structure of X₃PO and X₃NO compounds (with X = F, Cl, CH₃), using the semi-empirical CNDO/2 method. All the calculations have been made with and without 3d functions on the phosphorus atom. The comparison between the calculated and experimental values, especially in the case of bond length, dipole moment, and orbital level order, shows the influence of the 3d orbitals in the PO bond, which contains a sigma donation P → O and a pπ(O)-dπ(P) back bonding. The NO bond has sigma character in trimethylamine oxide, but is partially a double bond in trifluoramine oxide.     

XPS study on valence band structures of transition-metal trisulfides,TiS3, NbS3, and TaS3

Transition-metal trisulfides, TiS₃, NbS₃, and TaS₃, with a quasi-one-dimensional structure are investigated by X-ray photoelectron spectroscopic (XPS) measurements to obtain information on the valence band structures. The band structures at the Fermi level of these compounds correspond well to their transport properties. A shoulder is observed at the top of the valence band in NbS₃ and TaS₃, suggesting that this band is made up of the metal d_z² electrons. The d_z² band is occupied in NbS₃ and TaS₂ and empty in TiS₃. The characteristic features at the top of the valence band in NbS₃ imply the occurrence of d_z² band separation, which leads to a semiconducting nature.     

A new structural type in ternary chalcogenide chemistry: Structure and properties of Nb2Pd3Se8

A study of the NbPdSe system has afforded a new phase, Nb₂Pd₃Se₈. The structure of this phase has been established through single-crystal X-ray measurements. The compound crystallizes in space group D⁹_2h-Pbam with two formula units in a cell of dimensions a = 15.074(6), b = 10.573(4), c = 3.547(2)Å. In this unusual structure there are two chains of edge-sharing selenium trigonal prisms centered by niobium atoms. These chains conjoin through two types of palladium atoms—square planar and square pyramidal—each coordinated by selenium atoms. As a consequence of this conjunction tunnels extending along c result. Electrical conductivity measurements indicate that this material is a metallic-semiconductor.     

Polarized crystal spectra of the mixed metal salt [(CH3)3NH]MnxCu1−xCl3·2H2O

The electronic absorption spectra in two polarizations are reported for crystals of the dichroic salt, TMAMn_xCu_1?xCl₃·2H₂O where TMA represents the trimethylammonium cation, (CH₃)₃NH⁺. Although TMACuCl₃·2H₂O is monoclinic, the mixed metal salts in which x ≥ 0.20 adopt the orthorhombic structure of TMAMnCl₃·2H₂O. The bands observed in the near ir region are adequately explained as d-d transitions of the Cu(II) ion in D_2h symmetry. Other polarized bands which occur in the visible region and are neither Mn(II) nor Cu(II) d-d transitions are discussed.     

X-ray photoelectron emission studies of mixed selenides AgGaSe2 and Ag9GaSe6

Auger and direct electron specta from crystalline AgGaSe₂ and Ag₉GaSe₆ have been studied with X-ray photoelectron spectroscopy. It is shown that the AgM₅N_4,5N_4,5 and M₄N_4,5N_4,5 Auger spectra are more sensitive to the chemical environment than the Ag 3d direct photoelectron spectra. Furthermore the Auger parameter as defined by Wagner is used in order to characterize the chemical state of these compounds. Last, the XPS spectra of the valence-band region are investigated and chalcogen s and p and noble-metal d bands are clearly identified. The electronic structure of these two selenides does not seem to be determined predominantly by the crystal structure. As a whole, the spectral features are discussed in connection with the character of the chemical bonding and the physical properties of these compounds.     

Tin-magnesium substitution in Ir3Sn7—structure and chemical bonding in MgxIr3Sn7-x (x=0-1.67)

Well-shaped single crystals of binary Ir₃Sn₇ were obtained from a tin flux (starting composition Ir:Sn=1:10). The magnesium based stannides Mg_xIr₃Sn_7-x (x=0.61-1.67) were synthesized from the elements in glassy carbon crucibles in a water-cooled sample chamber of a high-frequency furnace. The samples were characterized by X-ray diffraction on powders and single crystals. All compounds crystallize with the cubic Ir₃Ge₇ type structure (space group Imm, Z=4). In this structure type the p-block atoms occupy the Wyckoff positions 12d and 16f and form two interpenetrating frameworks consisting of cubes and square antiprisms. The transition metal atoms center the square antiprisms and are arranged in pairs. With increasing magnesium substitution the lattice parameter of Ir₃Sn₇ (935.3 pm) decreases from 934.7 pm (x=0.61) to 930.6 pm (x=1.67) and the Ir-Ir distances decrease from 294 pm (Ir₃Sn₇) to 290 pm (Mg_1.67Ir₃Sn_5.33). In the ternary compounds Mg substitutes Sn on both framework sites. However, the 12d site shows a substantially larger preference for Mg occupation. By performing first-principles calculations we investigated the bonding situation in Ir₃Sn₇ and its alteration upon Mg incorporation. For binary Ir₃Sn₇ there are considerable bonding interactions between Ir and Sn atoms (d-p bonding) and between neighboring Sn atoms on the site 16f (p-p bonding). Both types of interactions diminish when substituting Sn for Mg. This explains the different site preference of Mg in Mg_xIr₃Sn_7−x: Mg occupation of the site 12d retains covalent p-p framework bonding between 16f atoms in the ternary compounds.     

Ferroelectric perovskite-type barium copper niobate: BaCu1/3Nb2/3O3

A perovskite-type BaCu_1/3Nb_2/3O₃ was prepared by high temperature reaction using BaCO₃, CuO and Nb₂O₅. The X-ray powder diffraction pattern of this compound was indexed with the tetragonal cell with the lattice parameters of a=4.0464(4) and c=4.1807(4) Å (c/a=1.033). This compound had the tetragonal perovskite-type structure in which the B site was occupied statistically by Nb and Cu atoms. From high temperature X-ray powder diffraction patterns this compound had a phase transition from the tetragonal to cubic symmetry in the temperature range of 500-600 °C. The P-E and S-E hysteresis loops occurred at room temperature and the apparent maximum in the temperature dependence of the dielectric constant was observed at 520 °C. The temperature dependence of the inverse of magnetic susceptibility exhibited paramagnetic behavior.     

Electronic and structural properties of A Al2Se4 (A=Ag, Cu, Cd, Zn) chalcopyrite semiconductors

We have studied the structural and electronic properties of defect chalcopyrite semiconductors A Al₂Se₄ (A=Ag, Cu, Cd, Zn) using density functional theory (DFT) based first principle technique within tight binding linear muffin-tin orbital (TB-LMTO) method. Our calculated structural parameters such as lattice constants a and c, tetragonal distortion (η=c/2a) are in good agreement with experimental work. Anion displacement parameters, bond lengths and bulk modulus are also calculated. Our band structure calculation suggests that these compounds are direct band gap semiconductors having band gaps 2.40, 2.50, 2.46 and 2.82 eV for A Al₂Se₄ (A=Ag, Cu, Cd, Zn) respectively. Calculated band gaps are in good agreement with other experimental and theoretical works within LDA limitation. We have made a quantitative estimation of the effect of p-d hybridization and structural distortion on the electronic properties. The reduction in band gap due to p-d hybridization is 19.47%, 21.29%, 0% and 0.7% for A Al₂Se₄ (A=Ag, Cu, Cd, Zn) respectively. Increment of the band gap due to structural distortion is 11.62%, 2.45%, 2.92% and 9.30% in case of AgAl₂Se₄, CuAl₂Se₄, CdAl₂Se₄ and ZnAl₂Se₄ respectively. We have also discussed the bond nature of all four compounds.     

Magnetic properties of Ca-doped SrRuO3 from full-potential calculations

We have carried out accurate generalized-gradient-corrected fully-relativistic full-potential calculations for Sr_1−xCa_xRuO₃ (x=0, 0.25, 0.5, 0.75, and 1) in para-, ferro-, and A-, C-, and G-type antiferromagnetic configurations. We have performed electronic structure calculations for the experimentally observed orthorhombic structure as well as the hypothetical cubic structure. Our results are analyzed with the help of total, site-, spin-, and orbital-projected density of states. The total-energy studies show that CaRuO₃ stabilizes in the G-type antiferromagnetic state. The octahedral tilting owing to the relatively small radius of Ca²⁺ leads to weak hybridization between Ru 4d and O 2p. This weak hybridization along with exchange splitting causes a pseudogap-like feature close to the Fermi level, which should stabilize G-type antiferromagnetic ordering in CaRuO₃. However, powder neutron diffraction data on CaRuO₃ taken at 8 and do not show any magnetic peaks, implying that CaRuO₃ exhibits a spin-glass-like state with dominant short-range antiferromagnetic interaction. The calculated magnetic ground state of Sr_1−xCa_xRuO₃ is found to be consistent with the experimental findings. We have also calculated optical spectra as well as X-ray and ultra-violet photoemission spectra and Ru and O K-edge X-ray absorption spectra for G-type CaRuO₃ and found good agreement with available experimental spectra.     

Preparation and structure of (NbSe4)3.33 I. [I3Nb10Se40]

The structural determination of a novel linear chain compound (NbSe₄)_3.33 I is reported. The structure was solved by means of Patterson and Fourier syntheses and refined toR = 0.030, R_W = 0.040, for 566 independent reflexions [I ≥ 3σ (I)]. The structure is built up of infinite [NbSe₄] chains along the c axis (tetragonal symmetry). Iodine atoms are located within the channels between these chains. The iodine atoms do not show the same distribution within all the channels as is the case for related compounds such as (NbSe₄)₃I, (TaSe₄)₂I. This new compound undergoes a phase transition at 285 K associated with a change density wave (CDW) origin.     

HNbO3 and HTaO3: New cubic perovskites prepared from LiNbO3 and LiTaO3 via ion exchange

The synthesis of HNbO₃ and HTaO₃ from LiNbO₃ and LiTaO₃ via ion exchange in hot aqueous acid solutions is reported. This reaction is accompanied by a topotactic structural transformation from the rhombohedral LiNbO₃ structure to the cubic perovskite structure; cell constants are a = 3.822(1) Å for HNbO₃ and 3.810(2) Å for HTaO₃. These new compounds have been characterized by powder X-ray diffraction, thermogravimetric analysis, and solid-state NMR. They are electronic insulators and have low ionic conductivity. Evidence of partially proton-exchanged phases Li_1?xH_xMO₃ was also seen. The possible significance of this ion exchange reaction for devices using LiNbO₃ or LiTaO₃ is discussed.