On the Electronic Structure of the New Intermetallics LnNi2B2C

The electronic structure of the new series of intermetallic compounds LnNiZBzC (Ln = lanthanide element) is calculated and discussed in terms of local chemical bonding in order to elucidate how superconductivity may be possible for some of these materials. The Fermi level occurs in a region that allows for a second-order Jahn-Teller mixing and can contribute to significant electron-phonon coupling in these systems. Moreover, the trend in the a lattice parameter for different Ln cations shifts the position of the x2-y2 band, which significantly changes its occupation and affects the superconducting transition temperature. We also compare the electronic structures of LnNizBzC with those from the related compounds LnNiBC

Ternary Metal-Rich Phosphides: Structure, Bonding, and Site Preferences in ZrNbP and Hf1+xMo1-xP

Semiempirical electronic structure calculations are utilized to assess the bonding and metal atom arrangement in the recently discovered ternary phosphide ZrNbP, which adopts the Co2Si structure type. These same calculations reveal that ZrMoP should form in the Fe2P structure type due primarily to metal-metal interactions within each system. Related structural alternatives like the Cu2Sb-type and the Ni2In-type are also examined for their stability ranges as a function of valence electron concentration (vec). Synthesis and structural characterization of Hf1.MM00.94P by single-crystal X-ray diffraction are also reported and confirm the prediction of stability of the Fe2P structure type for this vec. Hf1.06Mo0.9494P crystallizes in the space group P62

Hexamolybdenum Octatelluride, Mo6Te8

The title compound, isotypic with Mo6Ses, contains Mo6Te8 clusters. All eight faces of a distorted octahedral Mo cluster [Mo---Mo 2.699 (2), 2.785 (2) ,~,] are capped by Te atoms, six of which also provide bridges to neighboring clusters

Dimensional diversity in transition metal trihalides

Structural variations of the second- and third-row transition metal trihalides are rationalized via tight-binding band calculations and evaluation of Madelung energetic factors. The observed structure for a given metal halide is controlled by both the coordination geometry at the anion and the d electron configuration at the metal. As the polarizability of the halide increases, the M-X-M angle, in general, decreases so that three-dimensional frameworks occur for the fluorides, while layer and chain structures are found for the chlorides, bromides, and iodides. Within a particular halide system, systematic structural trends also occur as the d electron configuration changes

Synthesis and Structure of Ta4SI11: Disorder and Mixed Valency in the First Tantalum Sulfide Iodide

The new compound Ta4SI11 has been prepared by direct reaction of the elements at 430 °C for 2 weeks in evacuated Pyrex ampules and characterized by single-crystal X-ray diffraction, X-ray photoelectron spectroscopy, magnetic susceptibility measurements, and semiempirical electronic structure calculations. Ta4SI11 crystallizes with orthorhombic symmetry in space group Pmmn; a = 16.135(3) Å, b = 3.813(1) Å, c = 8.131(2) Å, and Z = 1. The disordered structure involves two crystallographically distinct sites for Ta atoms, both of which are 50% occupied as well as a bridging anion site that is 50% S and 50% I. Magnetic susceptibility above 100 K gives μeff = 1.53 μB to suggest one unpaired electron per formula unit. X-ray photoelectron spectroscopy and extended Hückel calculations suggest that the structure consists of Ta3 triangles and “isolated” Ta atoms, leading to the formulation (Ta3)9+(Ta4+)(S2-)(I-)11 and we hypothesize that each Ta3 is capped by a sulfur atom

New Co–Pd–Zn γ-Brasses with Dilute Ferrimagnetism and Co2Zn11 Revisited: Establishing the Synergism between Theory and Experiment

A synergism between electronic structure theory and the targeted synthesis of new ternary γ-brass compounds is demonstrated in the Co–Zn system. Co2Zn11, which adopts a cubic γ-brass structure, is shown to be at the Zn-rich end of a homogeneity range that varies from 15.4 to 22.1 atom % Co. Four samples were examined by single-crystal diffraction, all of which crystallize in space group I4̅3m with the lattice parameter ranging from 8.9851(1) to 8.8809(1) Å as the Co content increases. In the 26-atom γ-brass clusters, Co atoms preferentially occupy the outer tetrahedron (OT) sites and then replace Zn atoms at the octahedron (OH) sites at higher Co concentrations. In addition, a small fraction of vacancies occurs on the inner tetrahedron (IT) sites. The electronic structure of Co2Zn11 shows two distinct pseudogaps near the Fermi level: one at 292 valence electrons per primitive unit cell and the other at 302–304 valence electrons per primitive unit cell. Using molecular orbital arguments applied to the body-centered cubic packing of the 26-atom Co4Zn22 γ-brass cluster, these pseudogaps arise from (i) splitting among the valence s and p orbitals, which gives rise to the Hume–Rothery electron counting rule, and (ii) splitting within the manifold of Co 3d orbitals via Co–Zn orbital interactions. Co2Zn11 is Pauli paramagnetic, although the density of states at the Fermi level is large, whereas Curie–Weiss behavior emerges for higher Co concentrations. Because Pd has a size and an electronegativity similar to those of Zn, and inspired by the pseudogaps in the electronic density of states curve of Co2Zn11, Pd-doped γ-brass compounds were designed and two new γ-brass compounds were obtained: Co0.92(2)Pd1.08Zn11 and Co2.50(1)Pd2.50Zn8. In these, the site preferences for Co and Pd can be rationalized by electronic structure calculations. The densities of states indicate that Co 3d states are the major contributors near their Fermi levels, with the Pd 4d band lying ∼2–3 eV below this. The magnetic properties of the Co–Pd–Zn γ-brasses are quite different from those of Co2Zn11: a giant magnetic moment on the Co atom is induced by the Pd atom, and Co2.50(1)Pd2.50Zn8 shows magnetization consistent with a dilute ferrimagnet. The results of first-principles calculations on two different models of the 26-atom γ-brass clusters indicate that intracluster Co–Co exchange is ferromagnetic, whereas intercluster Co–Co exchange is antiferromagnetic. These different magnetic exchange interactions provide rationalization for the high-temperature magnetization behavior of Co2.50(1)Pd2.50Zn8

Safety in the Chemistry Curriculum at Iowa State University

At Iowa State University we have developed and are continuing to design programs into our undergraduate and graduate curricula that will establish a sound practice of proper chemical hygiene in the laboratory. Our efforts include: (1) for all entering graduate students, an introductory graduate course to teach proper laboratory procedures and handling of chemicals; (2) for advanced graduate students, a chemistry safety committee which interacts with our university\u27s EH&S department to learn about EPA and OSHA requirements so as to maintain a high level of safety awareness throughout our research labs; and (3) for our undergraduate introductory chemistry students, supplementary readings and prelaboratory quizzes designed with chemical safety in mind, e.g., waste handling procedures, first aid, MSDS\u27s, and the use of safety equipment. The fundamental goals of our approach are not only to teach students appropriate safety measures, but also to involve them in the enforcement of basic prudent practices in the laboratory

Rhombohedral Distortion of the Cubic MgCu2-Type Structure in Ca2Pt3Ga and Ca2Pd3Ga

Two new fully ordered ternary Laves phase compounds, Ca2Pt3Ga and Ca2Pd3Ga, have been synthesized and characterized by powder and single-crystal X-ray diffraction along with electronic structure calculations. Ca2Pd3Ga was synthesized as a pure phase whereas Ca2Pt3Ga was found as a diphasic product with Ca2Pt2Ga. Electronic structure calculations were performed to try and understand why CaPt2 and CaPd2, which crystalize in the cubic MgCu2-type Laves phase structure, distort to the ordered rhombohedral variant, first observed in the magneto-restricted TbFe2 compound, with the substitution of twenty-five percent of the Pt/Pd with Ga. Electronic stability was investigated by changing the valence electron count from 22e−/f.u. in CaPd2 and CaPt2 (2x) to 37e−/f.u. in Ca2Pd3Ga and Ca2Pt3Ga, which causes the Fermi level to shift to a more energetically favorable location in the DOS. The coloring problem was studied by placing a single Ga atom in each of four tetrahedra of the cubic unit cell of the MgCu2-type structure, with nine symmetrically inequivalent models being investigated. Non-optimized and optimized total energy analyses of structural characteristics, along with electronic properties, will be discussed