Walter Jung

Synthese und Kristallstrukturen der Calcium-Iridiumsilicide Ca3Ir4Si4 und Ca2Ir2Si†

Die neuen Verbindungen Ca3Ir4Si4 und Ca2Ir2Si wurden durch Umsetzung der Elemente in verschweisten Tantalampullen bei 1200 °C dargestellt. Ihre Strukturen wurden mit Rontgen-Einkristalldaten bestimmt. Ca3Ir4Si4(kubisch, Raumgruppe I4¯3m, a = 7, 4171(2)A, Z = 2) kristallisiert im Na3Pt4Ge4-Typ. Ca2Ir2Si (monoklin, Raumgruppe C2/c, a = 9, 6567(5)A, b = 5, 8252(2)A, c = 7, 3019(4)A, β = 100, 212(2)°, Z = 4) bildet eine neue Struktur aus. Stark verzerrte SiIr4-Tetraeder (Ir-Si: 2, 381 und 2, 414A) bilden durch Kantenverknupfung Ketten, die uber kurze Ir-Ir-Kontakte (2, 640A) zu einem sperrigen Ir/Si-Gerust verbunden sind. Darin befindet sich ein dreidimensionaler Verband aus Calciumatomen (Ca-Ca: 3, 413 bis 3, 948A). Synthesis and Crystal Structures of the Calcium Iridium Silicides Ca3Ir4Si4 and Ca2Ir2Si The new compounds Ca3Ir4Si4 und Ca2Ir2Si were prepared by reaction of the elemental components in sealed tantalum ampoules at 1200 °C. Their structures were determined from X-ray single crystal data. Ca3Ir4Si4(cubic, space group I4¯3m, a = 7.4171(2)A, Z = 2) crystallizes with the Na3Pt4Ge4 type structure. For Ca2Ir2Si (monoclinic, space group C2/c, a = 9.6567(5)A, b = 5.8252(2)A, c = 7.3019(4)A, β = 100.212(2)°, Z = 4) a new structure was found. Chains of edge sharing, heavily distorted SiIr4-tetrahedra (Ir-Si: 2.381 and 2.414A) are connected via short Ir—Ir-contacts (2.640A) to form an open Ir/Si-framework accommodating a three-dimensional arrangement of calcium atoms (Ca—Ca: 3.413 - 3.948A).

X-Ray Photoemission Studies of the Ternary Intermetallic Compounds Li2MGa and LiMGa2 (M = Rh, Pd, Ir, Pt)

X-ray photoelectron and x-ray excited Auger spectra were measured for the intermetallic compounds LiMGa2 and Li2MGa (M = Rh, Pd, Ir, Pt). The valence band spectra exhibit characteristic differences in the location of the M d-band between group 9 elements (Rh, Ir) and group 10 elements (Pd, Pt) on one side and between LiMGa2 and Li2MGa on the other. The experimentally observed differences are in excellent agreement with results from band structure calculations. The combination of binding energy shifts with Auger kinetic energy shifts allowed a separation of initial and final state contributions. Core hole screening is very efficient in accordance with the metallic character of the investigated phases. The magnitude of the screening correlates with the theoretically predicted composition of the density of states at the Fermi level. Application of Wertheims electrostatic model allowed to estimate the charge distribution for LiRhGa2 and Li2RhGa. The sign of the charges agrees with expectations that result from the Extended Zintl Concept. The results show, how dangerous it is to draw conclusions on the chemistry of such systems from photoemission data alone. Rontgen-Photoemissions-Untersuchungen an den ternaren intermetallischen Verbindungen Li2MGa und LiMGa2 (M = Rh, Pd, Ir, Pt) Die intermetallischen Verbindungen LiMGa2 und Li2MGa (M = Rh, Pd, Ir, Pt) wurden mit hochenergetischer Photoelektronen- und mit rontgenangeregter Augerelektronenspektroskopie untersucht. Die Valenzbandspektren zeigen sowohl hinsichtlich der Elemente der Gruppen 9 (Rh, Ir) und 10 (Pd, Pt) als auch zwischen LiMGa2 und Li2MGa charakteristische Unterschiede in der Lage des d-Bandes von M. Die beobachteten Unterschiede sind in voller Ubereinstimmung mit den Ergebnissen von Bandstrukturrechnungen. Die Kombination von Bindungsenergieverschiebungen mit der Verschiebung von Auger-Linien ermoglicht eine Trennung von Anfangs- und Endzustandseffekten. Die Locher in den inneren Schalen werden in Ubereinstimmung mit dem metallischen Charakter der untersuchten Phasen stark abgeschirmt. Die Grose der Abschirmung entspricht der Zusammensetzung der Zustandsdichte am Ferminiveau. Fur die Verbindungen LiRhGa2 and Li2RhGa war es moglich, die Ladungsverteilung mit Hilfe des elektrostatischen Modells von Wertheim zu bestimmen. Die Vorzeichen der Ladungen stimmen mit den Erwartungen uberein, die sich aus dem erweiteren Zintl Konzept ergeben. Die Ergebnisse zeigen auch, wie gefahrlich es ist, Aussagen uber die Chemie derartiger Systeme alleine aus Photoemissionsdaten abzuleiten.

Das Kupfer(II)‐indium‐oxidphosphat CuInOPO4 mit α‐Fe2OPO4‐Struktur, dargestellt durch die Oxidation einer Cu/In/P‐Legierung im Sauerstoffstrom

Einkristalle von CuInOPO4 (orthorhombisch, Pnma, a = 774,5(1) pm, b = 661,0(1) pm, c = 730,6(1) pm) wurden durch Oxidation von Cu/In/P-Legierungen mit Sauerstoff dargestellt. Das grune Oxidphosphat kristallisiert isotyp zu α-Fe2OPO4. Uber trans-Kanten verknupfte CuO6-Oktaeder bilden isolierte, langs [010] verlaufende [CuO4/2O2]-Ketten, die durch in [100]-Richtung verlaufende [InO4O2/2]-Ketten aus trans-eckenverknupften InO6-Oktaedern und Phosphatgruppen untereinander verbunden werden. The Copper(II) Indium Oxide Phosphate CuInOPO4 with α-Fe2OPO4 Structure, prepared by Oxidation of a Cu/In/P Alloy with Oxygen Green single crystals of CuInOPO4 (orthorhombic, Pnma, a = 774.5(1) pm, b = 661.0(1) pm, c = 730.6(1) pm, Z = 4) were prepared by reaction of Cu/In/P-alloys with oxygen. The compound is isotypic with α-Fe2OPO4. The structure contains isolated [CuO4/2O2]-chains running along [010] formed by CuO6-octahedra which are connected by common edges. The chains are linked by phosphate groups and by [InO4O2/2]-chains along [100] consisting of trans vertex connected InO6-octahedra.

BaRh2Si9 – a new clathrate with a rhodium–silicon framework

The semiconducting compound BaRh2Si9 is a new kind of intermetallic clathrate. It was obtained by reacting BaSi, Rh and Si at 950 °C. The crystal structure (space group C2/c; Pearson symbol mC48, a = 6.2221(5) Å, b = 21.399(2) Å, c = 6.2272(5) Å, β = 90.306(7)°) displays a covalently bonded Rh-Si framework, in which four-connected Si atoms partly show unusually small bond angles. The Ba atoms are encapsulated in large polyhedral cages formed by 18 Si and 4 Rh atoms. The compound is a diamagnetic p-type semiconductor, which is in agreement with band structure calculations resulting in a band gap of 0.12 eV. Quantum chemical calculations reveal positively charged Ba atoms (Ba(+1.3)) and negatively charged Rh atoms (Rh(-1)). Si atoms with neighboring Rh atoms are positively charged, while those connected only with Si atoms are negatively charged.

Mg5Pd10Si16 und Mg5Pt10Si16, Magnesium-Platinmetall-Silicide mit Tetraedern und gekappten Tetraedern aus Silicium-Atomen / Mg5Pd10Si16 and Mg5Pt10Si16, Magnesium Platinum Metal Silicides with Si4 Tetrahedra and Si12 Truncated Tetrahedra

The ternary silicides Mg5Pd10Si16 and Mg5Pt10Si16 have been prepared by reaction of magnesium with the binary platinum-metal silicides in sealed tantalum containers (Pt-compound: 1200°C, 3 d, up 20 °/h, down 5 °/h; Pd-compound: 1000°C, 2 d, up and down 50 °/h). In the case of the Pd-compound contact with the tantalum had to be avoided by using a boron nitride crucible. The isotypic compounds crystallize in the cubic space group F4̄3m with 4 formula units per unit cell. The crystal structures were determined from single crystal data, lattice constants from Guinier patterns. The following data were obtained: a = 1258.81(8) pm for Mg5Pd10Si16 and a = 1256.94(9) pm for Mg5Pt10Si16. Short distances in the three-dimensional platinum-metal silicon network indicate strong, covalent Pd(Pt)-Si-bonding (d(Pd-Si) = 240.2 to 256.1 pm; d(Pt-Si) = 237.1 to 258.5 pm). In addition, homonuclear bonding seems to be important, resulting in the formation of Si4-tetrahedra (d(Si-Si) = 250.4 pm (Mg5Pd10Si16) and 255 pm (Mg5Pt10Si16)), empty Si12-polyhedra with the shape of truncated tetrahedra (d(Si- Si): 234.5 and 248.2 pm (Mg5Pd10Si16); 236 and 248.2 pm (Mg5Pt10Si16)), and Mg-centered Pd(Pt)10-clusters with the shape of adamantane (d(Pd-Pd) = 282.3 pm; d(Pt-Pt) = 284.5 pm). Furthermore, Mg4-tetrahedra with Mg-Mg-distances of 360 pm are formed. The structure may be described by an expanded cubic “close” packing of MgPd(Pt)10-units in which the Si4-tetrahedra occupy the octahedral holes while the Si12-polyhedra and the Mg4-tetrahedra reside in one half of the tetrahedral holes each.