Markus Tampier

Kristallstrukturen und chemische Bindung von AM2GeSe4 (A = Sr, Ba; M = Cu, Ag)

Drei neue quaternare Selenogermanate wurden durch Erhitzen der Elemente auf 973–1073 K synthetisiert und ihre Strukturen mit Einkristallmethoden bestimmt. Die dunkelroten Halbleiter kristallisieren in azentrischen Raumgruppen. SrCu2GeSe4 (Ama2, a = 10,807(4) A, b = 10,735(4) A, c = 6,541(2) A, Z = 4) bildet einen neuen Strukturtyp, BaCu2GeSe4 (P31, a = 6,490(1) A, c = 16,355(3) A, Z = 3) und BaAg2GeSe4 (I222, a = 7,058(1) A, b = 7,263(1) A, c = 8,253(2) A, Z = 2) kristallisieren isotyp zu analogen Thiostannaten. Wesentliche Strukturmerkmale sind fast regulare GeSe4- und verzerrte CuSe4- bzw. AgSe4-Tetraeder, die zu dreidimensionalen Gerusten verknupft sind. In den Lucken liegen die achtfach von Selen koordinierten Erdalkalimetallatome. Die elektronischen Strukturen und Bindungsverhaltnisse wurden mit selbstkonsistenten LMTO-Bandstrukturrechnungen und der COHP-Methode untersucht. Elektronendichteverteilungen und die Elektronenlokalisierungsfunktion ELF zeigen, dass die Cu–Se Bindungen starker ionisch, die Ge–Se Bindungen dagegen kovalent zu formulieren sind. Damit sind die GeSe4-Tetraeder formal als quasi molekulare Bausteine oder Pseudoteilchen anzusehen. Die Anordnungen der GeSe4-Einheiten entsprechen in allen Strukturen den Motiven dichtester Kugelpackungen. In den Tetraeder- und Oktaederlucken dieser Tetraederpackungen liegen die Metallatome, so dass sich die quaternaren Selenogermanate strukturchemisch als quasiternare Varianten von Li3Bi und Ni2In interpretieren lassen.

Strontium Selenogermanate(III) and Barium Selenogermanate(II,IV): Synthesis, Crystal Structures, and Chemical Bonding

The new selenogermanates Sr2Ge2Se5 and Ba2Ge2Se5 were synthesized by heating stoichiometric mixtures of binary selenides and the corresponding elements to 750 degrees C. The crystal structures were determined by single-crystal X-ray methods. Both compounds adopt previously unknown structure types. Sr2Ge2Se5 (P2(1)/n, a = 8.445(2) A, b = 12.302 A, c = 9.179 A, beta = 93.75(3) degrees, Z = 4) contains [Ge4Se10]8- ions with homonuclear Ge-Ge bonds (dGe-Ge = 2.432 A), which may be described as two ethane-like Se3Ge-GeSeSe2/2 fragments sharing two selenium atoms. Ba2Ge2Se5 (Pnma, a = 12.594(3) A, b = 9.174(2) A, c = 9.160(2) A, Z = 4) contains [Ge2Se5]4- anions built up by two edge-sharing GeSe4 tetrahedra, in which one terminal Se atom is replaced by a lone pair from the divalent germanium atom. The alkaline earth cations are arranged between the complex anions, each coordinated by eight or nine selenium atoms. Ba2Ge2Se5 is a mixed-valence compound with GeII and GeIV coexisting within the same anion. Sr2Ge2Se5 contains exclusively GeIII. These compounds possess electronic formulations that correspond to (Sr2+)2(Ge3+)2(Se2-)5 and (Ba2+)2- Ge2+Ge4+(Se2-)5. Calculations of the electron localization function (ELF) reveal clearly both the lone pair on GeII in Ba2Ge2Se5 and the covalent Ge-Ge bond in Sr2Ge2Se5. Analysis of the ELF topologies shows that the GeIII-Se and GeIV-Se covalent bonds are almost identical, whereas the GeII-Se interactions are weaker and more ionic in character.

BaCu6Ge2S8 -Ein Thiogermanat als Variante der Li3Bi-Struktur / BaCu6Ge2S8 -A Thiogermanate as a Variant of the Li3Bi Structure

Abstract BaCu6Ge2 S8 was synthesized by direct reaction of the elements at 750°C and characterized by X-ray single crystal techniques. The thiogermanate crystallizes in a new orthorhombic structure type (a = 6.122(1) Å, b = 12.084(3) Å, c = 17.614(5) Å; Pbcm, Z = 4). Isolated [GeS4]4- tetrahedra form a slightly distorted cubic face-centered (fee) arrangement. Baand Cu-atoms each occupy half the octahedral gaps (OG) of this “tetrahedra packing” . Further Cu atoms fill the tetrahedral gaps ( TG) completely. The compound can be written as Ba2(1/2OG)(Cu2)2(1/2OG)Cu8(TG)(GeS4)4 . Thus the structure of BaCu6Ge2S8 can be derived from the Li3Bi type. The sulfur coordination of the metal atoms (Cu tetrahedral or trigonal, Ba with CN 8) are realized by the spatial orientation of the [GeS4]4- tetrahedra. Their centers nearly maintain the fcc-arrangement. This structure interpretation of BaCu6 Ge2S8 from the viewpoint of a “filled tetrahedra packing” is discussed for further known thiogermanate compounds.

Crystal Structures and Properties of Eu2GeSe4 and Eu2Ge2Se5

Europium selenogermanate(IV) and Europium selenogermanate(III) were synthesized by direct reactions of the elements at 1023 K and their crystal structures determined by single crystal methods. α-Eu2GeSe4 (P21, a = 6.964(1), b = 7.055(2), c = 8.400(2) Å , β = 108.12(2)˚, Z = 2) crystallizes as a polar variant of the monoclinic Sr2GeS4-type at room temperature. At 673 K, the structure is centrosymmetric (β-Eu2GeSe4, P21/m, a = 6.969(1), b = 7.059(2), c = 8.516(2)Å , β = 107.99(2)˚, Z = 2), and a phase transition P21→P21/m as known from Eu2GeS4 is highly probable. Eu2Ge2Se5 (P21/n, a = 8.421(4), b = 12.235(4), c = 9.127(3) Å,β = 93.67(4)˚, Z = 4) crystallizes in the Sr2Ge2Se5 type, mainly characterized by complex [Ge4Se10]8- anions with homonuclear Ge-Ge bonds. Both compounds are deep red Zintl phases according to (Eu2+)2Ge4+(Se2-)4 and (Eu2+)2(Ge3+)2(Se2-)5. Magnetic susceptibility measurements show paramagnetic behavior above 20Kwith magnetic moments of 8.00(5) μB/Eu and 8.10(5) μB/Eu, respectively, indicating Eu2+. Eu2Ge2Se5 orders antiferromagnetically at TN = 4.2(2) K and undergoes a metamagnetic transition at 2 K at a critical field of 0.5(3) T. The saturation moment at 2.2 K and 5.5 T is 7.00(2) μB/Eu. Eu2GeSe4 is a ferromagnet with TC = 5.8(2) K and a saturation moment at 2.2 K and 5.5 T of 6.92(2) μB/Eu. 151Eu Mössbauer spectroscopic measurements of Eu2GeSe4 and Eu2Ge2Se5 at 78 K show isomer shifts of -12.43(4) mm/s and -12.69(5) mm/s, respectively, in accordance with divalent europium.