Professor Herbert Jacobs zum 65. Geburtstag gewidmet Inhaltsu È bersicht. Drei neue quaterna È re Selenogermanate wurden durch Erhitzen der Elemente auf 973±1073 K synthetisiert und ihre Strukturen mit Einkristallmethoden bestimmt. Die dunkelroten Halbleiter kristallisieren in azentrischen Raumgruppen. SrCu 2 GeSe 4 (Ama2, a = 10,807(4) A Ê , b = 10,735(4) A Ê , c = 6,541(2) A Ê , Z = 4) bildet einen neuen Strukturtyp, BaCu 2 GeSe 4 (P3 1 , a = 6,490(1) A Ê , c = 16,355(3) A Ê , Z = 3) und BaAg 2 GeSe 4 (I222, a = 7,058(1) A Ê , b = 7,263(1) A Ê , c = 8,253(2) A Ê , Z = 2) kristallisieren isotyp zu analogen Thiostannaten. Wesentliche Strukturmerkmale sind fast regula È re GeSe 4 -und verzerrte CuSe 4 -bzw. AgSe 4 -Tetraeder, die zu dreidimensionalen Geru È sten verknu È pft sind. In den Lu È cken liegen die achtfach von Selen koordinierten Erdalkalimetallatome. Die elektronischen Strukturen und Bindungsverha È ltnisse wurden mit selbstkonsistenten LMTO-Bandstrukturrechnungen und der COHP-Methode untersucht. Elektronendichteverteilungen und die Elektronenlokalisierungsfunktion ELF zeigen, dass die Cu±Se Bindungen sta È rker ionisch, die Ge±Se Bindungen dagegen kovalent zu formulieren sind. Damit sind die GeSe 4 -Tetraeder formal als quasi molekulare Bausteine oder Pseudoteilchen anzusehen. Die Anordnungen der GeSe 4 -Einheiten entsprechen in allen Strukturen den Motiven dichtester Kugelpakkungen. In den Tetraeder-und Oktaederlu È cken dieser Tetraederpackungen liegen die Metallatome, so dass sich die quaterna È ren Selenogermanate strukturchemisch als quasiterna È re Varianten von Li 3 Bi und Ni 2 In interpretieren lassen.New Noncentrosymmetric Selenogermanates. I. Crystal Structures and Chemical Bonding of AM 2 GeSe 4 (A = Sr, Ba; M = Cu, Ag) Abstract. Three new quaternary selenogermanates were synthesized by heating the elements at 983±1073 K. Their crystal structures were determined by single crystal X-ray methods. The dark red semiconductors crystallize in noncentrosymmetric space groups. SrCu 2 GeSe 4 (Ama2, a = 10.807(4) A Ê , b = 10.735(4) A Ê , c = 6.541(2) A Ê , Z = 4) forms a new structure type, whereas BaCu 2 GeSe 4 (P3 1 , a = 6.490(1) A Ê , c = 16.355(3) A Ê , Z = 3) and BaAg 2 GeSe 4 (I222, a = 7.058(1) A Ê , b = 7.263(1) A Ê , c = 8.253(2) A Ê , Z = 2) crystallize in structures known from thiostannates. Main structural features are almost regular GeSe 4 -, but distorted CuSe 4 -or AgSe 4 -tetrahedra sharing corners or edges. Eight selenium atoms coordinate the alkaline earth atoms in the voids of these three dimensional tetrahedral networks. Chemical bond-ing and the electronic structure are elucidated by self-consistent band structure calculations and the COHP method. The electron density and the electron localization function ELF of SrCu 2 GeSe 4 reveal a significant stronger covalent character for the Ge±Se bonds compared with the Cu±Se bonds. For this reason the GeSe 4 tetrahedra appear as quasi molecular entities, arranged spatially according to the motifs of closest packing. The metal atoms oc...
Eu2SnS4, Sr2SnS4, and Sr2GeSe4 have been synthesized by heating the elements or binary precursors at 1073 K and their crystal structures were determined by single crystal methods. Eu2SnS4 crystallizes with a new structure type (Pnma, a = 11.187(2), b = 8.768(2), c = 7.538(2)Å, Z = 4), consisting of distorted [SnS4]4— tetrahedra and sevenfold coordinated Eu2+ ions. Sr2SnS4 and γ‐Sr2GeSe4 are isostructural and form a new structure type likewise (Ama2, Z = 4, Sr2SnS4: a = 9.977(1), b = 10.311(2), c = 7.243(1)Å, Sr2GeSe4: a = 10.284(2), b = 10.543(2), c = 7.411(1)Å) with more regular tetrahedral anions and strontium coordinated by seven and eight chalcogen atoms, respectively. Eu2SnS4 is a Curie‐Weiss paramagnet (7.80(2) μB/Eu; θ = 4.2(2) K) and orders antiferromagnetically at TN = 5.5(2) K with a magnetization of 6.56(5) μB/Eu at 5.5 T. The divalent nature of europium is also evident from 151Eu Mössbauer spectra which show a single signal at an isomer shift of —12.2(1) mm/s at 78 K. Full magnetic hyperfine field splitting (19.5(2) T at the europium nuclei) is observed at 4.2 K. The 119Sn spectrum shows one signal at 1.25(9) mm/s subject to quadrupole splitting of 0.76(2) mm/s. At 4.2 K a transferred hyperfine field of 3(1) T is detected at the tin site.
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.
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.
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