The new compounds SrPt 2 As 2 (a ϭ 4.482(1), b ϭ 4.525(1), c ϭ 9.869(2) Å ) and EuPt 2 As 2 (a ϭ 4.435(2), b ϭ 4.467(2), c ϭ 9.784(3) Å ) have been prepared by reaction of the elements at 1050°C. Satellite reflections observed in X-ray diffraction experiments on single crystals revealed that both compounds have modulated crystal structures at room temperature. The average structures can be derived from the CaBe 2 Ge 2 type and described in the orthorhombic space group Pmmn (Z ϭ 2) with splitted positions for half the Pt and As atoms. Based on the average structure of SrPt 2 As 2 , the modulated structure has been solved and refined using the (3ϩ1)-dimensional superspace group Pmmn(Ͱ00)0s0 and a modulation vector q ϭ 0,62 · a * . The modulation appears in the layers of the PtAs 4 tetrahedra with occupational and displacive components. The other atoms are also slightly affected by the modulation. With increasing pressure, SrPt 2 As 2 undergoes a phase transition at about 60 kbar. The crystal structure of the high pressure phase (HP-SrPt 2 As 2 : a ϭ 9.752(2), b ϭ 4.398(1), c ϭ 8.727(2) Å , β ϭ 90.34(3)°; 66(5) kbar) is a monoclinic variant (P2 1 /c; Z ϭ 4) of the Einleitung Von den möglichen ternären Erdalkalimetall-und Europiumphosphiden und -arseniden der Platinmetalle mit 1:2:2-Stöchiometrie (AM 2 X 2 ) sind bereits eine ganze Reihe strukturell charakterisiert und zum Teil auf ihre physikalischen Eigenschaften hin näher untersucht worden [1]. Sie kristallisieren in der Mehrzahl im ThCr 2 Si 2 -Typ, der von Schichten kantenverknüpfter MX 4 -Tetraeder geprägt wird, zwischen denen sich die Erdalkalimetall-bzw. Europiumatome befinden (M: Platinmetall; X: P, As). Die bevorzugte Bildung dieses Strukturtyps ist auf seine Fähigkeit zurückzuführen, sich unterschiedlichen Atomgrößen nahezu perfekt anzupassen und das Gleichgewicht zwischen kovalenten
ARh 2 As 2 (A: Eu, Sr, Ba) and BaZn 2 As 2 . -Single crystals of the title compounds are grown from the elements in a Pb or Pb/Bi flux at 1100 °C. EuRh2As2 and BaRh2As2 crystallize with the well-known ThCr2Si2 type structure (tetragonal, space group I4/mmm, Z = 2). Due to the rigid layers of RhAs4 tetrahedra and to the atomic size of Eu and Ba, the As-As distances between the layers are very long (2.97 and 3.66 Å, resp.). SrRh 2 As 2 is polymorphic and undergoes two phase transitions at about 190 and 282 °C. Main features of the three crystal structures are also layers of RhAs4 tetrahedra. At room temperature α-SrRh2As2 probably crystallizes with the BaNi2Si2-type (space group Cmcm, Z = 4), while β-SrRh2As2 (space group Fmmm, Z = 4) forms a new orthorhombically distorted variant of the ThCr2Si2-type. γ-SrRh2As2 crystallizes with the ThCr 2 Si 2 type structure. The same is true for the high temperature modification of BaZn2As2 (β-phase), whereas the already known α-BaZn2As2 phase exhibits the α-BaCu 2 S 2 type structure (space group Pnma, Z = 4) consisting of a three-dimensional-network of edge-and corner-sharing ZnAs4 tetrahedra wit Ba atoms in the voids of this network. -(HELLMANN, A.; LOEHKEN, A.; WURTH, A.; MEWIS*, A.; Z. Naturforsch., B: Chem. Sci. 62 (2007) 2, 155-161; Inst. Anorg. Chem. Strukturchem., Heinrich-Heine-Univ., D-40225 Duesseldorf, Germany; Ger., Abstr. Eng.) -Schramke 18-007
Abb. 1 Mg 2 Co 12 As 7 : Koordination der Atome (Mg: groûe, Co: mittlere, As: kleine Kreise; schwarze Kreise: y = 1/4; weiûe Kreise: y = 3/4; die Nummerierung der Atome entspricht Tab. 2) Abb. 2 Mg 2 Co 12 As 7 : Verknu È pfung der Koordinationspolyeder (Mg: groûe, Co: schwarze, As: kleine Kugeln)Abb. 3 Mg 2 Co 12 As 7 : [010]-Projektion (Mg: groûe, Co: mittlere, As: kleine Kreise; schwarze und weiûe Kreise sind um b/2 gegeneinander verschoben)
Four new arsenides of rhodium and zinc were prepared by heating mixtures of the elements at high temperatures (1000 -1200 • C) and investigated by single crystal X-ray methods. EuRh 2 As 2 (a = 4.067(1), c = 11.319(2)Å) and BaRh 2 As 2 (a = 4.053(1), c = 12.770(3)Å) crystallize with the well-known ThCr 2 Si 2 -type (I4/mmm; Z = 2). Due to the rigid layers of RhAs 4 tetrahedra, and to the atomic size of europium and barium, the As-As distances between the layers with values of 2.97 and 3.66Å, respectively, are very long. SrRh 2 As 2 is polymorphic and undergoes two phase transitions at about 190 and 282 • C. Main features of the three crystal structures are also layers of RhAs 4 tetrahedra. At room temperature α-SrRh 2 As 2 (a = 5.676(1), b = 6.178(2), c = 11.052(2)Å) probably crystallizes with the BaNi 2 Si 2 -type (Cmcm; Z = 4), whereas β -SrRh 2 As 2 (a = 5.760(3), b = 6.067(4), c = 11.264(5)Å, Fmmm, Z = 4) forms a new orthorhombically distorted variant of the ThCr 2 Si 2 -type. Single crystals grown in a flux of lead and quenched at high temperature show that the γ-phase (a = 4.112(1), c = 11.431(6)Å) crystallizes with the ThCr 2 Si 2 -type. The same is true for the high temperature modification of BaZn 2 As 2 (β -phase; a = 4.120(1), c = 13.578(1)Å), whereas the already known α-BaZn 2 As 2 forms the α-BaCu 2 S 2 -type (Pnma; Z = 4) consisting of a 3D-network of edge-and vertex-sharing ZnAs 4 tetrahedra with Ba atoms in the voids of this network.
Six new germanides Ln 3 Pt 4 Ge 6 with Ln ϭ PrϪDy were synthesized by heating mixtures of the elements at 900°C, annealing the inhomogeneous powders at 1050-1100°C for six days and then cooling down from 700°C in the course of two months.
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