Inkommensurabel modulierte Kristallstrukturen und Phasenumwandlungen – Die Verbindungen SrPt₂As₂ und EuPt₂As₂

Abstract

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

Coexistence of Superconductivity and Charge Density Wave in SrPt2As2

“…23) In accordance with this prediction, SrPt 2 As 2 exhibits a CDW transition at about 470 K. Imre et al reported that a structural modulation develops with a modulation vector of q ¼ 0:62a à in the -As-Pt-As-layers with PtAs 4 tetrahedra below about 470 K, whereas the -Pt-As-Pt-layers with Pt 4 As tetrahedra remain intact. This modulation leads to a structural distortion from the high-temperature tetragonal phase with the CaBe 2 Ge 2 -type structure to the low-temperature CDW phase with the average structure of the orthorhombic space group Pmmn (#59).…”

“…SrPt 2 As 2 crystallizes in a tetragonal CaBe 2 Ge 2 -type structure with the space group P4=nmm (#129), 23) as shown in Fig. 1(a).…”

“…22) Our observation suggests that Fermi-surface nesting is a key ingredient of the relatively high superconducting transition temperature of SrPt 2 As 2 . SrPt 2 As 2 crystallizes in a tetragonal CaBe 2 Ge 2 -type structure with the space group P4=nmm (#129), 23) as shown in Fig. 1(a).…”

Superconductivity up to 35 K in the Iron Platinum Arsenides (CaFe1−xPtxAs)10Pt4−yAs8 with Layered Structures

“…We considered the fact that a second transition metal should be one that can adopt coordination geometries other than tetrahedral. Platinum appeared promising, because Pt is known to be very flexible and forms arsenides with octahedral, tetrahedral, trigonal and square coordination in compounds like PtAs 2 , [10] SrPt 2 As 2 , [11] SrPtAs, [12] and Cs 2 PtAs 2 , [13] respectively. Recently, Nohara et al [14] mentioned superconductivity in the system Ca-Fe-Pt-As, but the structure and composition of the compound have not been reported.…”

Lead-flux Growth of Eu4Ir8As7 Crystals

“…When it comes to the iridium arsenides, only the phases REIr 2 As 2 (RE = La -Nd [21], K, Rb [11], Cs [10], Sr [23]), Mg 4 Ir 7 As 6 [24], and Ca 4 Ir 8 As 7 [13] are known. In continuation of our work on rare earth metal-iridium-phosphides, we tested the leadflux technique also for the arsenides.…”