Synthesis, structure and chemical bonding of CaFe2−xRhxSi2 (x=0, 1.32, and 2) and SrCo2Si2

Abstract

The finding of superconductivity in Ba0.6K0.4Fe2As2 put the attention on the investigation of compounds that crystallize with ThCr2Si2 structure type such as AT2X2 (A=alkali/alkaline earth/rare earth element; T=transition metal and X=element of the 13–15th group). In this context the silicides CaFe2Si2, CaFe0.68(6)Rh1.32(6)Si2, CaRh2Si2 and SrCo2Si2 have been synthesized by reaction of the elements under an argon atmosphere. Single crystals were obtained by special heat treatment in welded niobium/tantalum ampoules. The compounds were investigated by means of powder and single crystal X-ray diffraction. All compounds crystallize in the ThCr2Si2-type structure with space group I4/mmm (No. 139): a=3.939(1) Å, c=10.185(1) Å, R1=0.045, 85 F2 values, 8 variable parameters for CaFe2Si2; a=4.0590(2) Å, c=9.9390(8) Å, R1=0.030, 90 F2 values, 10 variable parameters for CaFe0.68(6)Rh1.32(6)Si2; a=4.0695(1) Å, c=9.9841(3) Å, R1=0.031, 114 F2 values, 9 variable parameters for CaRh2Si2; and a=3.974(1) Å, c=10.395(1) Å, R1=0.036, 95 F2 values, 8 variable parameters for SrCo2Si2. The structure of SrCo2Si2 contains isolated [Co2Si2]2− 2D-layers in the ab-plane whereas in CaFe2−xRhxSi2 the [T2Si2] layers (T=Fe and Rh) are interconnected along the c-axis via Si3Si bonds resulting in a three-dimentional (3D) [T2Si2]2− polyanions and therefore belong to the so-called collapsed form of the ThCr2Si2-type structure. The SrCo2Si2 and CaRh2Si2 are isoelectronic to the parent 122 iron–pnictide superconductors AeFe2As2 (Ae=alkaline earth elements), whereas CaFe2Si2 is a full substituted variant (As/Si) of CaFe2As2. The crystal chemistry and chemical bonding in the title compounds are discussed in terms of LMTO band structure calculations and a topological analysis using the Electron Localization Function (ELF).