Synthesis and structure of SrCo2Si2 and CaRh2Si2 - isoelectronic   variants of the parent superconductors AeFe2As2 and study of the influence of   the valence electron count in CaFe2-xRhxSi2
  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 melting mixtures of the elements in an arc-furnace under an argon atmosphere. Single crystals were obtained by special heat treatment in welded niobium 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) {\AA}, c = 10.185(1) {\AA}, R1 = 0.045, 85 F2 values, 8 variable parameters for CaFe2Si2 ; a = 4.0612(3) {\AA}, c = 9.9373(9) {\AA}, R1 = 0.030, 90 F2 values, 10 variable parameters for CaFe0.68(6)Rh1.32(6)Si2; a = 4.0695(1) {\AA}, c = 9.9841(3) {\AA}, R1 = 0.031, 114 F2 values, 8 variable parameters for CaRh2Si2; a = 3.974(1) {\AA}, c = 10.395(1) {\AA}, 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, Rh) are interconnected along the c axis via Si Si bonds resulting in a three-dimentional (3D) [T2Si2]2- 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, 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). 
Andrea V. Hoffmann, Viktor Hlukhyy, Thomas F. F\"assler
cond-mat.supr-con