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The finding of superconductivity in Ba0.6K0.4Fe2As2 put the attention on theinvestigation of compounds that crystallize with ThCr2Si2 structure type suchas AT2X2 (A = alkali/ alkaline earth/ rare earth element; T = transition metaland 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 meltingmixtures of the elements in an arc-furnace under an argon atmosphere. Singlecrystals were obtained by special heat treatment in welded niobium ampoules.The compounds were investigated by means of powder and single crystal X-raydiffraction. All compounds crystallize in the ThCr2Si2-type structure withspace 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 forCaFe0.68(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 abplane whereas in CaFe2-xRhxSi2 the [T2Si2] layers (T = Fe, Rh) areinterconnected along the c axis via Si Si bonds resulting in athree-dimentional (3D) [T2Si2]2- and therefore belong to the so-calledcollapsed form of the ThCr2Si2-type structure. The SrCo2Si2 and CaRh2Si2 areisoelectronic to the parent 122 iron-pnictide superconductors AeFe2As2, whereasCaFe2Si2 is a full substituted variant (As/Si) of CaFe2As2. The crystalchemistry and chemical bonding in the title compounds are discussed in terms ofLMTO band structure calculations and a topological analysis using the ElectronLocalization Function (ELF).

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