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

TL;DR

This study synthesizes and characterizes SrCo2Si2 and CaRh2Si2, isoelectronic variants of parent superconductors, analyzing how valence electron count influences their structure and bonding, with implications for understanding related superconducting materials.

Contribution

It reports the synthesis, structural analysis, and electronic structure investigation of SrCo2Si2 and CaRh2Si2, highlighting their relation to iron-based superconductors and the effect of electron count.

Findings

All compounds crystallize in ThCr2Si2-type structure.

SrCo2Si2 has isolated [Co2Si2]2- layers, CaRh2Si2 has interconnected layers.

Electronic structure analyzed via LMTO and ELF topological analysis.

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 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).