CaFe2Ge2 with square-planar iron layers – Closing a gap in the row of CaT2Ge2 (T = Mn–Zn)

Abstract

The intermetallic compound CaFe2Ge2 has been synthesized by ball-milling a stoichiometric mixture of pre-melted “FeGe” alloy with elemental Ca and subsequent annealing in welded niobium ampoule in a resistance furnace. Its crystal structure has been investigated by powder X-ray diffraction method: ThCr2Si2 structure type (space group I4/mmm, Z = 2), a = b = 3.99366(4) Å, c = 10.7060(1) Å. Like iron-based superconductors, the main structural motif in CaFe2Ge2 is a square-planar layer of Fe atoms that are tetrahedrally coordinated by Ge atoms. The (Fe2Ge2) layers are alternating along the c-axis with the layers of Ca2+ cations and are interconnected via Ge–Ge bonds, resulting in a three-dimensional (Fe2Ge2)2– polyanionic framework. The topological analysis of the Electron Localization Function (ELF) confirms 3D character of (Fe2Ge2) layers similar to reported 122 iron tetrelides YFe2Ge2 and CaFe2Si2. CaFe2Ge2 fills the gap in the row of CaT2Ge2 (T = Mn–Zn), being a fully-substituted variant (Ge/As) of the well-known superconducting collapsed tetragonal CaFe2As2. Magnetic measurements revealed the occurrence of ferromagnetic order in CaFe2Ge2 below 50 K.