Abstract
Superconductivity were observed in the new 122-layer compounds CaIr2Ge2 (Tc = 5.4 K) and CaPd2Ge2 (Tc = 2.5 K) with the BaFe2As2-type body-centered-tetragonal structure (bct, space group I4/mmm). For the pseudoternary Ca(Ir1-xPtx)2Ge2 system, superconducting transition Tc decreases from 5.4 K for CaIr2Ge2 (x = 0), to 3.8 K for x = 0.1, 3.0 K for x = 0.2, 2.2 K for x = 0.5, and below 2 K for x ﹥0.5. In addition to bct phase, x-ray powder differation pattern show the appearance of non-superconducting monoclinic 122-phase (space group P21, a 90o). No Tc above 2 K was observed for the monoclinic compound CaPt2Ge2. Higher Tc in the bct CaIr2Ge2 is due to a strong quasi-2D Ir-Ge-Ir 5dxz,yz-4p-5dxz,yz hybridization in the Ir-Ge layer with the squeezed-along-c-axis IrGe4 tetragonal crystal field and the Ir-5d7 spin-orbital interaction. For the orthorhombic (11) layer system (Ir1-xPtx)Ge (space group Pnma), Tc decreases from 4.8 K for IrGe (x = 0), to 3.6 K for x = 0.1, 2.3 K for x = 0.2 and below 2 K for x ≧ 0.3, with low Tc of 0.4 K reported for PtGe.
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