Superconductivity and structural variation of the electron-correlated layer systems Sr(Pd _1-xT _x) ₂Ge ₂ (T = Co, Ni, Rh; 0≤x≤1)

Superconductivity variations deduced from the x-ray diffraction and the magnetic and heat-capacity measurements in the pseudoternary Sr(Pd _1-xT _x) ₂Ge ₂ layer system [Pd(4d8), T= Co(3d7), Ni(3d8), or Rh(4d7); 0≤x≤1] are reported. For the BaFe ₂As ₂-type tetragonal structure, the degenerate nd7 or nd8 orbitals of transition metal T are split by c-axis squeezed TGe ₄ tetrahedral crystal field in the T-Ge layer. For the isoelectronic Sr(Pd _1-xNi _x) ₂Ge ₂ system, the superconducting transition temperature T _c decreases monotonically from 3.12 K for 4d-band SrPd ₂Ge ₂ to 0.92 K for 3d-band SrNi ₂Ge ₂, where major contributions of conduction electrons are from the half filled dispersive three-dimensional (3D)-like upper-lying nd _xz,yz bands. For the Sr(Pd _1-xRh _x) ₂Ge ₂ system, T _c decreases to 2.40K with 25% of 4d7 Rh substitution. For the Sr(Pd _1-xCo _x) ₂Ge ₂ system, T _c decreases sharply to 2.58 K with only 3% of 3d7 Co substitution. No superconductivity is expected for SrRh ₂Ge ₂ and SrCo ₂Ge ₂ with lower density of states in d _xz,yz bands due to down shift of Fermi energy E _F by one less electron per transition metal. The lower T _c of the present electron-overdoped (nd7 or nd8) compound is due to dispersive 3D-like nd _xz,yz conduction bands with weak electron correlation, in comparison with the less-electron-doped (3d6.1) 22-K superconductor BaFe _1.8Co _0.2As ₂ or the hole-doped (3d5.9) 38-K superconductor Ba _0.6K _0.4Fe ₂As ₂ where electron contribution is from less dispersive 2D-like lower-lying 3d _xy conduction band with stronger electron correlation.