Superconductivity and Structural Variation of the Electron-Correlated Layer Systems Sr(Pd1−xTx)2Ge…

Superconductivity variations deduced from the x-ray diffraction and the magnetic and heat-capacity measurements in the pseudoternary Sr(Pd${}_{1\ensuremath{-}x}{T}_{x}$)${}_{2}$Ge${}_{2}$ layer system [Pd($4{d}^{8}$), $T$ $=$ Co($3{d}^{7}$), Ni($3{d}^{8}$), or Rh($4{d}^{7}$); $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$] are reported. For the BaFe${}_{2}$As${}_{2}$-type tetragonal structure, the degenerate $n{d}^{7}$ or $n{d}^{8}$ orbitals of transition metal $T$ are split by $c$-axis squeezed $T$Ge${}_{4}$ tetrahedral crystal field in the $T$-Ge layer. For the isoelectronic Sr(Pd${}_{1\ensuremath{-}x}$Ni${}_{x}$)${}_{2}$Ge${}_{2}$ system, the superconducting transition temperature ${T}_{c}$ decreases monotonically from 3.12 K for $4d$-band SrPd${}_{2}$Ge${}_{2}$ to 0.92 K for $3d$-band SrNi${}_{2}$Ge${}_{2}$, where major contributions of conduction electrons are from the half filled dispersive three-dimensional (3D)-like upper-lying $n{d}_{xz,yz}$ bands. For the Sr(Pd${}_{1\ensuremath{-}x}$Rh${}_{x}$)${}_{2}$Ge${}_{2}$ system, ${T}_{c}$ decreases to 2.40 K with 25$%$ of $4{d}^{7}$ Rh substitution. For the Sr(Pd${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$Ge${}_{2}$ system, ${T}_{c}$ decreases sharply to 2.58 K with only 3$%$ of $3{d}^{7}$ Co substitution. No superconductivity is expected for SrRh${}_{2}$Ge${}_{2}$ and SrCo${}_{2}$Ge${}_{2}$ 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 ($n{d}^{7}$ or $n{d}^{8}$) compound is due to dispersive 3D-like $n{d}_{xz,yz}$ conduction bands with weak electron correlation, in comparison with the less-electron-doped ($3{d}^{6.1}$) 22-K superconductor BaFe${}_{1.8}$Co${}_{0.2}$As${}_{2}$ or the hole-doped ($3{d}^{5.9}$) 38-K superconductor Ba${}_{0.6}$K${}_{0.4}$Fe${}_{2}$As${}_{2}$ where electron contribution is from less dispersive 2D-like lower-lying $3{d}_{xy}$ conduction band with stronger electron correlation.