Local Structure and Site Occupancy in Cd- and Hg-doped Ce$T$In$_{5}$ ($T$=Co, Rh, Ir)

Local structure measurements using the extended x-ray absorption fine-structure (EXAFS) technique were performed from the In $K$, Cd $K$, and Hg $L_3$ edges on samples of CeCo(In$_{1-x}$Cd$_x$)$_5$ (0.5\%$\geq x \geq$3.1\%) and Ce$T$(In$_{1-x}$Hg$_x$)$_5$ ($T$=Co, Rh, Ir, 0.7\%$\geq x \geq$3.5\%). Fits indicate no measurable change in the bulk local structure with these substituents. In contrast, the local structure data around the substituent atoms indicates about $f_{Cd}^{Co}=44(3)\%$ of Cd atoms reside on In(1) sites, similar to previous results [$f_{Sn}^{Co}=55(5)\%$] for Sn in CeCo(In$_{1-x}$Sn$_x$)$_5$. Mercury has an even stronger preference for the In(1) site, with $f_{Hg}^{Co}=71(5)\%$, $f_{Hg}^{Rh}=97(3)\%$, and $f_{Hg}^{Ir}=55(5)\%$, although other phases appear to be present in the CeIr(In$_{1-x}$Hg$_x$)$_5$ material. Small distortions from the parent structure are also observed around the substituent atoms. These results will be related to the sharp decrease in the superconducting transition temperature with $x$.