  The study of (quantum) phase transitions in heavy-fermion compounds relies on
a detailed understanding of the microscopic control parameters that induce
them. While the influence of external pressure is rather straight forward,
atomic substitutions are more involved. Nonetheless, replacing an elemental
constituent of a compound with an isovalent atom is---effects of disorder
aside---often viewed as merely affecting the lattice constant. Based on this
picture of chemical pressure, the unit-cell volume is identified as an
empirical proxy for the Kondo coupling. Here instead, we propose an "orbital
scenario" in which the coupling in complex systems can be tuned by
isoelectronic substitutions with little or no effect onto cohesive properties.
Starting with the Kondo insulator Ce$_3$Bi$_4$Pt$_3$, we consider---within
band-theory---isoelectronic substitutions of the pnictogen (Bi$\rightarrow$Sb)
and/or the precious metal (Pt$\rightarrow$Pd). We show for the isovolume series
Ce$_3$Bi$_4$(Pt$_{1-x}$Pd$_x$)$_3$ that the Kondo coupling is in fact
substantially modified by the different radial extent of the $5d$ (Pt) and $4d$
(Pd) orbitals, while spin-orbit coupling mediated changes are minute. Combining
experimental Kondo temperatures with simulated hybridization functions, we also
predict effective masses $m^*$, finding excellent agreement with many-body
results for Ce$_3$Bi$_4$Pt$_3$. Our analysis motivates studying the so-far
unknown Kondo insulator Ce$_3$Sb$_4$Pd$_3$, for which we predict
$m^*/m_{band}=\mathcal{O}(10)$.
