Ab initio design of enhanced and sign-reversible spin Hall and spin Nernst conductivity in skutterudite materials

The search for novel materials capable of robust spin-current generation remains an active area of research. Here, we investigate the intrinsic spin Hall and spin Nernst effects in Co- and Rh-based skutterudites and explore their enhancement through heavy-metal substitution and site-selective doping. Using first-principles calculations and Wannier-based tight-binding models, we analyze the spin Berry curvature and spin-transport responses of pristine MX${}_3$, where M = Co, Rh and X = As, Sb, as well as chemically modified systems. Despite sizable momentum-resolved spin Berry curvature, the pristine compounds exhibit relatively small net spin Hall responses due to substantial cancellation across the Brillouin zone. Isoelectronic Ir and Bi substitution at 8$c$ Wycoff M site produces little enhancement near the E${}_F$, whereas non-isoelectronic Pt substitution yields a relatively better response. Most interestingly, the incorporation of Pt at the interstitial $2a$ Wycoff site of RhAs${}_3$ nearly doubles the spin Hall conductivity to $\sim 400(\mathrm{\hslash }\mathrm{/}e)(\mathrm{\Omega },\mathrm{c}\mathrm{m}{)}^{-1}$ near the E${}_F$ when compared to Pt substitution at 8$c$ position. At 300 K, the same interstitially filled system exhibits a spin Nernst conductivity of $\sim 1.46(\mathrm{\hslash }\mathrm{/}e),\mathrm{A},{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at the E${}_F$, nearly an order of magnitude larger than that of $M$-site Pt substitution. The enhanced responses arise from Pt $5d$ states, strong spin-orbit coupling, and avoided crossings that generate pronounced spin-Berry-curvature hot spots. These results demonstrate that site-selective chemical engineering can effectively overcome momentum-space cancellation resulting in enhanced spin Hall and spin Nernst responses in skutterudites. Our results provide a possible route towards robust spin-current generation in this class of materials.