Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels

The valence electronic structure of magnetic centers is one of the factors that determines the characteristics of a magnet. It may refer to orbital degeneracy, as for $j_{\text{eff}}=1\mathrm{/}2$ Kitaev magnets, or near-degeneracy, e.g. involving the third and fourth shells in cuprate superconductors. Here we explore the inner structure of magnetic moments in group-5 lacunar spinels, fascinating materials featuring multisite magnetic units in the form of tetrahedral tetramers. Our quantum chemical analysis reveals a very colorful landscape, much richer than the single-electron, single-configuration description applied so far to all group-5 Ga$M_4{X}_8$ chalcogenides, and clarifies the basic multiorbital correlations on $M_4$ tetrahedral clusters: while for V strong correlations yield a wave-function that can be well described in terms of four V${}^{4+}$V${}^{3+}$V${}^{3+}$V${}^{3+}$ resonant valence structures, for Nb and Ta a picture of dressed molecular-orbital-like $j_{\text{eff}}=3\mathrm{/}2$ entities is more appropriate. These internal degrees of freedom likely shape vibronic couplings, phase transitions, and magneto-electric properties in each of these systems.