Homopolar Bond Formation in ${\mathrm{ZnV}}_2{\mathrm{O}}_4$ Close to a Metal-Insulator Transition

Electronic structure calculations for spinel vanadate ${\mathrm{ZnV}}_2{\mathrm{O}}_4$ show that partial electronic delocalization in this system leads to a structural instability, with the formation of V-V dimers along the [011] and [101] directions, and readily accounts for the intriguing magnetic structure of this material. The formation of V-V bonds is a consequence of the proximity to the itinerant-electron boundary and is not related to orbital ordering. We discuss how this mechanism naturally couples charge and lattice degrees of freedom in magnetic insulators close to such a crossover.

Figure 1

Schematic representation of the “dimerized” structure resulting from our ab initio, all-electron calculations for realistic values of $U$ in ${\mathrm{ZnV}}_2{\mathrm{O}}_4$. Bold (thin) line represents short (long) in-chain bonds. The magnetic structure is indicated by arrows.

Figure 2

Temperature dependence of the resistivity in the ${\mathrm{A}}^{2+}{\mathrm{V}}_2{\mathrm{O}}_4$ series. Small differences in the resistivity for ${\mathrm{MgV}}_2{\mathrm{O}}_4$ and ${\mathrm{ZnV}}_2{\mathrm{O}}_4$ could be due to intergrain boundary scattering in the ceramic pellets (their room temperature resistivity differs only by $\simeq 8\Omega \mathrm{cm}$). In any case, their activation energy ($\Delta E$) is practically identical, as it should be, given the similar V-V distance in both compounds.

Figure 3

Stabilization energies (in meV/V) of the “dimerized” structure with respect to the “standard” one as a function of $U_{\mathrm{eff}}$. Our proposed “dimerized” structure is more stable for values of $U_{\mathrm{eff}}\lesssim 3\mathrm{eV}$. Error bars for $U_{\mathrm{eff}}=0$ account for the various functionals used (see text).