We report a systematic enhancement of the pressure dependence of $T_N$ in $A^{2+}[{\mathrm{V}}_2]{\mathrm{O}}_4$ spinels as the V-V separation approaches the critical separation for a transition to itinerant-electron behavior. An intermediate phase between localized and itinerant-electron behavior is identified in $\mathrm{Zn}[{\mathrm{V}}_2]{\mathrm{O}}_4$ and $\mathrm{Mg}[{\mathrm{V}}_2]{\mathrm{O}}_4$ exhibiting mobile holes as large polarons. Partial electronic delocalization, cooperative ordering of V-V pairs in $\mathrm{Zn}[{\mathrm{V}}_2]{\mathrm{O}}_4$ below $T_s\approx T_N$ and $d{T}_N/dP<0$, signals that lattice instabilities associated with the electronic crossover are a universal phenomenon.

• Received 4 June 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.187201