Observation of Metal–Insulator Transition in Hollandite Vanadate, K₂V₈O₁₆

Abstract

We have synthesized hollandite vanadate, K 2 V 8 O 16 , by a solid-state reaction under 4 GPa at 1473 K for one hour, and investigated its structural and electromagnetic properties. We found a metal–insulator transition with a two-step jump of resistivity of about three orders at around 170 K. The magnetic susceptibility is reduced to a small value at the transition, suggesting the formation of V 4+ –V 4+ singlet pairs and V 3+ –V 3+ pairs in the low-temperature insulator phase. The transition is of first order accompanied by a structural change from a tetragonal to a monoclinic structure. The low-temperature phase has a superlattice of \(\sqrt{2}a\times\sqrt{2}a\times 2c\), where a and c denote the parameters of the primitive cell for the simple tetragonal hollandite structure, suggesting a charge ordering of V 4+ and V 3+ . We construct a possible charge order model from the obtained results, in which two kinds of double-chain formed by V 3+ and V 4+ chains and by single V 4+ chains order in a manner that gives a superlattice of \(\sqrt{2}\times\sqrt{2}\) in the a – b plane. The V 4+ –V 4+ and V 3+ –V 3+ pairs are formed in each chain along the c -axis, resulting in a duplication of the c -axis.