Hollandites: a novel class of oxides with unusual properties

Hollandite with a general chemical formula $A_{x}M_{8}$O$_{16}$ ($x\le $2) is one of minerals. Its crystal structure consists of the $M_{8}$O$_{16}$-framework and $A$-cations at tunnel sites in the $M_{8}$O$_{16}$-framework. The most popular crystal symmetry is a tetragonal $I$4/$m$ in which the crystallographycally independent $M$ site is unique, hence, $A^{+}_{2}M_{8}$O$_{16}$ is a mixed valent oxide with $M^{3+}$/$M^{4+}$ = 1/3. We have synthesized K$_{2}$V$_{8}$O$_{16}$ [1] and K$_{2}$Cr$_{8}$O$_{16}$ [2] in both powder and single crystal form under high pressure and found metal-insulator (MI) transitions. The manner of MI transitions, however, is very different between both compounds. K$_{2}$V$_{8}$O$_{16}$ exhibits a first order MI transition 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 accompanied by the structural change from a tetragonal to a monoclinic structure. The low temperature phase has a superlattice of $\surd $2 x $\surd $2 x 2, suggesting a charge ordering of V$^{4+}$ and V$^{3+}$. On the other hand, K$_{2}$Cr$_{8}$O$_{16}$ is a ferromagnetic metal (or half-metal) with $T_{C}$ = 180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K$_{2}$Cr$_{8}$O$_{16}$ is quite unique in three aspects: It has a rare mixed valence of Cr$^{3+}$ and Cr$^{4+}$; it has a metal (or half-metal) to insulator transition in a ferromagnetic state; and the resulting low temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems. \\[4pt] References:\\[0pt] [1] M. Isobe, S. Koishi, N. Kouno, J. Yamaura, T. Yamauchi, H. Ueda, H. Gotou, T. Yagi, and Y. Ueda, J. Phys. Soc. Jpn. \textbf{75} (2006) 073801. \\[0pt] [2] K. Hasegawa, M. Isobe, T. Yamauchi, H. Ueda, J-I. Yamaura, H. Gotou, T. Yagi, H. Sato, and Y. Ueda, Phys. Rev. Lett. \textbf{103 }(2009) 146403.