Hollandite type oxides, K₂M₈O₁₆ with a mixed valence of M³⁺/M⁴⁺ =1/3 have intensively investigated, expecting novel itinerant properties such as metal-insulator (MI) transition, charge order and so on. Actually, K₂V₈O₁₆ was found to undergo the metal-insulator transition at 170K, accompanied by the structural transition and charge order [1, 2]. A very rare ferromagnetic MI transition was discovered in K₂Cr₈O₁₆ [3]. This MI transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the c-direction, leading to the formation of tetramers of Cr ions below the transition temperature [4].

Fig. 1. Electron diffraction pattern observed at 100 K and schematic illustration of charge order between Mn⁴⁺ and Mn³⁺ for K_1.6Mn₈O₁₆. The electron beam incidence is along the [001] direction of the monoclinic lattice. The superlattice reflections with five-fold periodicity along the b-axis (tunnel direction) are clearly observed, indicating K-vacancies ordering. The extra electrons are trapped by Mn ions adjacent to K-vacancies, leading to charge order between Mn⁴⁺ and Mn³⁺.

The crystal structure consists of the M₈O₁₆-framework and K-cations. The M₈O₁₆-framework is constructed from the double-chains (zigzag-chains) formed by sharing the edges of MO₆ octahedra. The M₈O₁₆-framework has rectangular tubes surrounded by four double-chains, and K-cations occupy the sites within each rectangular tube and act as electron donor.

K_xMn₈O₁₆ was prepared by high-pressure synthesis using a cubic anvil press [5]. Unfortunately Mn-hollandite was obtained only in K-deficient form. The maximum K-composition is x=1.6, in which K-vacancy molar fraction is 1/5 and Mn³⁺ molar fraction is also the same 1/5. K_1.6Mn₈O₁₆ is not a metal and a canted antiferromagnet with Néel temperature of 50 K. Resistivity well obeys a one-dimensional variable range hopping manner. K_1.6Mn₈O₁₆ shows successive structural transitions of tetragonal to monoclinic at 370 K and monoclinic to monoclinic at 250 K. The superlattice reflections with five-fold periodicity along the b-axis (tunnel direction) as shown in Fig. 1, are observed by electron diffraction of TEM (transmission electron microscope) below 250 K. Such a five-fold periodicity is consistent with the fraction of K-vacancies, 0.4/2=1/5, suggesting vacancy-ordering at 250 K. At room temperature, similar but rather diffusive super-reflections are observed. These results suggest that in K_1.6Mn₈O₁₆, the charge differentiation into Mn³⁺ and Mn⁴⁺ occurs even at higher temperature than at least 370 K and a short range order of K-vacancies progresses below 370 K, followed by a sudden long range order at 250 K. Such charge differentiation in K_1.6Mn₈O₁₆ could be due to K-deficiency, because the fraction of Mn³⁺ is 1/5 which coincides with the fraction of K-vacancies. Namely, extra e_g-electrons are trapped at Mn ions close to K-vacancies, giving rise to Mn³⁺ ions, namely an ordering between Mn³⁺ and Mn⁴⁺ takes place in cooperation with K-vacancy ordering. An origin for K-deficiency in hollandite manganese oxide could be in its rather shorter b-axis compared with those of K₂M₈O₁₆ (M = Ti, V, Cr). The electrostatic repulsion between K⁺ ions in the tunnel would not allow the full occupancy of K-sites. Much higher pressure would be necessary for the synthesis of the stoichiometric K₂Mn₈O₁₆.

References

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• [2] A. C. Komarek, M. Isobe, J. Hemberger, D. Meier, T. Lorenz, D. Trots, A. Cervellino, M. T. Fernández-Díaz, Y. Ueda, and M. Braden, Phys. Rev. Lett. 107, 027201 (2011).
• [3] K. Hasegawa, M. Isobe, T. Yamauchi, H. Ueda, J-I. Yamaura, H. Gotou, T. Yagi, H. Sato, and Y. Ueda, Phys. Rev. Lett. 103, 146403 (2009).
• [4] T. Toriyama. A. Nakao, H. Nakao, Y. Murakami, K. Hasegawa, M. Isobe, Y. Ueda, A. V. Ushakov, D. I. Khomskii, S. V. Streltsov, T. Konishi, and Y. Ohta, Phys. Rev. Lett. 107, 266402 (2011).
• [5] T. Kuwabara, M. Iosbe, H. Gotoh, T. Yagi, D. Nishio-Hamane, and Y. Ueda, J. Phys. Soc. Jpn. 81, 104701 (2012).

Authors

• M. Isobe, H. Gotoh, D. Nishio-Hamane, and Y. Ueda