Magnetic insulators are functional materials with potential applications in spintronics and multiferroics. The hollandites A x M 8 O 16 , which contain mixed-valent transition metals, have demonstrated ferromagnetism combined with insulating behavior and provide a new platform for exploring the effects of magnetic frustration due to their "folded" triangular lattice. We have tuned the hollandite Ba x Mn 8 O 16 from a complex antiferromagnet with Neél temperature (T N ) = 25 K to a ferrimagnet with Curie temperature (T C ) = 180 K via partial cobalt substitution for manganese. Both Ba x Mn 8 O 16 and Ba x Co y Mn 8-y O 16 were prepared by salt flux methods, and combined neutron and X-ray diffraction confirm a distorted hollandite-type structure for both oxides. X-ray photoelectron spectroscopy reveals that the Co 2+ substitution drives the average Mn oxidation state from 3.7+ to nearly 4.0+, thereby changing its d-electron count. Magnetization and resistivity measurements show that the cobalt-doped hollandite is a ferrimagnetic insulator, with a high T C of 180 K. On the basis of neutron diffraction measurements, we provide the first solution of the magnetic structure of Ba x Mn 8 O 16 , which consists of a complex antiferromagnet with a large magnetic unit cell. Upon substituting cobalt for manganese, the magnetic structure changes dramatically, destroying the previously large magnetic unit cell and promoting ferromagnetic alignment along the hollandite tunnel direction. The observed hysteresis at base temperature for Ba x Co y Mn 8−y O 16 is explained as arising from uncompensated spins aligned along the (200) crystallographic planes.

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We present the electronic band structures of microporous titanates with the hollandite-type structure.