Ferrite Materials Containing Kagomé Layers: Chemistry of Ba₂Fe₁₁Ge₂O₂₂ and K₂Co₄V₉O₂₂ Hexaferrites

Abstract

Hexaferrites have a wide range of technological applications as well as a rich and complicated structural chemistry, with iron oxide layers inducing highly anisotropic ferrimagnetism that remains intact to high temperatures (>400 K). Most hexaferrites (i.e., M-type BaFe12O19) contain the dominant magnetic vectors normal to the hexagonal planes forming hard ferrimagnets, while the much less common Y-type investigated here typically contains the ferrimagnetic vectors in the iron oxide planes, creating soft ferrimagnets suitable for RF applications. To study the structural chemistry and magnetism of this less common hexaferrite phase, single crystals of K2Co4V9O22 (I) and Ba2Fe11Ge2O22 (II) were prepared. The structure of these compounds is a derivative of the sophisticated mineral greenwoodite, having a complex assembly of transition metal octahedra and tetrahedra, notably featuring Kagomé layers in spinel-type blocks that are magnetically isolated from one another. In particular, compound II provides a pathway to developing iron-rich hexaferrites where the magnetic ions are not diluted by site substitution of nonmagnetic ions. This results in an exceptionally high magnetic ordering temperature of 855 K to a canted antiferromagnetic state.

Hydrothermal Synthesis and Crystal Structure of Vesuvianite Compounds, Ca19Al13Si18O71(OH)7 and Sr19Fe12Ge19O72(OH)6

“…We were also curious if the vesuvianite structure type could be expanded into a new phase space through the substitution of the larger tetrahedral Ge 4+ ion for Si 4+ . We have recently been examining the role of germanate as a building block in transition metal oxides and have observed some complex structural chemistry [23,24]. In the present case, we found that the larger germanate can accommodate the larger Fe 3+ ion in the trivalent Al 3+ sites, as well as the larger Sr 2+ ion in the Ca 2+ sites.…”