Niobium pentoxide block structures

The CS planes are associated with oxygen loss and serve to reduce the composition of the parent comer-shared octahedral structure from NbC 3 to Nb2Os, while retaining octahedral coordination for the Nb5+ cations. The centers of the blocks have a composition Nb03 and the block boundaries have a composition Nb02. 

The brownmillerite structure type can be described as a sequence. .. OTOT... where O stands for the octahedral sheet and T the tetrahedral sheet. A number of other phases closely related to this structure have been characterized, including Ca2LaFe308, with a stacking sequence. .. OOTOOT... and Ca4Ti2Fe20i 1 and 

La4Mn40n, with a stacking sequence. .. OOOTOOOT These phases are the 

The oxide Ba2In205 is another well-studied phase that adopts the brownmillerite structure. This material disorders above 930°C to a perovskite-type structure containing oxygen vacancies. Both the Sr-Fe and Ba-In oxides are of interest for electrochemical applications in fuel cells and similar devices (Section 6.10). 

The oxygen composition of doped brownmillerite structure materials is not always exactly 5.0 and depends upon a number of factors, especially the surrounding oxygen partial pressure. Thus, the oxide formed when Fe is replaced by Co has a broad 

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Figure 4.32 Niobium pentoxide block structures (a) H-Nb2Os, (b) Ti2Nb10O29 monoclinic, and (c) W3Nb14044. The shaded squares represent MOg octahedra at two different levels and the shaded spheres represent cations in tetrahedral coordination.

Figure 4.33 Electron micrograph showing disorder in a niobium pentoxide block structure. The phase was synthesized from Nb2Os and CoF3 (which is analogous to W03). The region at the lower center of the micrograph consists of blocks of 4 x 4 octahedra with the W3Nb14044 structure with a block size of approximately 1.5 nm. (Courtesy of Dr. J. Hutchison, University of Oxford.)...

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