Structures of the reduced niobium oxides Nb{sub 12}O{sub 29} and Nb{sub 22}O{sub 54}

The crystal structure of Nb{sub 22}O{sub 54} is reported for the first time, and the structure of orthorhombic Nb{sub 12}O{sub 29} is reexamined, resolving previous ambiguities. Single crystal X-ray and electron diffraction were employed. These compounds were found to crystallize in the space groups P2/m (a=15.7491(2)A, b=3.8236(3)A, c=17.8521(2)A, {beta}=102.029(3){sup 0}) and Cmcm (a=3.8320(2)A, b=20.7400(9)A, c=28.8901(13)A), respectively and share a common structural unit, a 4x3 block of corner sharing NbO{sub 6} octahedra. Despite different constraints imposed by symmetry these blocks are very similar in both compounds. Within a block, it is found that the niobium atoms are not located in the centers of the oxygen octahedra, but rather are displaced inward toward the center of the block forming an apparent antiferroelectric state. Bond valence sums and bond lengths do not show the presence of charge ordering, suggesting that all 4d electrons are delocalized in these compounds at the temperature studied, T=200 K. - Graphical abstract: The crystal structure of Nb{sub 22}O{sub 54} is reported, and the structure of orthorhombic Nb{sub 12}O{sub 29} is reexamined. Bond valence sums do not show the presence of charge ordering, suggesting that all 4d electrons are delocalized in these compounds at T=200 K, despite the well-known magnetic behavior of these materials.