Structures of the reduced niobium oxides Nb₁₂O₂₉ and Nb₂₂O₅₄

The crystal structure of Nb₂₂O₅₄ is reported for the first time, and the structure of orthorhombic Nb₁₂O₂₉ 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) Å, b = 3.8236 (3) Å, c = 17.8521 (2) Å, β = 102.029 (3)^{{ring operator}}) and Cmcm (a = 3.8320 (2) Å, b = 20.7400 (9) Å, c = 28.8901 (13) Å), respectively and share a common structural unit, a 4×3 block of corner sharing NbO₆ 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.