  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) A,
b = 3.8236(3) A, c = 17.8521(2) A, beta = 102.029(3)) 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 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.
