Structures of the reduced niobium oxides Nb12O29 and Nb22O54

TL;DR

This paper reports the crystal structures of Nb22O54 and reexamines Nb12O29, revealing shared structural units, niobium displacement indicating potential antiferroelectricity, and delocalized electrons without charge ordering at 200 K.

Contribution

First report of Nb22O54 structure and reexamination of Nb12O29, clarifying their structures and electronic properties.

Findings

Nb22O54 structure determined for the first time

Shared 4x3 NbO6 octahedral blocks in both compounds

Niobium atoms displaced inward, suggesting antiferroelectric behavior

Abstract

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.