Structure of Nd₁₀W₂₂O₈₁ from high-resolution electron microscopy and X-ray powder diffraction

The crystal structure of Nd₁₀W₂₂O₈₁, neodymium tungstate, has been determined using a combination of high-resolution transmission electron microscopy and X-ray powder diffraction methods. The unit-cell dimensions determined from X-ray data are a  =  3.8613 (1), b  =  35.9431 (1), c  =  21.8900 (1) Å, V = 3038.05 ų, Z  =  2, space group Pbcm. The structure is built up of pentagonal columns (PCs) connected to ReO₃-type fragments consisting of three octahedra, thus forming W₉O₃₂ units. These units form `pillars' along a. These `pillars' are joined by pairs of tilted octahedra (W₂O₁₁ groups) to form corrugated layers perpendicular to b. The Nd atoms are located in the space between these layers and form the only link between them. Interstitial O atoms are located between two of the Nd atoms. The formula Nd₁₀W₂₂O₈₁ can thus be alternatively given as Nd₅W₁₁O_39 + x (x ≃ 1.5), where x represents the interstitial O atoms. Nd₁₀W₂₂O₈₁ is a new type of PC structure.