Date Published: July 01, 2012
Publisher: International Union of Crystallography
Author(s): Abderrahmen Guesmi, Ahmed Driss.
In the title compound, trisodium dicobalt arsenate diarsenate, Na3Co2AsO4As2O7, the two Co atoms, one of the two As and three of the seven O atoms lie on special positions, with site symmetries 2 and m for the Co, m for the As, and 2 and twice m for the O atoms. The two Na atoms are disordered over two general and special positions [occupancies 0.72 (3):0.28 (3) and 0.940 (6):0.060 (6), respectively]. The main structural feature is the association of the CoO6 octahedra in the ab plane, forming Co4O20 units, which are corner- and edge-connected via AsO4 and As2O7 arsenate groups, giving rise to a complex polyhedral connectivity with small tunnels, such as those running along the b- and c-axis directions, in which the Na+ ions reside. The structural model is validated by both bond-valence-sum and charge-distribution methods, and the distortion of the coordination polyhedra is analyzed by means of the effective coordination number.
For related structures, see: Ruiz-Valero et al. (1996 ▶); Ben Smail & Jouini (2005 ▶); Guesmi & Driss (2002a ▶,b ▶). For bond-valence analysis, see: Brown (2002 ▶); Adams (2003 ▶). For the charge distribution method, see: Nespolo et al. (2001 ▶); Nespolo (2001 ▶); Guesmi et al. (2006 ▶).