Abstract
In the title compound, trisodium dicobalt arsenate diarsenate, Na
3
Co
2
AsO
4
As
2
O
7
, 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 CoO
6
octahedra in the
ab
plane, forming Co
4
O
20
units, which are corner- and edge-connected
via
AsO
4
and As
2
O
7
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.