Redetermination of diammonium trivanadate, (NH4)2V3O8

Abstract

The crystal structure of (NH4)2V3O8 has been reported twice using single-crystal X-ray data [Theobald et al. (1984). J. Phys. Chem. Solids, 45, 581–587; Range et al. (1988). Z. Naturforsch. Teil B, 43, 309–317]. In both cases, the orientation of the ammonium cation in the asymmetric unit was poorly defined: in Theobald's study, the shape and dimensions were constrained for NH4+, while in Range's study, H atoms were not included. In the present study, we collected a highly redundant data set for this ternary oxide, at 0.61 Å resolution, using Ag Kα radiation. These accurate data reveal that the NH4+ cation is disordered by rotation around a non-crystallographic axis. The rotation axis coincides with one N—H bond lying in the mirror m symmetry element of space-group type P4bm, and the remaining H sites were modelled over two disordered positions, with equal occupancy. It therefore follows that the NH4+ cations filling the space available in the (001) layered structure formed by (V3O8)2– ions do not form strong N—H...O hydrogen bonds with the mixed-valent oxidovanadate(IV,V) anions. This feature could have consequences for the Li-ion intercalation properties of this material, which is used as a cathode for supercapacitors.

Keywords

• crystal structure
• vanadate
• ammonium
• high-resolution data
• redetermination