Redetermination of di-ammonium trivanadate, (NH 4 ) 2 V 3 O 8 .

The crystal structure of (NH 4 ) 2 V 3 O 8 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 NH 4 + , 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 K α radiation. These accurate data reveal that the NH 4 + 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 P 4 bm , and the remaining H sites were modelled over two disordered positions, with equal occupancy. It therefore follows that the NH 4 + cations filling the space available in the (001) layered structure formed by (V 3 O 8 ) 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 inter-calation properties of this material, which is used as a cathode for supercapacitors.