Ammonium Hexavanadate, (NH4)2V6O16: Preparation, Crystal Structure, Infrared Spectra and High-Pressure Reactions

Abstract

Abstract Following a re-examination of several published methods for the preparation of ammonium hexavanadate, single crystals of (NH4)2V6O16 have been grown by a reaction of V2O5 with NH4Cl in hot aqueous solution. They are monoclinic, space group P 21/m , with a = 7.858(2), b = 8.412(2), c = 4.995(1) Å , β = 96.43(2)°, and Z = 1. The structure was refined to R = 0.060, wR = 0.039 for 745 unique reflections. It comprises layers built of V2O8 double square pyramids ( V - O = 1.595- 1.988 Å) and distorted VO6 octahedra ( V - O = 1.589-2.287 Å). The infrared bands of the NH4 +- and ND4 +- groups in (NH4)2V6O16 and its deuterated analogue can be assigned with a fair amount of certainty at 90 K under the space group P 21/m (C2h 2). The N - D stretching modes of isotopically diluted NH3D+-groups in (NH4)2V6O16 are in agreement with the predicted splitting into Cs, Cs and C1(2) components. The frequencies and shapes of these modes seem to suggest that bifurcated hydrogen bonds are formed, closely resembling the corresponding bonds in NH4VO3. The NH4 + ions are dynamic in character in (NH4)2V6O16 and remain so down to temperatures of 90 K. At high temperatures in a closed high-pressure system a redox reaction between vanadium (V ) and ammonia takes place which yields VO2 as main product. Intermediates have been found, but up to now their com position is still unknown.