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

Abstract Following a re-examination of several published methods for the preparation of ammonium hexavanadate, single crystals of (NH 4 ) 2 V 6 O 16 have been grown by a reaction of V 2 O 5 with NH 4 Cl in hot aqueous solution. They are monoclinic, space group P 2 1 /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 V 2 O 8 double square pyramids ( V - O = 1.595- 1.988 Å) and distorted VO 6 octahedra ( V - O = 1.589-2.287 Å). The infrared bands of the NH 4 + - and ND 4 + - groups in (NH 4 ) 2 V 6 O 16 and its deuterated analogue can be assigned with a fair amount of certainty at 90 K under the space group P 2 1 /m (C 2h 2 ). The N - D stretching modes of isotopically diluted NH 3 D + -groups in (NH 4 ) 2 V 6 O 16 are in agreement with the predicted splitting into C s , C s and C 1 (2) components. The frequencies and shapes of these modes seem to suggest that bifurcated hydrogen bonds are formed, closely resembling the corresponding bonds in NH 4 VO 3 . The NH 4 + ions are dynamic in character in (NH 4 ) 2 V 6 O 16 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 VO 2 as main product. Intermediates have been found, but up to now their com position is still unknown.