Order-disorder phase transition in the peroxidovanadium complex NH 4 [VO(O 2 ) 2 (NH 3 )]

Abstract Complex NH 4 [VO(O 2 ) 2 (NH 3 )] ( 1 ) undergoes an order-disorder phase transition at T c  ~ 258 K. This transition is accompanied by change in the space group of the orthorhombic lattice and also by significant structural rearrangements of the constituent molecules, which are pertinent mostly to their NH 4 + ions and their ammonia ligands. The low-temperature solid state IR and Raman spectra of 1 were corroborated by solid-state computations that employed Gaussian functions as the basis set. Results of these computations yielded excellent agreement with experimental data. On the curves of temperature dependence of vibrational modes, the phase transition is expressed by an abrupt change of the slope above T c .