Complex NH4[VO(O2)2(NH3)] (1) undergoes an order-disorder phase transition at Tc~258K. 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 NH4+ 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 Tc.

Department of Chemistry Faculty of Education J Selye University Bratislavská 3322 945 01 Komárno Slovakia

Department of Dielectrics Institute of Physics ASCR v v i Na Slovance 2 182 21 Praha 8 Czech Republic

Department of Inorganic Chemistry Faculty of Natural Sciences Comenius University Mlynská dolina 842 15 Bratislava Slovakia

Department of Inorganic Chemistry Faculty of Science Charles University Hlavova 2030 8 128 43 Praha 2 Czech Republic

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