Phase transitions in [Co(NH3)6](ClO4)3 investigated by neutron scattering methods

Highlights: ► Structure and molecular reorientations in [Co(NH3)6](ClO4)3 at phase transitions. ► IR, RS, IINS spectra calculated by DFT show good agreement with experimental data. ► High temperature phase is dynamically orientationally disordered crystal phase. ► Excess of elasticity in QENS below TC3 is observed. ► Crystal structure does not change drastically at the phase transition. - Abstract: [Co(NH3)6](ClO4)3 was investigated by neutron scattering methods in the temperature range of 20–290 K. Neutron powder diffraction revealed that the crystal structure does not change at the TC2 and TC3 phase transitions. High temperature phases are orientationally disordered. QENS study showed that NH3 ligands perform rapid reorientational motion around their 3-fold axes in a wide temperature range. However, below TC3 some excess of elastic scattering is observed, which can be explained by the assumption that only half of the NH3 groups reorient fast enough to contribute to the QENS broadening, whereas the remaining NH3 groups reorient more slowly. The activation energy for NH3 jumps, estimated from the temperature dependence of QENS broadening, is small but comparable with the results obtained for other hexaammine complexes. Quantum chemical calculation, carried out on the basis of DFT for isolated [Co(NH3)6]3+ cation and ClO4- anion, and experimental (IR, RS, IINS) spectra are in good agreement