Phase transition in [Ca(NH3)6](ClO4)2 studied by neutron scattering methods and far infrared spectroscopy

Highlights: → FT-FIR spectra and diffraction patterns vs. temperature showed that phase transition is probably associated with a change of the crystal structure. → Fast stochastic molecular reorientational motions of NH3 ligands do not change their character in the wide temperature range. → High temperature phase is dynamically orientationally disordered phase. → Low value of the activation energy for NH3 ligands reorientation suggests that this motion proceeds through the translation-rotation coupling. - Abstract: The results of inelastic and quasi-elastic incoherent neutron scattering with simultaneous neutron powder diffraction and of Fourier transform far infrared spectroscopy measurements carried out for [Ca(NH3)6](ClO4)2 between 20-220 K and 8.5-295 K, respectively, are reported. The far infrared spectrum on cooling the substance indicates splitting of some degenerated vibrational modes, which suggests lowering of the crystal structure at the phase transition temperature TCc=122.0K, revealed by us earlier using differential scanning calorimetry. In turn, lack of simultaneous abrupt narrowing of some broad infrared bands suggests an existence of fast molecular reorientational motions, which do not change their character in the wide temperature range. On the other hand, the quasi-elastic neutron scattering peak registered at 110 K and also at higher temperatures, shows distinct broadening, which is typical for dynamically, orientationally disordered crystals (ODIC). Reorientational motion of NH3 ligands can be well described by a model of instantaneous 120o jumps of protons around the 3-fold axis of Ca-N bonds, with the reorientational correlation time τR of an order of picoseconds. However, the NH3 ligands did not drastically change either their jump rate or a character of their reorientational motions at the phase transition temperature (TC). The estimated mean value of activation energy for NH3 ligands reorientation in both phases Ea = 1.2 kJ mol-1. Such low value of Ea suggests that the reorientational jumps of NH3 probably proceed through the translation-rotation coupling. Notwithstanding, one can observe the distinct changes between the neutron diffraction patterns before and after the TCh=123.3K. All these facts suggest that the discovered phase transition is associated with a change of the crystal structure without a sudden change of the NH3 reorientational dynamics.