Phase transition, molecular motions, structural changes and low-frequency vibrations in [Cu(NH3)5](ClO4)2

The phase transition in [Cu(NH3)5](ClO4)2 at Tch=150.6K while heating and at Tcc=141.6K while cooling was determined by means of differential scanning calorimetry (DSC), by extrapolating the TpeakhandTpeakc vs. rate of sample heating and cooling to the scanning rate value of 0 K min−1, respectively. The presence of ca. 9 K hysteresis in the phase transition temperature suggests that the detected phase transition is of the first order type. Large transition entropy value indicates considerable disordering of high-temperature phase. Changes in spectra were observed and two solid phases were identified by Fourier transform far- and middle-infrared spectroscopy (FT-FIR and FT-MIR), which indicate that the detected phase transition is related to a crystal structure change. Structural character of this phase transition also confirms the existence of some kind of soft modes in the low-temperature phase. The results of X-ray and neutron powder diffraction measurements (XRPD and NPD) proved that the crystal structure change took place. The cubic phase (Fm3¯m) transforms into the monoclinic phase (P21/m). The inelastic, incoherent neutron scattering (IINS) results, compared with infrared and Raman spectra at low-frequency vibrations region, additionally authenticate that statement. The temperature dependence of the full-width at half-maximum (FWHM) of the band connected with δas(HNH) mode indicates that the observed phase transition is not related to dynamical order-disorder process of the NH3 ligands, which make fast reorientational motions in both detected phases. Reorientational correlation time becomes longer than 10−10 s only at 25 K, which was detected by quasi-elastic neutron scattering (QENS) measurements.