Phase transition and its connection with molecular motions in [Cu(NH3)5](BF4)2

The phase transitions in [Cu(NH3)5](BF4)2, at Tch=147.9K-while heating and at Tcc=141.7K-while cooling were determined by means of differential scanning calorimetry (DSC). The presence of ca. 6 K hysteresis at the phase transition temperature suggests that the detected phase transition is of the first order type. A large transition entropy (ΔS≈25.1 [J mol−1 K−1]) indicates considerable configurational disordering in the high temperature phase. The temperature dependences of the full width at half maximum (FWHM) of the two infrared bands, registered by Fourier transform far-infrared and middle-infrared spectroscopy (FT-FIR and FT-MIR), one connected with δd(FBF)E and the other with ρr(NH3) mode, indicate that, in the high temperature phase, there occurs an isotropic picosecond's reorientational motion of BF4− anions with an activation energy equal to ca. 4.7 kJ mol−1 and a uniaxial, picosecond's reorientational motion of the NH3 ligands with an activation energy equal to ca. 4.6 kJ mol−1. At the phase transition region, both these reorientational motions are suddenly slowed down. The splitting of some infrared bands was observed during the cooling (and heating) of the sample, which indicates that the detected phase transition is also related to a change in crystal structure.