Phase transition and molecular motions in [Cd(NH3)6](NO3)2 †

Abstract

One phase transition was detected by DSC in [Cd(NH3)6](NO3)2 between 95 and 297 K. The phase transition temperatures, at heating = 212.6 K and at cooling = 205.3 K, were calculated by extrapolating to zero scanning rate the and values obtained at different scanning rates. The presence of ca 7.3 K hysteresis suggests that the detected phase transition is of the first-order type and the associated entropy change ΔS = 39.4 J mol−1 K−1 indicates its configurational character. Characteristic changes in temperature dependences of the FWHM of the bands connected with ρ r(NH3)F 1u and δ as(HNH)F 1u modes suggest the existence of a new phase transition at ca 80 K. Thus the examined compound has at 20–300 K range three solid phases, in which NH3 ligands perform fast (picoseconds), uniaxial reorientation around the Cd–N bond, with effective activation energies equal to: E aI = 6.1 kJ mol−1, E aII = 2.1 kJ mol−1 and E aIII = 1.3 kJ mol−1. Very low values of the activation energies for the NH3 reorientation (especially in phase III) suggest that this reorientation occurs through the translation--rotation coupling, which makes the barrier to the rotation of the ammonia ligands fluctuate. The reorientational motion of NH3 is only slightly disturbed at the phase transitions due to the dynamical orientational order--disorder process of anions. †Dedicated to Prof. J. A. Janik on the occasion of his 80th birthday.