Phase transitions and H2O motions in [Ca(H2O)4](NO3)2 studied by infrared spectroscopy, inelastic/quasi-elastic incoherent neutron scattering and proton magnetic resonance, Part II

Abstract

Fourier transform middle infrared spectroscopy (FT-MIR) inelastic/quasi-elastic incoherent neutron scattering (IINS/QENS) and proton magnetic resonance (1H NMR) data for [Ca(H2O)4](NO3)2 in the temperature range of 20–300 K are reported. The temperature dependence of full width at half maximum (FWHM) of the infrared band (2νδ(H2O) mode) indicates that there exist a time (ps) fast re-orientational motions of H2O ligands, with a mean value of activation energy of ca. 23 kJ mol−1, which is only slightly disturbed in the both phase transitions region. The elastic peak of neutron inelastic scattering, registered in the intermediate and high-temperature phase, shows broadening, which can be well described by a model of 180° instantaneous stochastic jumps of protons around the twofold axis formed by the CaO bond, assuming that only one out of four H2O ligands per [Ca(H2O)4]2+ unit undergoes this fast reorientation. The 1H NMR spin-lattice relaxation time T1 decreases with increasing of temperature up to the phase transition temperature TC2h≈203K and next starts drastically grows up till the room temperature, but in the meantime indicates small discontinuity at phase transition temperature TC1h≈244K. The analysis of the second moment of 1H NMR line in the low temperature phase of [Ca(H2O)4](NO3)2 revealed that the 180° flip of water molecules causes only very small changes of M2. It was concluded that a reorientation of a whole [Ca(H2O)4]2+ cations (around the twofold symmetry axis going through two calcium atoms of the neighbouring [Ca(H2O)4]2+ cations) is the reason for the plateau of M2 = 11.3 × 10−8T2 observed at ca. 100 K. Above the phase transition at TC2h, the anisotropic reorientation of [Ca(H2O)4]2+ (around the pseudo threefold symmetry axis going through two calcium atoms of the neighbouring [Ca(H2O)4]2+ cations) is set in motion with a frequency of the order of a few kilo hertz and just above 275 K one can observe a second plateau of M2 = 3.0 × 10−8T2.