Phase transitions and H_2O motions in [Ca(H_2O)_4](NO_3)_2 studied by infrared spectroscopy, inelastic/quasi-elastic incoherent neutron scattering and proton magnetic resonance, Part II

摘要

Fourier transform middle infrared spectroscopy (FT-MIR) inelastic/quasi-elastic incoherent neutron scattering (IINS/QENS) and proton magnetic resonance (~1H NMR) data for [Ca(H_2O)_4](NO_3)_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 (2v_(delta)(H_2O) mode) indicates that there exist a time (ps) fast re-orientational motions of H_2O 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 deg instantaneous stochastic jumps of protons around the twofold axis formed by the Ca-O bond, assuming that only one out of four H_2O ligands per [Ca(H_2O)_4]~(2+) unit undergoes this fast reorientation. The ~1H NMR spin-lattice relaxation time T_1 decreases with increasing of temperature up to the phase transition temperature T_(C2)~h approx = 203 K and next starts drastically grows up till the room temperature, but in the meantime indicates small discontinuity at phase transition temperature T_(C1)~h approx = 244 K. The analysis of the second moment of ~1H NMR line in the low temperature phase of [Ca(H_2O)_4](NO_3)_2 revealed that the 180 deg flip of water molecules causes only very small changes of M_2. It was concluded that a reorientation of a whole [Ca(H_2O)_4]~(2+) cations (around the twofold symmetry axis going through two calcium atoms of the neighbouring [Ca(H_2O)_4]~(2+) cations) is the reason for the plateau of M_2 = 11.3 X 10~(-8) T~2 observed at ca. 100 K. Above the phase transition at T_(C2)~h, the anisotropic reorientation of [Ca(H_2O)_4]~(2+) (around the pseudo threefold symmetry axis going through two calcium atoms of the neighbouring [Ca(H_2O)_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 M_2 = 3.0 X 10~(-8)T~2.