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

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• 作者：Anna Migdał ; -Mikuli ; Edward Mikuli ; Joanna Hetmań ; czyk ; Ł ; ukasz Hetmań ; czyk ; Krystyna Hoł ; derna-Natkaniec ; Ireneusz Natkaniec
• 关键词：Tetraaquacalcium nitrate(V) ; Phase transitions ; Molecular reorientations ; Fourier transform middle infrared spectroscopy (FT-MIR) ; Inelastic and quasi-elastic incoherent neutron scattering (QENS and IINS) ; Proton magnetic resonance (¹H NMR)
• 刊名：Journal of Alloys and Compounds
• 出版年：2009
• 出版时间：5 February 2009
• 年：2009
• 卷：469
• 期：1-2
• 页码：73-81
• 全文大小：813 K

文摘

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 (2ν_δ(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 and next starts drastically grows up till the room temperature, but in the meantime indicates small discontinuity at phase transition temperature . 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 , 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.