Zakład Fizyki Wysokich Ciśnień - publikacje

Abstract: Fourier transform middle infrared spectroscopy, inelastic/quasielastic incoherent neutron scattering and proton magnetic resonance data for [Ca(H2O)4](ClO4)2 in the temperature range of 20-300 K and 110-300 K, respectively, are reported. Lack of infrared bands broadening with increasing temperature indicated that molecular reorientational correlation time is longer than 10-12 s. The elastic peak of neutron inelastic scattering registered in the high temperature phase shows broadening, which can be well described by a model of 180° instantaneous stochastic jumps of protons around the two-fold axis formed by the Ca{single bond}O bond, assuming that only two out of four H2O ligands per [Ca(H2O)4]2+ formula unit undergoes fast reorientation with reorientational correlation time τR(H2O) of an order of 10-11 s. The analysis of the second moment M2 of 1H NMR line in the low temperature phase of [Ca(H2O)4](ClO4)2 revealed that the reorientation via 180° flips (jumps) of water molecules causes a very small changes of M2. It confirms that reorientation of H2O is fast even at 110 K. The reason for the first plateau of M2 ≈ 9.5 × 10-8 T2, which was observed at ca. 110-190 K range, is the anisotropic reorientation of whole [Ca(H2O)4]2+ cations around the two-fold symmetry axis going through two calcium atoms of the neighbouring [Ca(H2O)4]2+ cations. Above the phase transition at TC 4h, the anisotropic reorientation of [Ca(H2O)4]2+ (around the pseudo three-fold symmetry axis formed by the Ca...O direction between [Ca(H2O)4]2+ cation and ClO4- anion, lying along the hydrogen bridge bond O{single bond}H...O) is setting on with a frequency of the order of a few kHz, consequently above TC 1h one can observe next plateau of M2 ≈ 2.5 × 10-8 T2. © 2009 Elsevier B.V. All rights reserved.