Zakład Fizyki Wysokich Ciśnień - publikacje

Abstract: Two solid phase transitions of [Cd(H 2O) 6](BF 4) 2 occurring on heating at T C2=183.3 K and T C1=325.3 K, with 2 K and 5 K hysteresis, respectively, were detected by differential scanning calorimetry (DSC). High value of entropy changes indicated large orientational disorder of the high temperature and intermediate phase. Nuclear magnetic resonance ( 1H NMR and 19F NMR) relaxation measurements revealed that the phase transitions at T C1 and T C2 were associated with a drastic and small change, respectively, of the both spin-lattice relaxation times: T 1( 1H) and T 1( 19F). These relaxation processes were connected with the "tumbling" motions of the [Cd(H 2O) 6] 2+, reorientational motions of the H 2O ligands, and with the iso- and anisotropic reorientation of the BF 4 - anions. The cross-relaxation effect was observed in phase III. The line width and the second moment of the 1H and 19F NMR line measurements revealed that the H 2O reorientate in all three phases of the title compound. On heating the onset of the reorientation of 3 H 2O in the [Cd(H 2O) 6] +2, around the three-fold symmetry axis of these octahedron, causes the isotropic reorientation of the whole cation. The BF 4 - reorientate isotropically in the phases I and II, but in the phase III they perform slow reorientation only about three- or two-fold axes. A small distortion in the structure of BF 4 - as well as of [Cd(H 2O) 6] 2+ is postulated. The temperature dependence of the bandwidth of the O-H stretching mode measured by Fourier transform middle infrared spectroscopy (FT-MIR) indicated that the activation energy for the reorientation of the H 2O did not change much at the T C2 phase transition. © 2004 Elsevier Inc. All rights reserved.