Complex water dynamics in crystalline [Ca(H2O)2](ReO4)2, studied by the vibrational spectroscopy and proton magnetic resonance

Abstract

A 1H nuclear magnetic resonance study of [Ca(H2O)2](ReO4)2 has confirmed the existence of one phase transition at TCh=261.2 K (on heating) and TCc=231.2 K (on cooling), detected earlier by the DSC method. These transitions were reflected by changes in the temperature dependences of second moments of NMR line M2H. The study revealed anisotropic reorientations of whole [Ca(H2O)2]2+ cations, and the reorientations by 180° jumps of H2O ligands. It was found that the phase transition at TC is associated with the reorientation of H2O signed by oxygen O10 in crystal structure in high temperature phase. The oscillations of the cation as a whole unit around the axis crossing Ca and the middle of O9–O10 distance take place in both phases. From the temperature dependence of the full-width at half-maximum values (FWHM) of the bands connected with ρr (H2O) mode in IR spectra, we can conclude that the reorientational motions of H2O ligands contribute to the phase transition mechanism. The ligands perform fast (τR ≈ 10−11 to 10−13 s) stochastic reorientational motions in the high temperature phase. The estimated mean value of activation energy for the reorientation of the H2O ligands is ca. 11.1 kJ mol from Raman spectroscopy (RS) and 11.6 kJ mol from infrared spectroscopy, which is consistent with NMR results.The proton-weighted phonon density functions G(ν) obtained at 5 K show some separate peaks characteristic for ordered phase. Additionally, IR, RS and IINS spectra were calculated by the DFT method and an excellent agreement with the experimental data was obtained using CASTEP code.