Vibrational and reorientational dynamics and thermal properties in [Mg(H2O)4](ReO4)2 supported by periodic DFT study

Abstract

The polymorphism of the [Mg(H2O)4](ReO4)2 compound was investigated for the first time by us by means of differential scanning calorimetry (DSC). The measurements were performed in the temperature range of 295–130K on cooling and heating of the sample at different rates. One reversible phase transition of the investigated compound has been found at: Tch=285.1K (onset on heating) and Tcc=256.5K (onset on cooling). The large thermal hysteresis of the phase transition temperature Tc equal to ca. 28.6K and the heat flow anomaly sharpness suggest that the detected phase transition is a first-order one. The moderate entropy change (ΔS≈4.3Jmol−1K−1) connected with observed phase transition indicates some kind of dynamical disorder of the high temperature phase.X-ray single crystal and neutron powder diffraction results revealed that the phase transition discovered atTCc is associated with a small change of the crystal structure.Vibrational and reorientational motions of H2O ligands and ReO4− anions, in the high (I) and low (II) temperature phases, were investigated by Fourier transform far and middle-infrared and Raman light scattering spectroscopies (FT-IR and RS). The temperature dependences of the full-width at half-maximum values (FWHM) of the bands associated with: 2δ(H2O) mode (at 3236cm−1 in IR spectra) and νas(HO) mode (at 3463cm−1 in Raman spectra), suggest that the observed phase transition is not associated with a change of the H2O reorientational motions. Analysis of the FWHM vs temperature of the Raman band at 345cm−1 connected with the δ(OReO) mode, indicate that the observed phase transition is not connected with a change of the ReO4− reorientational dynamics. From the IR and RS band shape analysis it was found that H2O ligands and ReO4− anions perform fast (correlation time scale, τR≈10−11–10−13s) motions in the both phases and their reorientational motions do not contribute to the phase transition mechanism. The estimated mean value of activation energy for H2O ligands and ReO4− anions in the both phases (high and low) is: Ea(I/II)=8.50±0.37kJmol−1 and Ea(I/II)=6.78±0.37kJmol−1, respectively. The density functional theory plane wave calculations of the normal modes within the periodic boundary conditions (CASTEP code) were also performed in order to support band assignment. We have obtained good agreement between calculated and experimental data (IR and RS spectra).