Phase transition, structure and reorientational dynamics of H2O ligands and ReO4− anions in [Ba(H2O)3](ReO4)2⋅H2O

Abstract

One, low-temperature phase transition (PT) was detected for [Ba(H2O)3](ReO4)2⋅H2O, in the temperature range of 135–295 K by means of differential scanning calorimetry (DSC). The mean value of the PT temperature is TCh = 275.0 K (onset on heating) and TCc = 235.5 K (onset on cooling). Sharpness of the DSC peak as well as associated with this transformation hysteresis indicate that the detected PT is of the first-order type. X-ray single crystal diffraction (XRSCD) and neutron powder diffraction (ND) results showed that during the PT discovered at TCc the crystal structure of the studied compound does not change. Vibrational and reorientational motions of H2O ligands and ReO4− anions, in the high (I) and low (II) temperature phases, were investigated by Fourier transform middle-infrared and Raman light scattering spectroscopies (FT-MIR and RS). The temperature dependences of the full-width at half-maximum values (FWHM) of the band associated with δ(OReO) mode at 350 cm−1 in Raman spectra, suggests that the observed PT is not associated with a change of the perrhenate reorientational dynamics. However, the FT-MIR measurements showed that in the observed PT the reorientation dynamics of H2O ligands do contribute to the PT mechanism. Below the temperature of 225 K, a constant value of FWHM is observed, demonstrating the inhibition of reorientation of H2O molecules, and is associated only with so-called vibration relaxation. QENS measurements furnished evidence that H2O motion in high temperature phase (phase I) can be fairly accurately described by a simple model of 180° jumps around a twofold axis within a picosecond time scale with an assumption that three from four H2O molecules in [Ba(H2O)3]2+⋅H2O unit perform fast stochastic reorientations. Below PT QENS broadening is not observed. The QENS and IR methods clearly confirm that the observed PT is connected with slowing down of the reorientation dynamics of water molecules. The density functional theory plane wave calculations of the normal modes were performed in order to support band assignment. A good agreement between calculated and experimental data (IR and RS spectra) was obtained.