Dynamics of H 2 O ligands and ReO 4 anions at the phase transition in MnH 2 O 2 ReO 4 2 studied by complementary spectroscopic methods

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EarlyView Article

  • Published: Nov 13, 2017
  • Author: Joanna Hetmańczyk, Łukasz Hetmańczyk
  • Journal: Journal of Raman Spectroscopy

Abstract

The vibrational and reorientational motions of H2O ligands and ReO 4 anions in high‐ and low‐temperature phases were investigated by means of Fourier transform far‐ and mid‐infrared and Raman light‐scattering spectroscopy and neutron scattering (inelastic/quasi‐elastic incoherent neutron scattering) methods. The dynamics of H2O and ReO 4 molecules in high‐ (I) and low‐temperature (II) phases was investigated by means of band shape analysis performed for Raman and IR bands. The temperature dependencies of full width at half maximum values of the Raman bands at 874 (νas(ReO)) and 943 cm−1s(ReO)) exhibit changes in their behaviour in the vicinity of transformation, suggesting that the observed phase transition is connected with a change in the reorientational dynamics of ReO 4 . However, anions also perform fast (τR ≈ 10−12 to 10−13 s) stochastic reorientational motions in Phase II. The estimated mean value of activation energy for ReO 4 anions in the low‐temperature phase is Ea(II) = 8.1 kJmol−1. In infrared band shape analysis it was found that H2O ligands perform fast (picosecond correlation time scale) motions in both phases with a mean value of activation energies of 7.68 kJmol−1. These reorientational motions of H2O ligands do not contribute to the phase transition mechanism. Moreover, the observed phase transition was accompanied by the splitting of some bands, suggesting a reduction in crystal structure symmetry.

Quasi‐elastic neutron scattering measurements furnished evidence that H2O motion in Phase I can be fairly accurately described by a simple model of 180° jumps around a twofold axis within a picosecond time scale. Additionally, infrared, Raman light‐scattering spectra were calculated using the density functional theory method for the isolated equilibrium model Mn(H2O)2(ReO4)2, and qualitative agreement with the experimental data was achieved. The theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions using the VEDA 4 program.

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