Crystal structure, solid-solid phase transition and thermal properties of [Mn(H₂O)₂](ReO₄)₂

Abstract

Using differential scanning calorimetry, one phase transition at  = 285.0 K (on heating) and  = 284.5 K (on cooling) was detected for [Mn(H₂O)₂](ReO₄)₂ in the temperature range 120–295 K. Sharpness of the heat flow anomaly and thermal hysteresis associated with this anomaly suggest that the detected phase transition is a first-order one. The following thermodynamic parameters for phase II ↔ phase I transition were obtained: ΔH ≈ 2.0 kJmol⁻¹ and ΔS ≈ 6.9 Jmol^-1 K⁻¹. The latter quantity is close to Rln2, indicating some degree of molecular dynamical (configurational) disorder of the high temperature phase. X-ray single crystal diffraction and neutron powder diffraction results revealed that the phase transition discovered at is associated also with a change of the crystal structure from monoclinic (space group: I2/m, No. 12) to triclinic (P-1, No. 2). Moreover one hydrogen is disordered between two possible positions in the high temperature phase. Far infrared absorption spectra registered on cooling indicate at the vicinity of splitting of some degenerate vibrational modes. Additionally, one can observe a few new bands in the wavenumber ranges of 500–350 and 170–125 cm⁻¹.

Keywords:

• Diaquamanganese(II) rhenate(VII)
• thermal decomposition (TG/DTG/QMS)
• phase transition
• X-ray single crystal and neutron powder diffraction (XRSCD, NPD)
• Fourier transform far infrared spectroscopy (FT-FIR)

Acknowledgements

The X-ray structure was determined in the Advanced Crystal Engineering Laboratory (aceLAB) at the Chemistry Department of the University of Warsaw by Dr Łukasz Dobrzycki. We would like to thank MScEng. Magdalena Malik from Wrocław University of Science and Technology for Raman measurements. The infrared absorption (FT-MIR and FT-FIR) research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). The project was supported by grant of the Polish Plenipotentiary to JINR and JINR Directorate from 27.01.2015, No. 47, p. 17.