Phase transitions and water dynamics of [Mn(H2O)(6)](ClO4)(2) studied by differential scanning calorimetry and neutron scattering methods

DSC measurements performed at 95 - 290 K have shown that [Mn(H2O)6](ClO4)(2 ) possesses, besides a high-temperature phase, existing above 323 K, four l ow-temperature solid phases. The inelastic incoherent neutron scattering (I INS) spectra and neutron powder diffraction (NPD) patterns registered at 20 - 290 K have supported the DSC results and provided evidence that the inve stigated substance possesses even more than five solid phases. The IINS spe ctra have shown that in the room-temperature phase, water molecules perform fast stochastic reorientation at the picosecond scale. The orientational d isorder characteristic for the room-temperature phase can be easily overcoo led and frozen. Even by relatively slow cooling at ca. 40 K/hour a metastab le, orientational (protonic) glass phase is formed below ca. 160 K. Below c a. 100 K, a structural phase transition was observed by the NPD, however th e IINS spectra indicate existence of the pure ordered low-temperature phase only after annealing the sample for a few hours at 100 K. On heating, a st ructural phase transition takes place at ca. 120 K, and at ca. 225 K water molecules begin fast reorientation.