Thermal properties of polycrystalline [Mn(NH3)6](ClO4)2

The X-ray powder diffraction (XRPD) pattern of [Mn(NH3)(6)](ClO4)(2) at room temperature can be indexed in the regular (cubic) system (Fmm space group) with lattice cell parameter a = 11.5729 and with four molecules per unit cell. The XRPD pattern at 100 K is evidently different, and it indicates lowering of the crystal structure. One phase transition at = 143.5 K (on heating) and at = 137.6 K (on cooling) was detected by DSC for [Mn(NH3)(6)](ClO4)(2) in the range of 120-300 K. The following thermodynamic parameters for phase I a dagger" phase II transition were obtained: a dagger H = 3.1 +/- A 0.2 kJ mol(-1) and a dagger S = 21.6 +/- A 0.3 J mol(-1) K-1. The large value of entropy changes (a dagger S) indicates considerable configurational disorder in the high temperature phase. The presence of 5.9 K hysteresis of the phase transition temperature at T (C) and sharpness of the heat flow anomaly attest that the detected phase transition is of the first-order type. Appearing of the two new bands (splitting of delta (s)(HNH)F (1u) mode) in FT-MIR spectra at the vicinity of 1200 cm(-1) suggests that the phase transition undergoes a change of the crystal structure.