The X-ray powder diffraction (XRPD) pattern of [Mn(NH 3 ) 6 ](ClO 4 ) 2 at room temperature can be indexed in the regular (cubic) system (Fm 3m space group) with lattice cell parameter a

11.5729 A ̊ and with four mole- cules per unit cell. The XRPD pattern at 100 K is evidently different, and it indicates lowering of the crystal structure. One phase transition at T h C1

143.5 K (on heating) and at T c C1

137.6 K (on cooling) was detected by DSC for [Mn(NH 3 ) 6 ](ClO 4 ) 2 in the range of 120–300 K. The fol- lowing thermodynamic parameters for phase I $ phase II transition were obtained: D H

3.1 ± 0.2 kJ mol

1 and D S

21.6 ± 0.3 J mol

1 K

1 . The large value of entropy changes ( D S ) indicates considerable configurational disor- der 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 d 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.