Phase transitions and reorientational motions in [Al(OS(CH3)2)6](NO3)3 and its deuterated analog

Two phase transitions were found in solid [Al(OS(CH3)2)6](NO3)3 at TC1 = 343.1 K by differential scanning calorimetry and at TC2 = 34.3 K by adiabatic calorimetry. The transition at TC1 is accompanied by large thermal hysteresis (ca. 30 K) and a large entropy change (76 J K−1 mol−1), suggesting an order–disorder-type transition, whereas that at TC2 has a considerably smaller entropy change (3.4 J K−1 mol−1). The phase behavior of the deuterated compound is essentially the same. IR and Raman spectra reveal a reduction in symmetry below TC1 and a change in the reorientational dynamics of CH3 groups and NO3− anions, indicating that the highest temperature phase is orientationally disordered. The transition at TC2 can be explained by a librational soft-mode theory and all complex cations change their orientation in a cooperative manner.

► Phase transitions are found at 343.1 and 34.3 K for [Al(DMSO)6](NO3)3.
► The high-temperature transition is first-order and of the order–disorder type.
► Reorientational dynamics of CH3 and NO3− is involved in the mechanism.
► Deuteration shifts the transition temperature to 350.5 K.
► A librational soft-mode theory explains the low-temperature transition.