Low-temperature phase transitions in [Cd(DMSO)6](BF4)2 studied by differential scanning calorimetry, X-ray single crystal diffraction and infrared absorption spectroscopy

• We examine phase polymorphism of [Cd(DMSO)6](BF4)2.
• The crystal structures of Cr 3 and Cr 1 phase are different.
• The fast reorientational motions of the BF4− and CH3 are slowed down below 218 K.

The differential scanning calorimetry (DSC) measurements made for [Cd(DMSO)6](BF4)2, in the temperature range of 113–298 K revealed existence of two low-temperature solid–solid phase transitions: phase Cr 3 ↔ phase Cr 2 at Tc2c = 218 K and phase Cr 2 ↔ phase Cr 1 at Tc1c = 246 K. X-ray single crystal diffraction studies of [Cd(DMSO)6](BF4)2 have shown that these transitions are related to a crystal symmetry reduction from an orthorhombic crystallographic system (Fdd2, No. 43) to a monoclinic one (Cc, No. 9). The [Cd(DMSO)6](BF4)2 compound undergoes also series of reversible high temperature phase transitions but they are not a subject of this work and will be presented in our next paper. The characteristic changes of the FT-FIR, FT-MIR and FT-RS spectra of [Cd(DMSO)6](BF4)2 at the phase transitions’ temperatures confirmed that phase transitions phase Cr 3 ↔ phase Cr 2 ↔ phase Cr 1 are related to the crystal structure change. It was also found that the reorientation of the BF4− anions and DMSO ligands freezes below 218 K.

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