Solid‐phase transitions in TlHSO 4 crystal

Polycrystalline TlHSO 4 was investigated by calorimetry (253–423 K) and by infrared (10–300 K) and Raman (10–420 K) spectroscopy. Two phase transitions are detected at 320 and 388 K. A complete assignment of the spectra of the room‐temperature phase (III) is given and the spectroscopic results are consistent with infinite chains of hydrogen bonds. This structure appears disordered above 320 K (phase II). The high‐temperature phase (I) presents the characteristic features of a plastic phase according to the spectroscopic manifestations of an increasing orientational disorder of sulphate ions. Above 360 K, a metastable phase (I′) showing a complex hydrogen‐bonded framework is identified.