Crystalline MgSO 4 •7H 2 O Reference terahertz spectra of crystalline MgSO 4 •7H 2 O in the solid state were acquired over a range of temperatures in order to characterise the vibrational features that are characteristic of the crystalline form. Three strong vibrational features were identified at 1.2, 1.6 and 2.8 THz in the spectral range that was accessible using the THz-TDS setup (Figure ).