THz-TDS measurements of crystalline MgSO 4 •7H 2 O in the solid state A sample of MgSO 4 •7H 2 O (Sigma-Aldrich, Gillingham, UK) was ground using an agate mortar and a pestle, and then mixed with polyethylene (PE) powder to 2.5 w/w% by gentle mixing. The powder mixture was compressed into a pellet of 13 mm diameter using a hydraulic press (Specac Ltd., Kent, UK) at a load of 2 tons. A blank PE pellet was used as a reference. A terahertz time-domain spectrometer TeraPulse 4000 (Teraview, Cambridge, UK) was equipped with a cryostat (Janis, Massachusetts, USA) and an attached heater and controller (Lakeshore 330, Ohio, USA). The setup was capable of performing temperaturevariant transmission measurements. For each measurement, 1000 waveforms were acquired and averaged and a spectral resolution of 0.94 cm -1 was achieved per spectrum.