The THz-TDS spectra were acquired using a commercial TeraPulse 4000 instrument across the spectral range of 0.2 -2.2 THz (TeraView, Cambridge, UK). The sample temperature (90 -360 K) was controlled using a continuous flow cryostat with liquid nitrogen as the cryogen (Janis ST-100, Wilmington MA, USA) as outlined previously. The cryostat cold finger accommodated both the reference (two z-cut quartz windows) as well as the sample (quartz/sample/quartz sandwich structure as described above). The two z-cut quartz windows that were used for the reference (same thickness and diameter dimensions as sample)