Optical transmission was measured with a Perkin Elmer FTIR spectrometer in the range 1.8–25 μm using samples of approximately 1 mm thickness. Refractive index measurements were done using a J.A. Woollam Variable Angle Spectroscopic Ellipsometer (VASE) in the wavelength range 290–1700 nm. At least three measurements were done on these samples and the average value was computed as a final result. Error bars indicate the scatter in the data. The Raman spectra were measured using a Horiba Jobin Yvon double monochromator Raman spectrometer at room temperature using an Ar-ion laser line at 488 nm.