Optical properties of single-cryst

We report reflectance measurements of single-crystal (Formula presented)(Formula presented)(Formula presented) over the frequency range between 30 (Formula presented) and 40 000 (Formula presented). Using crystals of 10 μm to 100 μm thickness we were also able to perform transmittance measurements at frequencies below the lowest phonon mode as well as between the highest phonon mode and the charge-transfer gap. The measurements were done on the (001) surface with ab polarization at temperatures between 10 K to 360 K. With decreasing temperature the phonon modes shift to higher frequencies; the 340 (Formula presented) stretching mode splits below 300 K. The most notable feature in the transmittance spectrum is a weak electronic absorption, possibly magnetic in origin. This feature is not resolved in the reflectance. At the charge-transfer gap there is an increase in the conductivity by four orders of magnitude. The charge-transfer gap energy increases with decreasing temperatures. In addition there is a temperature-dependent absorption band in the low-frequency tail of the fundamental absorption edge.