The measured optical constants for the principal directions in these crystals are presented for the infrared out to about 20 µ. A comparison is made, using an Argand diagram, with simple free-electron theory and it is concluded that such an interpretation is not applicable to the results. Interband transitions may be stronger at these wavelengths than was previously believed.
A. P. LENHAM and D. M. TREHERNE, "Optical Constants of Single Crystals of Mg, Zn, Cd, Al, Ga, In, and White Sn," J. Opt. Soc. Am. 56, 752-756 (1966)