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The far infrared optical constants of four crystalline materials at room temperature and at 1.5 K are reported. The materials are crystal quartz, sapphire, germanium, and silicon. The first two of these are birefringent (uniaxial) and both sets of optical constants are reported. The measurements extend over the range from 30 cm−1 to 350 cm−1.
Loewenstein et al. (1973) studied this question.