Polycrystalline silicon, windows

Polycrystalline silicon and germanium are difficult to obtain (monocrystalline materials, especially germanium, are much more readily available but they rupture by cleavage under stress). Moreover, since they are opaque in the visible, they do not allow safe seating of the windows (see below). 

Above 2000 cm, and up to 50 000 cm, with the maximum accessible wavenumber depending on the quality of the specific single-crystalline material, sapphire is unrivalled as a high-pressure, high-temperature, window material. Moreover sapphire can withstand appreciable chemical attack so long as the medium remains neutral or acidic. Additional and more detailed information about high-pressure window materials has been given by Ferraro and Basile and by Sherman and Stadtmuller. The optical cells described subsequently are usually equipped with sapphire windows. They are thus sufficiently transparent from 2000 to 50 (XX) cm. When fitted with windows of polycrystalline silicon, most of these cells have also been used in an extended... 

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