Intrinsic absorption threshold of stishovite and coesite

The optical absorption spectra of the small mono-crystals samples of stishovite and coesite were studied at first. The intrinsic absorption threshold of stishovite is determined at 8.75 eV, being probably, highest in the family of different crystalline polymorph modifications of silicon dioxide. The absorption spectrum of stishovite is independent of temperature (studied in the range 290–450 K). The intrinsic absorption threshold of coesite mono-crystal situated near 8.6 eV at 293 K, coincides within experimental errors with that of α-quartz crystal, and depends on temperature, as used to be for the tetrahedron structured silicon dioxide crystalline modifications. A broad absorption band with a first spread maximum near 7.6 eV sides with intrinsic absorption threshold was found in the stishovite mono-crystal sample. Its low intensity (about 10 cm−1) in an as-received sample shows on a defective nature of this band. No analogous band was detected in the sample of coesite.