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Microcrystalline samples of BC8 silicon III and hexagonal silicon IV, grown under high pressure in the diamond anvil cell, remain metastable at ambient pressure. Hall-effect and low-temperature resistivity measurements show Si III to be a hole semimetal with p510^20 cm^-3. Photoconductivity data show that Si IV, like Si I, is an intermediate-gap semiconductor. The observed behavior confirms theoretical predictions on these two new elemental materials.
Besson et al. (Mon,) studied this question.