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The electrical conductivity of black phosphorus has been measured as a function of temperature and pressure up to 350^ and 8000 kg/cm^2. The Hall constant of the same material has been measured as a function of temperature at atmospheric pressure. At low temperatures p-type impurity conduction is observed; at high temperatures the phosphorus is an intrinsic semiconductor with a gap width of 0. 33 ev. The mobilities at 27^ are 350 cm^2/volt sec and 220 cm^2/volt sec for the holes and electrons, respectively. Application of hydrostatic pressure decreases the gap at a rate VdWdV=8. 3 ev. The results are also interpreted in terms of a two-dimensional semiconductor model.
Robert W. Keyes (Sun,) studied this question.