Magnetic susceptibility of pure boron

The magnetic susceptibility of various β‐rhombohedral boron samples has been measured between 4 and 1300 °K. An unusual paramagnetic component, with an experimental number of centres dependent on the temperature of the form N = 2 N0 exp { ‐ E/kT}, is observed above 500 °K. The obtained experimental values are: ΔE≈ 0.41 eV, N0≈ ss 1.0 X 1022 cm−3. ΔE is assumed to be the consequence of a gap of about 0.82 eV between two narrow bands which do not seem to contribute to the conductivity at least for high temperatures (T > 500 °K). Moreover a second contribution is observed increasing linearly above 300 °K with rising temperature and becoming less diamagnetic. The slope of the second signal versus temperature is 2.8 x 10−11 cm3 g−1 deg−1. A Curie paramagnetic contribution appears below 77 °K, which can be accounted for by a part (5 ppm) of the residual impurities.