METAL-SEMICONDUCTOR TRANSITION IN YTTERBIUM AND STRONTIUM AT HIGH PRESSURE

It has been suggested that Yb and Sr undergo semimetal-to-semiconductor transitions under pressure. We have measured the electrical resistivity ρ from 298 to 2°K at different pressures up to 50 kbar. In Yb, ρ at 4.2°K increases by a factor of 6×104 at P=25 kbar and saturates at higher pressures. For P<25 kbar, we find the empirical relation at 4.2°K ln[ρ(P)ρ(1atm)]=0.45P, where P is the pressure in kbar. At P=25 kbar, the resistance ratio ρ(4.2K)ρ(298K)=220. In Sr the resistance rise is much less dramatic, ρ at 4.2°K increasing by a factor of 50 in 35 kbar. A small negative temperature coefficient of resistivity appears for P30 kbar in Sr. We suggest that Yb becomes a semiconductor at high pressures, whereas Sr remains a semimetal. These conclusions are compatible with the band-structure calculations of Vasvari, Animalu, and Heine for the fcc alkaline-earth metals, if the effects of spin-orbit coupling are included. We find no evidence for either an excitonic phase or a first-order transition in the neighborhood of the semimetal-to-semiconductor transition at low temperatures. However, no definitive statements can be made because of impurity effects. The Yb sample was 99.99% pure according to emission-spectrographic analysis. At higher pressures both Yb and Sr showed the transition to the bcc phase, and in this phase both materials behaved as a good metal and showed no evidence of a magnetic transition. © 1969 The American Physical Society.