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(1 atm)]=0.45 P, where P is the pressure in kbar. At $P=25$ kbar, the resistance ratio ρ(4.2^∘K)ρ(298^∘K)=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.