Metal-Semiconductor Transition in Ytterbium and Strontium at High Pressure

Abstract

It has been suggested that Yb and Sr undergo semimetal-to-semiconductor transitions under pressure. We have measured the electrical resistivity $\rho$ from 298 to 2°K at different pressures up to 50 kbar. In Yb, $\rho$ at 4.2°K increases by a factor of 6×${10}^4$ at $P=25\mathrm{}$ kbar and saturates at higher pressures. For $P<\mathrm{}25\mathrm{}$ kbar, we find the empirical relation at 4.2°K $\ln \left[\frac{\rho \mathrm{}\left(P\right)\mathrm{}}{\rho \left(1\mathrm{} \mathrm{atm}\right)}\right]=0.45\mathrm{} P$, where $P$ is the pressure in kbar. At $P=25\mathrm{}$ kbar, the resistance ratio $\frac{\rho \left({4.2}^{\circ }\mathrm{K}\right)}{\rho \left({298}^{\circ }\mathrm{K}\right)}=220\mathrm{}$. In Sr the resistance rise is much less dramatic, $\rho$ at 4.2°K increasing by a factor of 50 in 35 kbar. A small negative temperature coefficient of resistivity appears for $P\gtrsim 30$ 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.