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 $\ensuremath{\rho}$ from 298 to 2\ifmmode^\circ\else\textdegree\fi{}K at different pressures up to 50 kbar. In Yb, $\ensuremath{\rho}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K increases by a factor of 6\ifmmode\times\else\texttimes\fi{}${10}^{4}$ at $P=25$ kbar and saturates at higher pressures. For $P<25$ kbar, we find the empirical relation at 4.2\ifmmode^\circ\else\textdegree\fi{}K $\mathrm{ln}[\frac{\ensuremath{\rho}(P)}{\ensuremath{\rho}(1 \mathrm{atm})}]=0.45 P$, where $P$ is the pressure in kbar. At $P=25$ kbar, the resistance ratio $\frac{\ensuremath{\rho}({4.2}^{\ensuremath{\circ}}\mathrm{K})}{\ensuremath{\rho}({298}^{\ensuremath{\circ}}\mathrm{K})}=220$. In Sr the resistance rise is much less dramatic, $\ensuremath{\rho}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K increasing by a factor of 50 in 35 kbar. A small negative temperature coefficient of resistivity appears for $P\ensuremath{\gtrsim}30$ 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.