Properties of the Group VI B Elements under Pressure. II. Semiconductor-to-Metal Transition of Tellurium

The electrical resistance of tellurium was measured at pressures to 60 kbar and temperatures to 600°C in a tetrahedral anvil device. A semiquantitative resistance-pressure-temperature phase diagram was determined, showing three solid polymorphs and the liquid phase. The room temperature Te I-II transition pressure was found to be at about 43 kbar. The Te I-II phase boundary has a slope of -30°C/kbar. The triple point for coexistence of Te I, Te II, and liquid Te is at about 29 kbar and 445°C. The semiconductor-to-metal nature of the Te I-II phase change is well established by the isobaric temperature characteristics of the resistance. The magnitude of the resistance change at the Te I-II phase transformation was found to decrease significantly with increasing temperature. At room temperature the resistance of Te I just above the transition is smaller than the atmospheric-pressure resistance of Te by a factor of about ${10}^3$. The locus of points defining the pressure dependence of the electronic energy gap $E_g$ of Te I, determined from the resistance measurements, is convex toward the pressure axis, $E_g$ decreasing monotonically from 0.33 eV at atmospheric pressure to zero at the Te I-II phase boundary. The slope of this curve is about -0.017 eV/kbar at atmospheric pressure. The experimental data are used with theoretical expressions to calculate the electronic contribution to the thermal conductivity and an upper bound on the thermoelectric figure of merit as a function of pressure. The pressure-induced approach to the metallic state of the Group VI B elements is discussed.