Pressure-induced metallization transition in Mg2Ge

Abstract

Mg2Ge with anti-fluorite structure at ambient pressure is characterized as a narrow band semiconductor and increasing pressure results in a decrease of the gap. In this work, the band structure of anti-fluorite Mg2Ge under high pressure is studied by first principles calculations, which suggests that Mg2Ge becomes metallic at 7.5 GPa as a result of band gap closure. The enthalpy difference between anti-fluorite phase and anti-cotunnite phase under high pressure is calculated by the first-principles plane-wave method within the pseudopotential and generalized gradient approximation. The results show that Mg2Ge undergoes a phase transition from the anti-fluorite structure to anti-cotunnite structure at 11.0 GPa. Then we investigate experimentally the pressure-induced metallization of Mg2Ge by electric resistance measurement in strip anvil cell and Raman spectroscopy by diamond anvil cell. The pressure distribution is homogeneous along the central line of the strip anvil and the pressure is changed ccontinuously by using a hydraulically driven two-anvil press. Raman scattering experiment is performed at pressure up to 21.1 GPa on a back scattered Raman spectrometer. The wavelength of excitation laser is 532 nm. No pressure-transmitting is used and pressure is determined by the shift of the ruby luminescence line. It is found that neither a discontinuous change of electrical resistance at 8.7 GPa nor Raman vibration modes of Mg2Ge appear above 9.8 GPa. The disappearance of the Raman vibration mode is ascribed to the metallization since the the free carrier concentration rises after metallization has prevented the laser light from penetrating into the sample. We compare these results with those of resistivity measurements in diamond anvil cell. Li et al.[2015 Appl. Phys. Lett. 107 142103] reported that Mg2Ge becomes metallic phase at 7.4 GPa and is transformed into metallic anti-cotunnite phase at around 9.5 GPa. We speculate that the discontinuous change in electric resistance at 8.7 GPa is ascribed to the gap closure of anti-fluorite phase and Mg2Ge may transform into the anti-cotunnite phase above 9.8 GPa.