Pressure-induced metallization in the absence of a structural transition in the layered ferromagnetic insulator Cr2Ge2Te6

We report the crystallographic and electrical transport properties of single crystals of the ferromagnetic twodimensional (2D) material Cr2Ge2Te6 under high pressure. In contrast to previous studies, our high-pressure single-crystal x-ray diffraction under hydrostatic conditions shows prominent anisotropic compressibility in the layered structure of the crystalline R3 over bar phase of Cr2Ge2Te6 without any structural phase transitions up to 20 GPa. Our data confirm a distinct and irreversible crystalline-amorphous transformation in Cr2Ge2Te6. The loss of crystallinity starts at 20 GPa; however, the crystalline phase and amorphous state coexist even at the maximum pressure of 31.2 GPa. High-pressure powder x-ray diffraction data and electrical resistivity measurements of Cr2Ge2Te6 using NaCl as the pressure-transmitting medium reveal an insulator-to-metal transition in the absence of a phase transition at -3.9 GPa; at a considerably lower pressure than the previously reported (7-14 GPa). Density functional theory calculations demonstrate the density of states around the Fermi level are primarily dominated by Cr 3d and Te 5p states. Hence the large reduction of Cr-Te bond lengths within the CrTe6 octahedra under compression is most likely responsible for the band-gap closure. This study clarifies that the phase stability and onset of metallization pressure in the Cr2Ge2Te6 sample are sensitive to the hydrostatic environments and demonstrates how pressure can be used to tune the physical properties of 2D ferromagnetic compounds.