Pressure-induced metallization in the absence of a structural transition in the layered ferromagnetic insulator ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$

We report the crystallographic and electrical transport properties of single crystals of the ferromagnetic two-dimensional (2D) material ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ 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 $R\overline{3}$ phase of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ without any structural phase transitions up to 20 GPa. Our data confirm a distinct and irreversible crystalline-amorphous transformation in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. 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 ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ 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 ${\mathrm{CrTe}}_6$ 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 ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ sample are sensitive to the hydrostatic environments and demonstrates how pressure can be used to tune the physical properties of 2D ferromagnetic compounds.

Figure 1

(a) Lattice parameters of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ as a function of pressure. The inset indicates the pressure dependence of the $c/a$ ratio. (b) The third-order Birch-Murnaghan equations of states fit the unit-cell volume data. The inset displays a bulk single crystal of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ compressed in helium at 0.4 GPa. The powder x-ray data using NaCl as the PTM are indicated as unfilled symbols. (c) Changes in bond distances under compression. The coordination environments of Cr1 and Ge1 atoms are shown in the inset. (d) Evolution of Te-Cr-Te and Te-Ge-Te angles under pressure.

Figure 2

(a) Crystal structures of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ at 0.4 GPa viewed along the [010] and [001] directions. (b) Structures at 19.5 GPa viewed along the same directions as 0.4 GPa. (c) Single layer of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ viewed approximately along the $c$ axis. The edge-sharing ${\mathrm{CrTe}}_6$ octahedra are indicated.

Figure 3

(a) Electrical resistance of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ in the crystalline phase range (below 20 GPa). The inset indicates the data measured in a different single crystal at ambient pressure. (b) Electrical resistance data in the amorphous phase above 20 GPa. The inset shows the photograph of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ single crystal immersed in the NaCl medium at 0.9 GPa using the four-terminal method for the measurements. (c) Pressure-dependent electrical resistances of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ at room temperature. (d) $P\text{−}T$ phase diagram ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ based on the x-ray and electrical resistance data.

Figure 4

Calculated electronic band structures and density of states of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ at (a) 0.4 GPa, (b) 3.2 GPa, and (c) 6.5 GPa.