Pressure-Induced Metallization of Molybdenum Disulfide

X-ray diffraction, Raman spectroscopy, and electrical conductivity measurements of molybdenum disulfide ${\mathrm{MoS}}_2$ are performed at pressures up to 81 GPa in diamond anvil cells. Above 20 GPa, we find discontinuous changes in Raman spectra and x-ray diffraction patterns which provide evidence for isostructural phase transition from $2H_c$ to $2H_a$ modification through layer sliding previously predicted theoretically. This first-order transition, which is completed around 40 GPa, is characterized by a collapse in the $c$-lattice parameter and volume and also by changes in interlayer bonding. After the phase transition completion, ${\mathrm{MoS}}_2$ becomes metallic. The reversibility of the phase transition is identified from all these techniques.

Figure 1

Synchrotron x-ray diffraction patterns of ${\mathrm{MoS}}_2$ during the pressurization from ambient conditions to 81.0 GPa ($\lambda =0.3344\AA$). Arrows indicate a new peak, which appears at the phase evolution. The observed and Rietveld refined diffraction patterns at 1.5 and 81.0 GPa are shown for the $2H_c$ and $2H_a$ phases. The open circles represent the measured intensities, and the red lines are the results from profile refinements. The patterns shown in red correspond to the mixed phase region. The positions of the Bragg reflections are marked by vertical sticks and the signal from Au in olive sticks. The $R$ values are $R_p=0.34\%$, weighted profile $R_{\mathrm{wp}}=0.53\%$ at 1.5 GPa and $R_p=0.69\%$, weighted profile $R_{\mathrm{wp}}=1.15\%$ at 81.0 GPa.

Figure 2

Pressure dependence of the lattice parameters (upper panel) and volume per formula unit (lower panel) of two ${\mathrm{MoS}}_2$ samples in both $2H_c$ and $2H_a$ phases in both the compression (denoted by $\_c$) and decompression (by $\_d$) run. The experimental data points from Webb et al. [27] and Aksoy et al. [29], and the theoretical results of Hromadová et al. [24] are plotted for comparison. The arrows in the upper panel indicate the corresponding axes. The solid lines demonstrate the fitting data with respect to the equation of states of $2H_c$ and $2H_a$. The insets illustrate the atomic arrangement of the $2H_c$ and $2H_a$ structures where Mo is represented by small spheres and S by large spheres.

Figure 3

Raman spectra of ${\mathrm{MoS}}_2$ at various pressures up to 57 GPa in both the compression (denoted by $c$) and decompression (denoted by $d$) run. The insets represent in-plane phonon modes $E_{2g}^2$, $E_{1g}$, and $E_{2g}^1$ and the out-of-plane phonon mode $A_{1g}$.

Figure 4

Phonon frequencies of ${\mathrm{MoS}}_2$ as a function of pressure with in-plane $E_{2g}^2$ (a), $E_{1g}$ (b), and $E_{2g}^1$ modes and the out-of-plane $A_{1g}$ mode (c) and the frequency differences (d) with respect to the corresponding modes. The solid and open symbols are the data for the compression (denoted by ${}_{-}c$) and decompression (by ${}_{-}d$) run, respectively. The experimental (Refs. [30, 39]) and theoretical (Ref. [24]) data points were shown for comparison. $2H_c$ and $2H_a$ are the low-pressure and high-pressure phases, respectively.

Figure 5

Electrical resistivity of ${\mathrm{MoS}}_2$ as a function of pressure at room. Insets: Left bottom is the photograph of the sample in the diamond anvil cell with electrical leads at 5 GPa. Upper-right panel shows the temperature dependence of the resistivity of another sample at 28 and 60 GPa.