Pressure-Induced Electronic and Structural Phase Evolution in the van der Waals Compound ${\mathrm{FePS}}_3$

Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. Here we present work on a new metal-to-insulator transition system ${\mathrm{FePS}}_3$. This compound is a two-dimensional van der Waals antiferromagnetic Mott insulator. We report the discovery of an insulator-metal transition in ${\mathrm{FePS}}_3$, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure, and resistivity measurements show evidence of the onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.

Figure 1

Integrated diffraction patterns. The data have been scaled to the low angle background (giving arbitrary intensity, see right $y$ axis) and then the offset set to the pressure at which the data were collected (shown on the left $y$ axis). The 0.0 GPa data have been truncated for the two highest intensity peaks to allow all the patterns to be plotted together. The two phase transitions can be seen to take place over the region colored blue (PT1) and that colored black (PT2). The three patterns that we identified as being monophase are labeled with their pressures. The wavelength of the x rays was $\lambda =0.4246\AA$.

Figure 2

Schematics showing the evolution of the structure of ${\mathrm{FePS}}_3$ with pressure. The three refined structures at their corresponding pressures are drawn to the same scale and with respect to the lattice parameters for the structures given in Tables S2–S4 in the Supplemental Material [31]. The Fe atoms are shown in brown, the P atoms are shown in purple, and the S atoms are shown in yellow. The views show different projections of the same number of unit cells; hence the “sulfur” only figures show only those sulfurs between two adjacent $ab$ planes. Also shown are all interatomic bonds for $r\le 3.6\AA$. The illustrations were created using the vesta software [37].

Figure 3

Resistivity $\rho$ of ${\mathrm{FePS}}_3$ plotted against temperature at ambient pressure. The inset plots $\ln (\rho )$ against the reciprocal of temperature, showing good agreement with thermally activated Arrhenius-type behavior.

Figure 4

Resistance of ${\mathrm{FePS}}_3$ against temperature for 4 increasing pressures, estimated as (a) 4.0 GPa, (b) 4.5 GPa, (c) 5.5 GPa, and (d) 22.5 GPa. A transition from insulating to metallic behavior is seen as pressure is increased, as well as an upturn in the resistivity at low temperatures in the high-pressure measurements.

Figure 5

Resistivity of ${\mathrm{FePS}}_3$ plotted against temperature, at pressures from an estimated 3.0 GPa (blue, topmost) to 13.5 GPa (red) in a Bridgman anvil cell—reproducing the data shown in Fig. 4. The resistivity is drastically suppressed with applied pressure—note the logarithmic axis, and an upturn seen in the higher-pressure data.

Figure 6

Detail of Fig. 5. Resistivity of ${\mathrm{FePS}}_3$ plotted against temperature, at pressures from an estimated 11.0 GPa (blue, topmost) to 13.5 GPa (red) in a Bridgman anvil cell. Inset: Low-temperature data at 12.0 GPa, which levels off or saturates at the lowest temperatures.