Symmetry crossover in layered $M{\mathrm{PS}}_3$ complexes $(M=\mathrm{Mn}, \mathrm{Fe}, \mathrm{Ni})$ via near-field infrared spectroscopy

We employ synchrotron-based near-field infrared spectroscopy to reveal the vibrational properties of bulk, few-sheet, and single-sheet members of the $M{\mathrm{PS}}_3$ $(M=\mathrm{Mn}, \mathrm{Fe}, \mathrm{Ni})$ family of materials and compare our findings with complementary lattice dynamics calculations. ${\mathrm{MnPS}}_3$ and the Fe analog are similar in terms of their symmetry crossovers, from $C2/m$ to $P\overline{3}1m$, as the monolayer is approached. These states differ as to the presence of a $C_3$ rotation around the metal center. On the other hand, ${\mathrm{NiPS}}_3$ does not show a symmetry crossover, and the lack of a $B_u$ symmetry mode near 450 ${\mathrm{cm}}^{-1}$ suggests that $C_3$ rotational symmetry is already present, even in the bulk material. We discuss these findings in terms of local symmetry and temperature effects as well as the curious relationship between these symmetry transformations and those that take place under pressure.

Figure 1

(a) Van der Waals gap size and sheet thickness as a function of metal-site substitution. (b) Schematic of an AFM cantilever tip directing light to the sample surface. (c) High-resolution AFM image of few-layer ${\mathrm{FePS}}_3$.

Figure 2

Near-field amplitude and phase spectra for ${\mathrm{MnPS}}_3$, ${\mathrm{FePS}}_3$, and ${\mathrm{NiPS}}_3$ single crystals at room temperature. Calculated mode positions and assignments are shown in the top panel. The overall scale focuses on the currently available near-field frequency window (330–650 ${\mathrm{cm}}^{-1}$) of interest.

Figure 3

(a) and (b) Close-up view of the near-field infrared response highlighting the behavior of the $B_u$ symmetry vibrational mode in the $M$ = Mn and Fe materials. Below $n$ = 11 (Mn) and $n$ = 10 (Fe), the data are multiplied by a factor of 5. The symmetry crossover is indicated, below which the $B_u$ symmetry feature disappears. (c) Frequency vs layer number trends for the $M$ = Mn and Fe compounds showing how the $B_u$ symmetry mode hardens on the approach to the symmetry crossover. The dotted lines are a 1/${\mathrm{size}}^2$ fit to the data points. The model is discussed in the Supplemental Material. Error bars are smaller than the symbol size. A schematic view of the displacement pattern of the $B_u$ symmetry vibrational mode is also included. The motion is a P-P stretch combined with in-phase, out-of-plane ${\mathrm{PS}}_3$ translation.

Figure 4

Close-up view of the near-field infrared response in the region of the $A_u+B_u$ modes for the (a) Mn and (b) Ni materials at 300 K. The symmetry crossover is indicated by the change in color (violet or blue to green). (c) Frequency vs layer number trend for ${\mathrm{MnPS}}_3$ across the $C2/m\to$ $P\overline{3}1m$ transition for both experiment (violet) and theory (green). Error bars are smaller than the symbol size. (d) Theoretically predicted frequency shift of ${\mathrm{MnPS}}_3$ across the pressure-induced $C2/m\to$ $P\overline{3}1m$ sliding transition. Inflection points are observed at the symmetry crossovers.