Figure 2.

(a) Temperature-dependent Raman spectra of bulk FePS₃ below $T_{\text{N}}$. The ${\text{ψ}}_4$ mode appears below 100 K and its frequency dramatically increases with decreasing temperature. (b) Frequencies of the various modes (relative to frequency at 15 K). While the other $\text{ψ}$ modes and phonon modes only shift by 1 cm⁻¹ to 2 cm⁻¹, as is expected by lattice anharmonic effects, the frequency of ${\text{ψ}}_4$ shifts by as much as 8 cm⁻¹ before it is no longer measurable above 100 K.