Exploring Few and Single Layer CrPS₄ with Near-Field Infrared Spectroscopy

We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS₄ in bulk, few-, and single-layer form. Analysis of the mode pattern reveals a C2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS₃ where phonons shift as 1/size² and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P-P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap.