Abstract
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.
Highlights
Work on complex van der Waals systems has begun to encompass materials beyond the scope of traditional transition metal dichalcogenides leading to the discovery of diverse monolayer systems [1, 2]
We compare our findings with complementary lattice dynamics calculations as well as prior work on the MPS3 family of materials (M = Mn, Fe, Ni) revealing surprising differences in symmetry behavior stemming, in part, from the lack of a phosphorus–phosphorus dimer in CrPS4
We compare our findings with complementary lattice dynamics calculations and prior studies on the MPS3 family of materials (M = Mn, Fe, Ni), revealing surprising differences in symmetry behavior
Summary
Work on complex van der Waals systems has begun to encompass materials beyond the scope of traditional transition metal dichalcogenides leading to the discovery of diverse monolayer systems [1, 2]. CrPS4 is a closely related analog that differs from the MPS3 (M = Mn, Fe, Ni) family in (1) stoichiometry as well as (2) lack of a phosphorusphosphorus dimer. The latter promotes dissimilar stacking patterns and different symmetry effects as well as distinct local structure. An oxidation state analysis indicates charges of S2 − , Cr3 + , and P5 + , different from the situation in MnPS3 where we have S2 − , Mn2 + , and infrared light
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