Exploring few and single layer CrPS₄ with near-field infrared spectroscopy

Abstract

We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS 4 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 MnPS 3 where phonons shift as 1/size 2 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.

Controlling Spin Orientation and Metamagnetic Transitions in Anisotropic van der Waals Antiferromagnet CrPS4 by Hydrostatic Pressure

“…Therefore, we conducted a temperature‐dependent neutron diffraction experiment with the two typical diffraction patterns shown in Figure 1a,b at 20 and 35 K, respectively. The magnetic reflections at 20 and 35 K can both be indexed with the k = (0, 0, 0.5) propagation vector in the nonmagnetic unit cell of C 2 space group [ 31,32,38,39 ] (Note S1, Supporting Information), consistent with that of 1.7 K, indicating no magnetic group change. However, two magnetic structures are symmetry‐allowed with spins in the ac plane or along the b axis.…”

Metal Site Substitution and Role of the Dimer on Symmetry Breaking in FePS3 and CrPS4 under Pressure

“…The distortion away from C 2/ m is very weak. We note in passing that calculations require spin–orbit coupling to stabilize C 2 symmetry . In any case, the frequency vs pressure trends in CrPS 4 reveal two critical pressures: P C1 = 5.5 and P C2 = 16 GPa.…”

“…Before presenting our first-principles results, we comment first that the C 2 → C 2/ m transition at P C1 is subtle and somewhat difficult to capture within DFT, so here, we focus on the second transition at P C2 . Second, electron correlations have been incorporated with DFT calculations via two different flavors: (1) a rotationally invariant and simplified form of the DFT + U eff exchange-correlation functional and (2) a numerically efficient variant of the Strongly Correlated and Appropriately Normed (SCAN) semilocal functional as dubbed r 2 SCAN .…”

“…In addition to the many members of the M PS 3 family of materials, there is another layered transition metal compound that has attracted recent attention. CrPS 4 is a direct gap semiconductor with a 1.4 eV gap and an A-type antiferromagnetic order below the 36 K Néel temperature. ,− Due to its chemical composition, CrPS 4 is commonly included in structure–property comparisons with the M PS 3 compounds. , But a closer look at the crystal structure reveals several significant differences compared to the M PS 3 family Figure (a,b) displays the crystal structures of FePS 3 and CrPS 4 at ambient conditions.…”

“…20,32−34 Due to its chemical composition, CrPS 4 is commonly included in structure−property comparisons with the MPS 3 compounds. 11,35 But a closer look at the crystal structure reveals several significant differences compared to the MPS 3 family. 34 Figure 1(a,b) displays the crystal structures of FePS 3 and CrPS 4 at ambient conditions.…”