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† Corresponding author. E-mail:
We report a comprehensive Raman scattering study on layered MPS3 (M=Mn, Fe, Ni), a two-dimensional magnetic compound with weak van der Waals interlayer coupling. The observed Raman phonon modes have been well assigned by the combination of first-principles calculations and the polarization-resolved spectra. Careful symmetry analysis on the angle-dependent spectra demonstrates that the crystal symmetry is strictly described by C2h but can be simplified to D3d with good accuracy. Interestingly, the three compounds share exactly the same lattice structure but show distinct magnetic structures. This provides us with a unique opportunity to study the effect of different magnetic orders on lattice dynamics in MPS3. Our results reveal that the in-plane Néel antiferromagnetic (AF) order in MnPS3 favors a spin–phonon coupling compared to the in-plane zig-zag AF in NiPS3 and FePS3. We have discussed the mechanism in terms of the folding of magnetic Brillouin zones. Our results provide insights into the relation between lattice dynamics and magnetism in the layered MPX3 (M=transition metal, X=S, Se) family and shed light on the magnetism of monolayer MPX3 materials.
One of the current research interests in two-dimensional (2D) materials focuses on magnetic van der Waals (vdW) materials, due to the intriguing physical properties for both fundamental research and potential applications in optoelectronics, spintronics, and valleytronics. A number of potentially magnetic single-layer vdW materials have recently been proposed, including V-based dichalcogenides, CrGeTe3 ternary tritellurides, and CrX3 trihalides. Studies on CrI3 reveal that it has a stable ferromagnetic (FM) ordering in 2D form and a little change of layer-number dependence of Curie temperatures.[1–5] This has attracted a lot of attention in other magnetic vdW crystals that are similar to CrI3.[6–12]
Recently, many studies have focused on the magnetic van der Waals crystals MPX3, which resemble similar lattice structure and chemical composition.[13–16] MPX3 is a family of 2D vdW layered crystals that have been prepared and studied for two decades. The structure of bulk MPS3 is similar to CrI3 which adopts the monoclinic AlCl3 structure (point group C2h). The layer structure is anchored by (P2X6)4− bipyramids in a triangular lattice which provide enclosures for transition metal (TM) atoms arranged in a hexagonal array.[17–23] The monolayer MPS3 has a perfect honeycomb lattice similar to graphene. For magnetism in MPS3, the intralayer magnetic structures are antiferromagnetic (AF). However, depending on the variety of TM atoms, magnetic structures possess very rich types. For MnPS3 material, the AF phase transition temperature TN is 78 K. The magnetic moment is 3.5 **
Raman is a useful tool to study the structure and magnetic properties of ultrathin vdW single crystals. Meanwhile, there are several spectroscopic studies in FePS3 and NiPS3. In the previous studies of thin layer FePS3 and NiPS3, two different point groups, C2h and D3d, have been proposed when discussing the phonon modes and the lattice structure.[13,14] However, no consensus has yet been reached. The information on MnPS3 is still lacking. A comparative study of the phonon spectra in MPS3 (M=Mn, Fe, Ni) is important for the clarification of this issue. Moreover, the temperature dependence behaviors of magnetism in MPS3 have not been studied or discussed in detail. Therefore, this calls for a systematic research of the phonon spectra and magnetic properties in MPS3 materials with different transition metals.
In this paper, we present temperature-dependent and polarization-resolved Raman spectroscopy study on layered MPS3 (M=Mn, Fe, Ni) single crystals. Almost all of the Raman active phonons are observed, and the phonon mode assignments are made by symmetry analysis and comparison with first principle calculations. In particular, we conduct a comparative study of Raman spectra in MnPS3, FePS3, and NiPS3, which share identical lattice structure but exhibit distinct magnetic order, and their temperature evolution crosses the magnetic transition. Consequently, we provide comprehensive information on the relation of the lattice dynamics and magnetism in the MPS3 system.
Single crystals of MPS3 (M=Mn, Fe, Ni) were grown by chemical vapor transport (CVT) method. The crystal naturally cleaves along the (001) surface, forming MnPS3 flakes weakly bonded by van der Waals force. In this experiment, the MnPS3 flakes were cleaved from bulk single crystals, transferred on to silicon substrates, and capped with 90 nm silicon oxide (SiO2) because of the transparent characteristic. The SiO2 capping layer does not induce any extra signal in the energy range we are interested in and can enhance Raman scattering signal intensity from the sample. The FePS3 and NiPS3 single crystals were tested on the copper holder directly. The temperature-dependent and polarization-resolved Raman spectra were collected in a backscattering configuration using an HR800 spectrometer (Jobin Yvon) equipped with a liquid-nitrogen-cooled charge-coupled device (CCD) and volume Bragg gratings. The samples were placed in an ultra high vacuum (UHV) cryostat with a vacuum of **
First-principles calculations were employed to work out the Brillouin zone-center (Gamma point) phonon modes of MnPS3. The projector augmented wave (PAW) method implemented in the Vienna ab initio simulation package (VASP) package was used to describe core electrons.[24–28] For the exchange–correlation potential, the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof formula was adopted.[29] To describe the van der Waals interaction in layered systems not included in the conventional density functional theory, the vdW-optB86b functional was chosen.[30] The on-site Coulomb repulsion among the localized Mn 3d electrons was also included by using the formalism (GGA+U) of Dudarev et al.[31] with an effective U = 5 eV. The kinetic energy cutoff of the plane-wave basis was set to be 300 eV. The simulations were carried out with a triclinic cell containing two Mn atoms, two P atoms, and six S atoms, in which two Mn atoms took the antiferromagnetic order. An 8×8×8 k-point mesh for the Brillouin zone sampling and the Gaussian smearing with a width of 0.05 eV around the Fermi surface were employed. In structure optimization, both cell parameters and internal atomic positions were allowed to relax until all forces were smaller than 0.01 eV/Å. When the equilibrium structure was obtained, the phonon modes at Brillouin zone center were calculated by using the dynamic matrix method. The calculations with a 10-atom cell gave 27 optical modes. However, to illustrate the displacement patterns of phonon modes, we show our results in a 20-atom supercell deduced from the real-space translational invariance of the 10-atom cell; as in [32].
The MPS3 crystal has a monoclinic structure with C2h point group (space group C2/m). With 10 atoms in a primitive unit cell, we have a total of 30 phonon modes. At the **
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In Figs.
To find the origin of the remaining four phonon modes, we consider a change of the lattice symmetry in the 2D van der Waals materials. For monolayer MnPS3, the lattice point group is D3d. At the **
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The bulk crystals consist of ABC-stacked single layer assemblies that are held together by van der Waals forces. Thus, for bulk crystals, the C3 symmetry along the c axis has broken compared to the monolayer crystals. Because of the symmetry breaking, the Eg mode in D3d symmetry will degenerate into Ag and Bg modes which have similar energies in C2h symmetry. Based on the discussion above, for MnPS3, the observed phonons are: 115.5 cm−1 (
To relate the observed modes at certain frequencies to the specific atomic displacements, we conduct the first-principles calculations. The experimental and calculated mode frequencies are summarized in Table
![]() | Table 1.
Calculated and experimental optical phonon modes (in cm−1) for MnPS3. The atomic motions of the modes are also given (see Fig. |
Using the same analysis method for MnPS3, we determine the phonon modes for FePS3 and NiPS3. As shown in Fig.
In Fig.
We report the experimental and first-principles calculation for the temperature dependent and polarization resolved Raman spectroscopy on the lattice dynamics of MPS3 (M=Mn, Fe, Ni) single crystals. We identify 7 out of 11 Raman active phonons and the corresponding atomic displacements. In addition, we observe additional phonons appearing in FePS3 in the magnetic ordered state, while there is no new phonon mode in MnPS3 and NiPS3. This magneto-elastic effect is related to unique AF order along the c axis in FePS3, which doubles the unit cell and folds the zone boundary mode to the **
Computational resources have been provided by the Physical Laboratory of High Performance Computing at Renmin University of China. The atomic structures and vibrational displacement patterns were prepared with the XCRYSDEN program.[33]
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] | |
[26] | |
[27] | |
[28] | |
[29] | |
[30] | |
[31] | |
[32] | |
[33] |