Metal–metal bonding, electronic excitations, and strong resonance Raman effect in 2D layered α-MoCl₃

Abstract

Covalent bonding between transition metal atoms is a common phenomenon in honeycomb lattices of layered materials, which strongly affects their electronic and magnetic properties. This work presents a detailed spectroscopic study of α-MoCl3, 2D van der Waals material with covalently bonded Mo2 dimers, with a particular focus on the Mo–Mo bonding. Raman spectra of α-MoCl3 were studied with multiple excitation laser lines chosen in different parts of the absorption spectrum, while polarization measurements aided in the symmetry assignment of the observed modes. Furthermore, far-IR measurements and DFT phonon computations were performed to complete vibrational assignment. Polarized absorption, photoluminescence, and photoelectron spectroscopy supported by DFT calculations were employed to understand the consequences of the Mo–Mo bonding for the electronic structure and the localization/delocalization balance in d3–d3 interactions. A coupling of dimerization-related structural and electronic properties was revealed in the strong resonance Raman enhancement of the Mo–Mo stretching mode at 153 cm−1 when the excitation laser matched the electronic transition between σ-bonding and antibonding orbitals of the Mo2 dimer (σ→σ*). The deeper understanding of the metal-metal bonding and identification of the vibrational and electronic spectroscopic signatures of the dimerization will be of great use for the studies of electron delocalization in magnetic van der Waals materials.

Structural and Magnetic Transitions Caused by Dimer Formation in the CrCl3–MoCl3 Solid Solution

“…Further details can be found in the respective publication. 27 With increasing Cr 3+ content in the solid solution, all of these Mo-related excitations decrease in intensity, as is expected due to the decreasing Mo 3+ content. Finally, the spectra show the excitation spectrum of CrCl 3 in agreement with a previous work.…”

“…In the case of α-TiCl 3 as another example of a 2D TMTH with a similar crystal structure, an equal peak splitting was observed upon cooling as it undergoes a phase transformation from an undistorted honeycomb arrangement (like in CrCl 3 ) to a distorted arrangement with aligned Ti–Ti dimers (like in MoCl 3 ) . Recent theoretical and experimental investigation of the vibrational modes of MoCl 3 assigned this additional peak at 302 cm –1 to a stretching vibration of the Mo–Cl bonds along the Mo–Mo dimer bond, which can be differentiated from the Mo–Cl stretching vibration in the direction of nonbonded neighboring Mo 3+ …”

“…The spectra were normalized to the data in the range between 200 and 400 cm –1 . The vertical lines show the respective modes of CrCl 3 and MoCl 3 according to the literature , to assist the eye in following the shifts of modes. The color assignment of the legend is also true for (b).…”

“…9 Recent theoretical and experimental investigation of the vibrational modes of MoCl 3 assigned this additional peak at 302 cm −1 to a stretching vibration of the Mo−Cl bonds along the Mo−Mo dimer bond, which can be differentiated from the Mo−Cl stretching vibration in the direction of nonbonded neighboring Mo 3+ . 27 Following the trends of the peaks in this specific energy range for the solid solution samples (Figure 7b), the characteristic peak doublet in the case of MoCl 3 is observed in the range of 0.00 ≤ x ≤ 0.40, whereas the lower energy peak slightly shifts toward lower wavenumbers with increasing x. As the peak at 302 cm −1 is assigned to the specific Mo−Cl vibration along the dimer, the peak position of this vibration is expected to be relatively stable against other possible local lattice disorders.…”

“…(b, c) Zoom-in on the low-energy features. The transition assignments for the case of MoCl 3 were adopted from the literature,27 while they belong to crystal field excitations in the case of CrCl 3 30. The spectrum of CrCl 3 is adapted from the literature 30.…”