Faraji and Colleagues Shed Light On The Properties Of FeX Monolayer (Material Science)

M. Faraji and colleagues explored the lattice, dynamical stability, electronic and magnetic properties of FeTe bulk and FeX (X=S, Se, Te) monolayers using the density functional calculations. Their study recently appeared in the Journal of Applied Physics Letters.

The iron-chalcogenide crystals are a category of magnetic materials including FeS, FeSe, and FeTe. A variety of magnetic phenomena including antiferromagnetism, ferrimagnetism, and ferromagnetism can be approved by manipulating the chalcogen elements (Se, S, and Te). Furthermore, iron-chalcogenides often demonstrate several structural phases with different characteristics. However, the structural, electronic, and magnetic properties of iron dichalcogenides FeX (X=S, Se, Te) monolayers have not yet been fully understood. Thus, M. Faraji and colleagues shed light on this issue in their recent paper using first-principle calculations.

At first, they calculated lattice parameters for the FeS, FeSe, and FeTe monolayers which are found to be 3.56 Å, 3.62 Å, and 3.58 Å respectively. These lattice parameters are smaller than the corresponding lattice parameters of bulk structures. Then, they studied the mechanical properties and revealed that, all FeX monolayers have brittle nature. Furthermore, these structures are stable and as they move down the 6A group in the periodic table, i.e., from S, Se and Te.

They also found, the cohesive energies of FeS, FeSe, and FeTe monolayers, which are -11.57 eV/atom, -10.60 eV/atom, and -9.97 eV/atom, respectively. This finding indicated that the formation of FeS monolayer is more favorable than other monolayers due to the higher electronegativity of S as compared to the electronegativity of Se and Te.

Figure 1. Top and side wies of atomic structures of (a) FeS, (b) FeSe and (c) FeTe monolayers. Phonon band dispersion with corresponding structure is shown in the bottom panel. The primitive unit cell indicated by a black rectangular © Faraji et al.

In addition, they verified the dynamical stabilities of FeX monolayers by calculating their phonon band dispersions through the whole BZ which are presented at the bottom panel in Figs. 1 (a-c). Apparently, phonon branches are free from any imaginary frequencies indicating the dynamical stability of the structures.

Table 1: Structural and electronic parameters of FeX (X=S, Se, Te) monolayers including lattice constants a; bond length between Fe-X atoms d; thickness defined by the difference between the largest and smallest z coordinates of X atoms (t); bond angles between Fe-X-Fe atoms θ; cohesive energy per atom, (Ecoh); charge transfer (∆Q) from X atom to Fe atom; work function (Φ) magnetic moment Mtot; bulk modulus (B); shear modulus (S); Young’s modulus (Y); Poisson’s ratio (ν)); and bulk/shear ratio (BS), respectively. © Faraji et al.

Moreover, it has been found that FeX structure has 6 independent elastic constants, which verify the conditions of elastic stability criteria, meaning, the FeX structures have mechanical stability. They also found that the Bulk Modulus of FeS monolayer is the best one that can resist the compression as compared to the other monolayers. They confirmed this result by calculating Young’s modulus values.

In addition, they found that the FeTe monolayer has a total magnetization of 3.8 µB, which is smaller than the magnetization of FeTe bulk (4.7 µB). However, the FeSe and FeS are nonmagnetic monolayers.

Finally, they concluded that the FeTe monolayer can be a good candidate material for spin filter applications due to its electronic and magnetic properties. Their study highlighted the bright prospect for the application of FeX monolayers in electronic structures.

Featured image: Electronic band structure, density of states (DOS) and projected DOS (PDOS) © Faraji et al.

Reference: A. Bafekry, I. Abdolhosseini Sarsari, M. Faraji, M. M. Fadlallah, H. R. Jappor, S. Karbasizadeh, Chuong V. Nguyen, and M. Ghergherehchi, “Electronic and magnetic properties of two-dimensional of FeX (X = S, Se, Te) monolayers crystallize in the orthorhombic structures”, Appl. Phys. Lett. 118, 143102 (2021); https://doi.org/10.1063/5.0046721

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