BIBECHANA Vol. 22, No. 3, December 2025, 258-265 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar A DFT based study of structural, electrical and vibrational properties of LiFeAs – A pnictide superconductor Aditya M. Vora Department of Physics, University School of Sciences, Gujarat University, Navrangpura, Ahmedabad 380 009, Gujarat, India ∗Corresponding author. Email: voraam@gmail.com Abstract By the uses of ultrasoft pseudopotential and the generalized gradient approximation (GGA), we have looked at the structural, electrical and vibrational characteristics of the LiFeAs pnic- tide superconductor in the present work. There is a strong correlation between phonons and superconductivity, which is the major root of the superconductivity of iron pnictide super- conductors. The superconducting characteristics of materials can be predicted using phonon properties. According to studies using density functional theory (DFT), LiFeAs exhibits its metallic nature in the overlaying bands close to the Fermi level. The dynamical stability is supported by positive frequencies in the phonon dispersion curves. Keywords Density functional theory (DFT), Generalized gradient approximation (GGA), ultrasoft pseudopo- tential, Iron pnictide superconductors, Phonon dispersion curves. Article information Manuscript received: April 18, 2025; Revised: August 13, 2025; Accepted: August 16, 2025 DOI https://doi.org/10.3126/bibechana.v22i3.77717 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 1 Introduction The discovery of superconductivity in iron-based layered compounds by Kamihara et al. [1] sparked a fierce search for new pnictide and chalcogenide superconductors. In early reviews and theoreti- cal overviews, these materials were classified as a distinct class based on FeAs or FeSe layers, which are critical to superconductivity [2–10]. LiFeAs, a 111-type pnictide, is unique among them due to its intrinsic superconductivity without chemical dop- ing [11, 12]. Without the added complications of disorder, this material offers a clean platform for examining the underlying mechanisms. Extensive experimental and theoretical studies of the structural and magnetic properties of iron pnictides have revealed strong electronic correla- tions [13], unusual magnetic interactions [14], and the possible coexistence or competition between magnetism and superconductivity [14–21]. While spin fluctuations are often cited as a possible glue for pairing [22], other options include electron- phonon interaction or synergistic effects. However, optical and penetration-depth measurements fur- ther emphasize the complex interplay between the carrier dynamics and superconducting gap struc- 258 http://nepjol.info/index.php/BIBECHANA voraam@gmail.com https://doi.org/10.3126/bibechana.v22i3.77717 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Aditya M. Vora/ BIBECHANA 22 (2025) 258-265 259 ture [16,23]. LiFeAs has attracted a lot of attention within this broader family due to its unusual lack of the Fermi surface nesting [24], which calls into the ques- tion of conventional spin-density-wave-based pair- ing models. Comparative first-principles simula- tions with related compounds such as LiFeP [25,26] and high-pressure experiments [27] have shown that subtle changes in the pnictogen height and lattice properties have a substantial effect on the density of states at the Fermi level and, thus, the supercon- ducting capabilities. These findings have demon- strated how important it is to understand such ma- terials through accurate electronic structure calcu- lations and structural optimization, which density functional theory (DFT) provides. Despite these strides, gaps remain. Phonon dispersion data are also sparse relative to elec- tronic structure data, and measured lattice con- stants for LiFeAs vary between experiments as well as compared to computational studies. The respec- tive roles of spin fluctuations and lattice vibrations in superconductivity in LiFeAs remain controver- sial. Also, some phonon studies indicate insuffi- cient electron–phonon coupling to account for the observed Tc [8,28], whereas others point to possible cooperative effects in the examined materails. And yet, there still remains no integrated study of struc- tural, electronic, and vibrational properties within the same computational setting. This study con- ducts a systematic DFT analysis of LiFeAs, focus- ing on its structure, electronic band structure, and vibrational spectra in order to fill in the gaps. By integrating these factors, the study seeks to elu- cidate the fundamental properties of the material that influence its superconductivity and to estab- lish computational models that can guide later ex- perimental and theoretical investigations. LiFeAs crystallizes in the tetragonal P4/nmm space group and has a Matlockite structure. Li1+ forms distorted LiAs5 trigonal bipyramids when it is bonded to five equivalent As3 atoms. These trigonal bipyramids share corners with 12 equiva- lent FeAs4 tetrahedra, edges with four equivalent FeAs4 tetrahedra, and edges with eight equivalent LiAs5 trigonal bipyramids. All Li-As bonds have a length of 2.67Å. Fe2+ forms distorted FeAs4 tetra- hedra by bonding with four equivalent As3 atoms, which share corners with four equivalent FeAs4 tetrahedra, edges with four equivalent LiAs5 trig- onal bipyramids, and corners with twelve equiva- lent LiAs5 trigonal bipyramids. The average Fe-As bond length is 2.47Å. As3 is coupled to five equiva- lent Li1+ and four equivalent Fe2+ atoms in a nine- coordinate geometry [29]. Figure 1: Crystal structures of LiFeAs. Currently, it is observed that simulations based on Density Functional Theory (DFT) [30–39] are more accurate when examining the physical charac- teristics of different materials. Studying the struc- tural, electrical, vibrational, and superconducting properties of LiFeAs iron pnictide superconductors under DFT environment is the focus of the current work, which is motivated by the significance of the superconductivity phenomenon and its applications in many fields. 2 Theory Using the Quantum Espresso (QE) code [40], a DFT based computations are carried out to in- vestigate the structural, electrical, vibrational, and superconducting properties of LiFeAs. Because it favours density inhomogeneity, the General- ized Gradient Approximation (GGA) with Perdew- Burke-Einzerhof (PBE) [41] functional is used to analyze the exchange correlation effects. The plane waves with a charge density cutoff of 80 Ry and a kinetic energy cutoff of 20 Ry are considered here. The 4x4x4 k-point mesh is used to highlight integration over the Brillouin zone. In this work, the ultrasoft pseudopotentials [42] of three distinct elements—Li, Fe, and As are employed. Since convergence tests revealed insignificant energy changes outside of this mesh, a 4×4×4 Monkhorst–Pack k-point mesh was used for geom- etry optimizations in this work to lower computa- tional cost while maintaining accurate forces and stresses. A denser 6×6×6 mesh was used for the final electronic structure and DOS calculations in order to guarantee precise Brillouin zone integra- tion and fine resolution close to the Fermi level. Aditya M. Vora/ BIBECHANA 22 (2025) 258-265 260 3 Results and Discussion Figure 1 displays the optimization curves for the kinetic energy cutoff (Ecut), k-grid, and the cell parameter ratio (c/a) for LiFeAs. Using the vari- able cell relaxation (vc-relax) approach, the atomic location of said material and the lattice constants were first optimized. To confirm the findings, ration of the lattice parameters (c/a) was then adjusted. The relaxed parameters were achieved by apply- ing the GGA technique with ultrasoft pseudopo- tentials, as implemented in QE. Table 1 presents a comparison among the computed optimized lat- tice parameter results and the available experimen- tal outcomes [12]. In the present computation, the optimized lattice parameter is obtained a=3.790A, which found in good agreement with the available experimental data [12]. The current outcomes are found inconsistence with them with 0.1%-0.2% variations. (a) (b) (c) Figure 2: Optimization curves for (a) c/a, (b) E-cut, (c) k-grid for LiFeAs. In this work, the gnuplot [43] in Figure 2 is used to plot the band structure for the given material. On the high symmetry points, Γ→ Z → R → X → M → A→ Γ, the k-points path in reciprocal space is taken into consideration. Here, the energy cutoff considered is 80 Ry, while the charge density cutoff is 320 Ry. In order to integrate over the Brillouin zone, a 6×6×6 k-pint mesh is used. Because Fe is ferromagnetic by nature, a spin component is intro- duced for the computations in both circumstances. In other words, plots of the band structures for up and down spin are available. From the band struc- ture study, it is observed that, the LiFeAs has a metallic quality, as evidenced by the overlapping of band lines close to the Fermi region in Figure 2. Furthermore, no differences are seen between the situations of a minority and a majority. The Fermi energy (EF) is found 9.6985 eV from the band struc- ture data. (a) (b) Figure 3: Electronic band structure for LiFeAs (a) spin up (b) spin down. Aditya M. Vora/ BIBECHANA 22 (2025) 258-265 261 Table 1: Calculated and experimental lattice parameters for LiFeAs. Lattice Constants (Å) Presently Calculated Values Experimental Values [?] Deviation (%) a 3.790 3.791 0.1 c/a 1.6577 1.6599 0.2 The TDOS and PDOS for LiFeAs are displayed in Figures 3 and 4. In this case, the Fermi level electron density is seen at around 4.1 states/eV. At -10 eV, much below the EF, an electron density of about 1.8 states/eV is also detected. Furthermore, there is no difference in the TDOS for spin-up and spin-down situations, which would point to its su- perconducting nature. It is evident from the partial DOS displayed in Figure 4 that the 3d state of Fe predominates close to the Fermi area. The 4s or- bital of arsenic contributes significantly below the Fermi level. The density in the range of -5.0 eV to EF clearly suggests that Fe and As are covalently bonded through hybridization between the Fe(3d)- As(3p) orbitals in Fe2As2 blocks. Consequently, the pnictogen bands and the 3d states of Fe are what give LiFeAs its metallic nature. From the Figure 3, the highest Fermi level DOS is observed around 4.2 states/eV. (a) (b) Figure 4: TDOS of LiFeAs (a) spin up (b) spin down. (a) (b) (c) Figure 5: PDOS of LiFeAs (a) Li, (b) Fe and (c) As. Aditya M. Vora/ BIBECHANA 22 (2025) 258-265 262 In the present work, XCrySDen [44] software is used to analyze the Fermi surface topology of the material under investigation. The spin component is not included in the computation of the Fermi sur- face since there is no difference between the band structure and DOS for the spin-up and spin-down cases. Five bands intersect the Fermi energy level EF in the band structure of LiFeAs (Figure 2). These five bands are represented by their respective Fermi sur- faces. They confirm that FeAs superconductors are often two-dimensional, as proposed by Sadovskii, Ivanovskii and Izyumov [2–4]. Figures 5(a)–(e) de- pict the Fermi surfaces for individual bands that pass through the EF; Figures 5(f) and (g) show the Fermi surfaces for merged bands. The electron con- tribution is provided by the quasi-cylinders at the corners of the Brillouin zone in the M-A direction, while the hole-like concentric cylinders are located at the zone centre (). This way, the Fermi surfaces are visualized to discriminate between the occupied and unoccupied states. Hence, for the aforemen- tioned materials, Fermi surfaces are created in a way that ascertains the electron contribution at the corners and the hole contribution in centre of the Brillouin zone. The QE code's "PHonon" package is used to cal- culate phonons [40]. The energy cutoff of 80 Ry and the charge density cutoff of 320 Ry were taken into consideration while computing the phonon disper- sion of studied material. It was assumed that the k-mesh was 6x6x6. The phonon dispersion was cal- culated over the high symmetry sites (Γ→Z→R→Γ) of the Brillouin zone, taking into account the q-grid of 2×2×2. Due to their involvement in pairing interactions, electron and hole pockets in Fermi surface anal- ysis have a major influence on superconductivity. By permitting interband scattering between elec- tron and hole pockets, multiple pockets can im- prove superconductivity, particularly when spin or orbital fluctuations are involved. Unconventional ± wave pairing can be facilitated by mediating spin-density-wave-type fluctuations by nesting be- tween electron pockets at the zone corners and hole pockets at the Brillouin zone center. The density of states at the Fermi level is influenced by the size and shape of pockets, with higher N(EF ) typ- ically strengthening pairing. The superconducting gap structure is influenced by electron-hole asym- metry; fully gapped states are preferred by bal- anced pockets, whereas nodes or anisotropic gaps may result from severe imbalance. The momentum- space "playground" for pairing interactions is de- fined by Lifshitz transitions, which can result in abrupt changes in Tc. The phonon DOS and phonon dispersion curves for LiFeAs are shown in Figure 6. The dispersion curve has 18 phonon branches, including 2 TA, 1 LA, 5 LO, and 10 TO mode branches. A phonon dispersion of 0 cm-1 to 695 cm-1 is computed. The LiFeAs crystal structure's dynamical stability is suggested by the positive phonon frequencies. The appropriately degenerate states of different optical and acoustical branches at point R are visible at 100 cm-1, 125 cm-1, 190 cm-1, 255 cm-1, 275 cm-1, 295 cm-1, 510 cm-1, and 580 cm-1. Additionally, an overlapping of the acoustical and optical branches is visible at 125 cm-1. It is discovered that the highest phonon branches are very dispersive in the Z-R di- rection. Because of the overlapped, weakly disper- sive optical branches, the phonon DOS about 290 cm-1 has its maximum peak. There is a little band gap seen around 210 cm-1. Certain peaks are iden- tified in the phonon DOS at 100 cm-1, 120 cm-1, 15 cm-1, 210 cm-1, 230 cm-1, 290 cm-1, 300 cm-1, and 315 cm-1. From the Figure 6 it is observed that, the highest phonon frequency noted around at 698 cm-1 at R point. Figure 6: The Fermi surfaces for different bands (a)-(e) and for merged bands (f) and (g). Aditya M. Vora/ BIBECHANA 22 (2025) 258-265 263 Figure 7: (a) Phonon dispersion curve and (b) phonon DOS. 4 Conclusions Ultimately, we draw the conclusion that the struc- tural, electronic and vibrational properties of LiFeAs, an iron pnictide superconductor are re- ported in the present paper. It is observed that 3s state of ‘As’ predominates in the section far below the Fermi level and that the 3d states of ‘Fe’ and ‘Li’ are found near to and above the Fermi level, respectively. The metallic character of LiFeAs is demonstrated by the superimposing bands near to the Fermi level. Furthermore, dynamic stability of aforementioned structure is suggested by the posi- tive frequencies in the phonon dispersion curves. 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