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Strong electron–phonon coupling and predicted high superconducting transition temperature of MXenes revealed in 2H-Mo2N under biaxial stress
(2024-12-15)
Kotmool, Komsilp
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Tsuppayakorn-aek, Prutthipong
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Bovornratanaraks, Thiti
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Kaewmaraya, Thanayut
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Sakdanuphab, Rachsak
Extensive investigations have been conducted on Mo-based MXenes due to their high superconducting temperatures (T<inf>c</inf>). This work theoretically reports strong electron–phonon coupling (EPC) and high T<inf>c</inf> (≈38 K) of 2H-Mo<inf>2</inf>N under biaxial stress, with excellent mechanical properties. EPC and T<inf>c</inf> are elucidated upon dynamically stable strain range. At 0% strain, EPC constant (λ) and T<inf>c</inf> are 1.32 and 22.7 K, respectively. They are improved when subjected to biaxial stresses. Strong EPC with λ over 2.0 occurs at −4%, −2.5%, and 5% strains, yielding enhanced T<inf>c</inf>. The finding suggests that the energy levels of electronic bands at the Fermi level are enhanced by strain, which enhances EPC. Moreover, the impact of functional groups on superconductivity has been investigated. The λ’s of Mo<inf>2</inf>NH<inf>2</inf> and Mo<inf>2</inf>NO<inf>2</inf> are reduced, and T<inf>c</inf> drops nearly to 20.0 and 0.9 K, respectively. This work provides an example of designing 2D superconductors by tuning atomic recipes and strain-dependent engineering.
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Novel hard and unusual superconducting monoclinic phase of FeB2C2: An ab initio evolutionary study
(2024-06-14)
Kotmool, Komsilp
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Pinsook, Udomsilp
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Luo, Wei
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Ahuja, Rajeev
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Bovornratanaraks, Thiti
This study focuses on conducting an ab initio evolutionary investigation to search for stable polymorphs of iron diborocarbides with the formula FeB 2 C 2 . We also examined other forms of C contents, including FeB 3 C and FeBC 3 . Our findings reveal that the lowest energetic structure of FeB 2 C 2 is a semimetallic monoclinic phase with a space group (s.g.) of C2/m and a metastable metallic phase of FeB 2 C 2 is an orthorhombic structure with s.g. of Pmmm. In addition, structural and relative properties of FeB 3 C and FeBC 3 are performed and discussed to compare with FeB 2 C 2 . All predicted structures are dynamically and elastically stable, verified without negative phonon frequency and Born criteria, respectively. We also analyzed the energetic stability through calculated cohesive and formation energies, which showed that C2/m- FeB 2 C 2 is stable at low pressure. Interestingly, the C2/m and Pmmm phases of FeB 2 C 2 are hard materials with Vickers hardness ( H v ) of 22.40 and 27.52 GPa, respectively. Additionally, we examined the electron-phonon coupling of both FeB 2 C 2 phases. Unexpectedly, we found that the semimetallic C2/m- FeB 2 C 2 phase is a superconductor with a significant superconducting temperature ( T c ) exceeding 6 K. These findings provide some novel results for the Fe-B-C system and pave the way for investigating other metal borocarbides and related ternary compounds.
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Stability, thermodynamic, electronic, and thermoelectric properties of triclinic Cu2Se structure
(2024-06-01)
Kotmool, Komsilp
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Khammuang, Satchakorn
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Rudradawong, Chalermpol
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Thatsami, Niphat
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Kaewmaraya, Thanayut
The conversion of heat into electricity using the thermoelectric effect is a crucial issue to tackle the ever-increasing global energy consumption. Cu<inf>2</inf>Se is one of the high-performance thermoelectric materials due to its liquid-like atomic structure, which minimizes the phonon-derived thermal conductivity. Nevertheless, the unambiguous atomic structure of the low-temperature phase, α-Cu<inf>2</inf>Se, remains controversial. By employing a combination of theoretical approaches including an evolutionary algorithm for structural searching, density functional theory, and lattice dynamics, the α-Cu<inf>2</inf>Se phase is proposed to crystalize in the dynamically stable triclinic Cu<inf>2</inf>Se structure (s.g. P1) which can be regarded as a monoclinic polymorph (s.g. P2<inf>1</inf>/c<sup>†</sup>) through slight distortion. The corresponding heat capacity (C<inf>V</inf>) and Debye temperature (Θ<inf>D</inf>) are 0.37 J/g K and 276 K, respectively, indicating low thermal conductivity in the material. The semiconducting P1 phase possesses an indirect gap of 1.0 eV. Additionally, based on the Boltzmann transportation theory, fundamental thermoelectric parameters around the transition temperature (300–500 K) are investigated. Beyond the semiconducting phase, a novel metallic phase identified as the triclinic P1̄ phase is also discovered, exhibiting a two-dimensional (2D) crystal geometry.
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Biaxial stress and functional groups (T = O, F, and Cl) tuning the structural, mechanical, and electronic properties of monolayer molybdenum carbide
(2022-07-19)
Kotmool, Komsilp
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Kaewmaraya, Thanayut
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Hussain, Tanveer
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Ahuja, Rajeev
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Luo, Wei
MXenes are a family of novel two-dimensional (2D) materials attracting intensive interest because of the rich chemistry rooted from the highly diversified surface functional groups. This enables the chemical optimization suitable for versatile applications, including energy conversion and storage, sensors, and catalysis. This work reports the ab initio study of the crystal energetics, electronic properties, and mechanical properties, and the impacts of strain on the electronic properties of tetragonal (1T) and hexagonal (2H) phases of Mo<inf>2</inf>C as well as the surface-terminated Mo<inf>2</inf>CT<inf>2</inf> (T = O, F, and Cl). Our findings indicate that 2H-Mo<inf>2</inf>C is energetically more stabilized than the 1T counterpart, and the 1T-to-2H transition requires a substantial energy of 210 meV per atom. The presence of surface termination T atoms on Mo<inf>2</inf>C intrinsically induces variations in the atomic structure. The calculated structures were selected based on the energetic and thermodynamic stabilities (400 K). The O atom prefers to be terminated on 2H-Mo<inf>2</inf>C, whereas the Cl atom energetically stabilizes on 1T-Mo<inf>2</inf>C. Meanwhile, with certain configurations, 2H-Mo<inf>2</inf>CF<inf>2</inf> and 1T-Mo<inf>2</inf>CF<inf>2</inf> with slightly different energies could exist simultaneously. The Mo<inf>2</inf>CO<inf>2</inf> possesses the highest mechanical strength and elastic modulus (σ<inf>max</inf> = 52 GPa at ϵ<inf>b</inf> = 20% and E = 507 GPa). The nature of the ordered centrosymmetric layer and the strong bonding between 4 d-Mo and 2 p-O of 2H-Mo<inf>2</inf>CO<inf>2</inf> are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-Mo<inf>2</inf>CO<inf>2</inf> at a wide range of strains (−10% to 12%) are reported. Moreover, 2H-Mo<inf>2</inf>CF<inf>2</inf> is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-Mo<inf>2</inf>CF<inf>2</inf> and 1T-Mo<inf>2</inf>CCl<inf>2</inf> preserve their features under the ranges of the strain of −2% to 10% and −1% to 5%, respectively, beyond which they undergo the semiconductor-to-metal transitions. These findings would guide the potential applications in modern 2D straintronic devices.
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Structural Phase Transitions, Electronic Properties, and Hardness of RuB4under High Pressure in Comparison with FeB4and OsB4
(2020-07-09)
Kotmool, Komsilp
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Tsuppayakorn-Aek, Prutthipong
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Kaewmaraya, Thanayut
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Pinsook, Udomsilp
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Ahuja, Rajeev
We have employed an evolutionary algorithm with first-principles calculations to investigate the pressure-induced structural evolution of RuB4 up to 500 GPa. The ambient phase is predicted to be a hexagonal structure (P63/mmc). The novel phases consisting of monoclinic (C2/c) and orthorhombic (Immm) structures are proposed to be the high-pressure phases at the pressure intervals of 198-388 GPa and beyond 388 GPa, respectively. The stability of the predicted phases is confirmed by both dynamic and elastic calculations. The electronic and mechanical properties of the predicted phases are evaluated and mainly discussed compared to the isoelectronic metal tetraborides, i.e., FeB4 and OsB4. In contrast to FeB4 and OsB4, all the stable phases of RuB4 are metal or semimetal, and any semiconducting phases do not emerge in the transformation pathway of RuB4. The nature of chemical bonding investigated by ELF, MPA, and pCOHP calculations reveals that the atomic configurations and the degree of covalent bonding of the predicted phases are responsible for lower hardness compared to those of FeB4 and OsB4. The results of this work provide more understanding of the family of metal tetraboride for designing metal-boride-based hard/superhard materials.
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