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Exploring Hydrogen-Bearing Metallic Alloys: Phonon-Mediated Superconductivity in (Zr,Hf)H3 under High Pressure
(2025-01-09)
Tsuppayakorn-Aek, Prutthipong
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Sukmas, Wiwittawin
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Kotmool, Komsilp
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Luo, Wei
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Bovornratanaraks, Thiti
Advanced structural forecasting of alloy hydrides, particularly through cluster expansion combined with first-principles calculations, opens up new possibilities for discovering novel phases in transition metal alloy hydrides like (Zr,Hf)H<inf>3</inf>. Within this framework, significant findings have been made for compounds Zr<inf>7</inf>HfH<inf>24</inf>, Zr<inf>4</inf>Hf<inf>2</inf>H<inf>18</inf>, Zr<inf>2</inf>Hf<inf>2</inf>H<inf>12</inf>, and Zr<inf>2</inf>Hf<inf>4</inf>H<inf>18</inf>, which demonstrate thermodynamic stability at 100 GPa. All identified structures exhibit metallic properties, suggesting a promising pathway to superconductivity. In terms of superconducting properties, Zr<inf>7</inf>HfH<inf>24</inf>, Zr<inf>4</inf>Hf<inf>2</inf>H<inf>18</inf>, Zr<inf>2</inf>Hf<inf>2</inf>H<inf>12</inf>, and Zr<inf>2</inf>Hf<inf>4</inf>H<inf>18</inf> show critical temperatures (T<inf>c</inf>) of 15.9, 14.6, 8.2, and 12.8 K, respectively, at 100 GPa. Notably, Zr<inf>4</inf>Hf<inf>2</inf>H<inf>18</inf> achieves the highest T<inf>c</inf> within the (Zr,Hf)H<inf>3</inf> series, reaching approximately 17 K at 150 GPa. Our analysis of the superconducting state is based on H-rich criteria under specific conditions, revealing that hydrogen’s contribution to the partial density of states is lower than that of hafnium and zirconium. The investigation also finds that these structures lack H clathrate configurations or H<inf>2</inf>-like molecular units, suggesting they are unlikely to reach near-room-temperature T<inf>c</inf>. These results highlight how structural frameworks supported by H or H<inf>2</inf>-like molecules could potentially enhance superconductivity. Additionally, the alignment of the vibrational modes of the alloy with those observed in hafnium suggests that Hf-substituted Zr alloys support superconductivity and offer theoretical feasibility for achieving higher critical temperatures across a broader range of alloying combinations.
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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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Hydrogenation-induced superconducting properties of MgB2 investigated using Migdal–Eliashberg formalism: Insights from a first-principles study
(2024-09-01)
Tsuppayakorn-aek, Prutthipong
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Sukmas, Wiwittawin
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Pluengphon, Prayoonsak
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Petchsirivej, Sukanya
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Sakulkalavek, Aparporn
Theoretical investigation of hydrogenation processes has applied to magnesium diborides under ambient conditions, which identified two structurally stable phases, i.e, Mg<inf>4</inf>B<inf>6</inf>H<inf>2</inf> and Mg<inf>4</inf>B<inf>4</inf>H<inf>4</inf>. These identifications were evaluated through assessments of their lattice dynamics stability using density functional perturbation theory. Both phases exhibit metallic behavior within their electronic band structures. Our findings showcase the significant impact of anisotropic Migdal–Eliashberg calculations, enhancing the superconducting properties within this system and resulting in a notably higher T<inf>c</inf> of 34 K. Mg<inf>4</inf>B<inf>4</inf>H<inf>4</inf> exhibits superconductivity with a T<inf>c</inf> of 17 K under atmospheric conditions, as determined by anisotropic Migdal–Eliashberg calculations. Our study underscores the wide range of structural variations achievable through the hydrogenation of MgB<inf>2</inf> and highlights the crucial importance of hydrogen atom placement within these structures. In addition, the calculation result indicates the influence of band dispersion characteristics on Fermi velocity, a factor attributed to both anharmonicity and harmonicity, which plays a pivotal role in determining the superconducting properties of these materials.
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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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Thermodynamic and dynamic stability of NaLiC4: Exploring superconductivity in a layered hexagonal compound through first-principles calculations
(2024-06-01)
Pluengphon, Prayoonsak
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Sukmas, Wiwittawin
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Tsuppayakorn-aek, Prutthipong
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Kotmool, Komsilp
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Sakulkalavek, Aparporn
The layered hexagonal compound NaLiC<inf>4</inf> has been thoroughly investigated to assess its thermodynamic and dynamic stability in comparison to its parent compounds NaC<inf>2</inf> and LiC<inf>2</inf>. Utilizing first-principles calculations and phonon analyzes, NaLiC<inf>4</inf> has demonstrated remarkable stability within a pressure range of 10 to 100 GPa, surpassing the thermodynamic stability of NaC<inf>2</inf> and LiC<inf>2</inf>. It exhibits metallic behavior with distinctive electronic bands along high symmetry paths, suggesting a conducive environment for superconductivity. The superconducting transition temperature (T<inf>c</inf>) of NaLiC<inf>4</inf> under different pressures was estimated using the Allen–Dynes equation, with a maximum T<inf>c</inf> of 79 K observed at 10 GPa. However, as pressure increases, T<inf>c</inf> gradually decreases, indicating the significant impact of external pressure conditions on the superconducting properties. Notably, the in-plane E<inf>2g</inf> phonon mode originating from the layered hexagonal structure of carbon atoms plays a crucial role in facilitating electron–phonon coupling and influencing the superconducting behavior and T<inf>c</inf> of NaLiC<inf>4</inf>. These findings highlight the thermodynamic and dynamic stability of NaLiC<inf>4</inf> as a promising candidate for exploring superconductivity, offering insights into its electronic properties, pressure-dependent T<inf>c</inf> behavior, and the influence of specific phonon modes. Further theoretical investigations and experimental studies are necessary to fully unlock the potential of NaLiC<inf>4</inf> and its contribution to the development of high-performance superconductors.
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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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Role of anharmonicity in phonon-mediated superconductivity of quasi- van der Waals layered XP2 (X = As, Sb, Bi) structures: Insight from first-principles calculations
(2023-10-10)
Tsuppayakorn-aek, Prutthipong
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Pluengphon, Prayoonsak
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Sukmas, Wiwittawin
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Sakulkalavek, Aparporn
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Inceesungvorn, Burapat
It is of utmost importance to prioritize the undertaking of investigating superconductivity, as it represents one of the most intriguing and significant phenomena in the field of condensed matter physics. Our research endeavors to shed light on the metallic state of van der Waals layered XP<inf>2</inf> (X = As, Sb, Bi) structures, which is crucial in predicting their superconducting properties. Employing a first-principle evolutionary algorithm, we predicted the structures of phosphides, including AsP<inf>2</inf>, SbP<inf>2</inf>, and BiP<inf>2</inf>, and determined their energetically stable configurations under atmospheric pressure. To investigate the potential for superconductivity in these structures, we conducted a thorough examination of relevant electronic properties. One of the crucial factors we addressed was the role of anharmonic phonon properties in determining the superconducting state in van der Waals layered structures. Our findings suggest that the introduction of anharmonicity can significantly suppress the superconducting abilities of such structures. Hence, the anharmonicity present in AsP<inf>2</inf>, SbP<inf>2</inf>, and BiP<inf>2</inf> could be held accountable for the lower observed superconducting critical temperature in these van der Waals layered structures.
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Role of Lifshitz transition stabilized carbon hexagon structure from biaxial strain: A case of sodium carbide with superconducting condition
(2023-06-15)
Tsuppayakorn-aek, Prutthipong
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Pluengphon, Prayoonsak
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Sukmas, Wiwittawin
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Sukserm, Akkarach
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Kotmool, Komsilp
We theoretically report on an investigation of sodium carbide system by means of first-principles calculations based on density functional theory. Herein, the diverse sodium–carbon structures are predicted by taking carbon-rich compositions of NaC<inf>2</inf> with a carbon hexagon structure as a starting point. Metallic phases of NaC<inf>2</inf> at a pressure of 100 GPa are predicted to be stabilized by biaxial strain, culminating in the strain-induced electronic topological transitions, also known as the Lifshitz transitions. We found that the flat band accommodates localized electrons around the Fermi level, originating from the effect of biaxial strain, which results in low-velocity electrons forming up to at least 20% of Cooper pairs. According to the respective phonon-mediated superconductivity, NaC<inf>2</inf> is dynamically stable not only without the influence of biaxial strain but also with that of the biaxial tensile strain, indicating possible enhancement of the critical temperature superconductor (T<inf>c</inf>). Furthermore, the estimated T<inf>c</inf> reaches 29.5 K, slightly higher than 24.7 K for the case without biaxial strain. These findings suggest that the possibility of superconductivity is promoted by the applied biaxial tensile strain. Our findings pave the way for future investigation of high superconductivity in carbon-based materials and suggest the possibility of metal-carbides being synthesisable and exhibiting quite a high T<inf>c</inf> superconductivity.
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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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