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Author: search.filters.author.Kongnok, Thanundon

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    Pressure-induced phase transition and indirect band gap semiconductor in ZnSnN2: First Principles Calculation
    (2025-01-01)
    Sailuam, Wutthigrai
    ;
    Fongkaew, Ittipon
    ;
    Kongnok, Thanundon
    ;
    Kotmool, Komsilp
    In this study, we investigate the phase transition of ZnSnN<inf>2</inf> from Pna2<inf>1</inf> to Pmnb using Density Functional Theory (DFT) across a pressure range of 0–70 GPa. Our results show the enthalpy intersection of the Pna2<inf>1</inf> and Pmnb phases at 19.28 GPa, indicating a phase transition from Pna2<inf>1</inf> to Pmnb ZnSnN<inf>2</inf>. The decrease in H<inf>v</inf> of the Pna2<inf>1</inf> phase under pressure before the phase transition is attributed to the reduction of the G and weakening covalent bond of Sn–N pair. The new Pmnb phase exhibits an increased Vickers hardness, Debye temperatures, and brittleness. Moreover, the band gap is an indirect band gap of 1.41 eV due to a rearrangement of lower energy levels for Sn s and p states in conduction band minimum (CMB) and N s and p states in valence band maximum (VBM) at Γ-point. These characteristics make The Pmnb phase promising candidates for applications were longer carrier lifetimes are needed. The mechanical properties, dynamical behavior, and electron localization functions (ELFs) have been investigated and discussed.
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    Pressure and atomic size effects of IV cation on mechanical and electronic properties of Zn-IV-N2 (IV[dbnd]Si, Ge and Sn): First principles calculation
    (2024-09-01)
    Boonkhuang, Apiwat
    ;
    Kongnok, Thanundon
    ;
    Meethan, Weerachon
    ;
    Busayaporn, Wutthikrai
    ;
    Phacheerak, Kanoknan
    Zn-IV-N<inf>2</inf> compounds, incorporating Si, Ge, and Sn, have emerged as pivotal materials for their mechanical and electronic properties, influencing optoelectronic devices and photovoltaic applications. Employing density functional theory (DFT), we comprehensively investigate the structural, elastic, mechanical, and electronic characteristics of ZnIVN<inf>2</inf> (IV[dbnd]Si, Ge, Sn) under ambient and pressure conditions up to 20 GPa. Our findings suggest that a larger atomic size of the group IV cation can be more easily compressed than a smaller size. The mechanical stability criteria and the phonon dispersion show mechanical and dynamic stability in both ambient pressure and under high pressure up to 20 GPa. The ZnSiN<inf>2</inf> and ZnGeN<inf>2</inf> exhibit linear increments in bulk modulus (B), shear modulus (G), and Young's modulus (E) under pressure, while ZnSnN<inf>2</inf> experiences a decrease in G and E. Notably, the energy gap of ZnSiN<inf>2</inf>, ZnGeN<inf>2</inf>, and ZnSnN<inf>2</inf> (4.62 eV indirect, 2.82 eV, 1.16 eV, respectively) increases with pressure due to higher N s orbital energy, approaching the UV region. In the valence band, a hybridization of N p and Si/Ge/Sn p orbitals is observed, offering opportunities to tailor the band gap for optimal applications in optoelectronic devices. Preferentially adjusting group-IV elements over group-II elements is recommended for optimizing band gap modulation. The correlation between larger atomic size and decreased band gap energy highlights the potential to fine-tune material properties through controlled variations in group-IV elements.
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    Investigating the phase transition and properties of CaSiN2 under pressure based on first-principles calculations
    (2023-12-01)
    Meethan, Weerachon
    ;
    Kongnok, Thanundon
    ;
    Fongkaew, Ittipon
    ;
    Bootchanont, Atipong
    ;
    Saisopa, Thanit
    In this study, we apply first-principles calculations to examine the pressure-induced phase transformation of CaSiN<inf>2</inf> in a range of pressure of 0–100 GPa. Its pressure-induced transitions at 1.3 GPa, 15.3 GPa, and 55.8 GPa followed the order of α- CaSiN<inf>2</inf> → β- CaSiN<inf>2</inf> → α- CaSiN<inf>2</inf> → γ- CaSiN<inf>2</inf>, for α- CaSiN<inf>2</inf> → β- CaSiN<inf>2</inf>, β- CaSiN<inf>2</inf> → α- CaSiN<inf>2</inf>, and α- CaSiN<inf>2</inf> → γ- CaSiN<inf>2</inf>, respectively. The stability of the phases of CaSiN<inf>2</inf> was confirmed based on calculations of the Born criterion of elastic stability. Its behavior transitioned in the sequence of brittle (0–1.3 GPa) → ductile (1.3–55.8 GPa) → brittle (55.8–100 GPa). The structure of its projected orbital band reflected insulating behavior by CaSiN<inf>2</inf> under a range of pressure of 0–55.8 GPa with a direct band gap, which transformed into metallic behavior by the γ- CaSiN<inf>2</inf> phase under pressures higher than 55.8 GPa, due to a shift in energy to higher levels around the Γ point of the N p orbitals and Si p orbitals. The Si-N bonds in CaSiN<inf>2</inf> were found to be covalent, while ionic bonding dominated the Ca-Si and Ca-N bonds in the range of pressure of 0–100 GPa. We also investigate and discuss its mechanical properties, Vickers hardness (H<inf>v</inf>), average sound velocity v<inf>m,</inf> and Debye temperature (θ<inf>D</inf>).
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    Disentangling small-polaron and Anderson-localization effects in ceria: Combined experimental and first-principles study
    (2019-01-23)
    Kolodiazhnyi, Taras
    ;
    Tipsawat, Pannawit
    ;
    Charoonsuk, Thitirat
    ;
    Kongnok, Thanundon
    ;
    Jungthawan, Sirichok
    By comparison of the electrical conductivity of ceria doped with penta- and hexavalent ions, we separate the total electron localization energy into the two contributions originating from the small polaron effects and the Coulomb interaction with the donor ions. The upper bound of the itinerant small polaron hopping energy is estimated at 66±20 meV. The binding energy of the Ce3+-M5+/6+ defect complex increases from 121 meV for M=Nb5+/Ta5+ to 243 meV for M=W6+/U6+. The first-principles simulations are in qualitative agreement with the experimental findings. At low temperatures the f electrons bound to the donor defects show dielectric relaxation with the lowest activation energy of 2.7 and 17 meV for Nb(Ta)- and W-doped ceria, respectively. Remarkably, these energies are significantly smaller than the hopping energy of the itinerant small polarons. While both the electron-lattice and the electron-defect interactions cause the f electron localization in real-case ceria, the latter effects seem to be the dominant.