| 1. |
- Mopoung, Kunpot, et al.
(author)
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First-principles demonstration of band filling-induced significant improvement in thermodynamic stability and mechanical properties of Sc1-xTaXB2 solid solutions
- 2023
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In: Scientific Reports. - : NATURE PORTFOLIO. - 2045-2322. ; 13:1
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Journal article (peer-reviewed)abstract
- Mixtures of different metal diborides in the form of solid solutions are promising materials for hard-coating applications. Herein, we study the mixing thermodynamics and the mechanical properties of AlB2-structured Sc1-xTaXB2 solid solutions using the first-principles method, based on the density functional theory, and the cluster-expansion formalism. Our thermodynamic investigation reveals that the two diborides readily mix with one another to form a continuous series of stable solid solutions in the pseudo-binary TaB2 -ScB2 system even at absolute zero. Interestingly, the elastic moduli as well as the hardness of the solid solutions show significant positive deviations from the linear Vegards rule evaluated between those of ScB2 and TaB2. In case of Sc1-xTaXB2, the degrees of deviation from such linear trends can be as large as 25, 20, and 40% for the shear modulus, the Youngs modulus, and the hardness, respectively. The improvement in the stability as well as the mechanical properties of Sc1-xTaXB2 solid solutions relative to their constituent compounds is found to be related to the effect of electronic band filling, induced upon mixing TaB2 with ScB2. These findings not only demonstrate the prominent role of band filling in enhancing the stability and the mechanical properties of Sc1-xTaXB2, but also it can potentially open up a possibility for designing stable/metastable metal diboride-based solid solutions with superior and widely tunable mechanical properties for hard-coating applications.
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| 2. |
- Mopoung, Kunpot, et al.
(author)
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Investigation of in situ annealing effect on the thermal stability and crystallinity of IrMn thin films by X-ray diffraction and electron energy loss spectroscopy
- 2022
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In: Thin Solid Films. - : ELSEVIER SCIENCE SA. - 0040-6090 .- 1879-2731. ; 762
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Journal article (peer-reviewed)abstract
- The crystal reorganization of the IrMn thin film was observed by the in situ thermal annealing in the X-ray diffraction (XRD) and electron energy loss spectroscopy. From room temperature to 700 degrees C, the in situ annealing measurements identify the different Mn diffusion effects in three temperature ranges. First, between room temperature and 300 degrees C, the XRD profiles show the reorganization of Mn atoms from interstitials to IrMn lattice points. Second, between 300 and 400 degrees C, we observed the diffusion of Mn atoms from the IrMn thin film to the surface, causing the atomic Mn/Ir ratio to drop from 10 to 7. The MnO appeared on the film surface in this temperature range. Third, from 400 to 700 degrees C, the O content in the IrMn thin film increases to 8%, while the Mn/ Ir ratio continuously decreases from 7 to 5. The scanning transmission electron microscopy images also show that the crystal structure of IrMn thin film completely degrades to another structure.
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| 3. |
- Mopoung, Kunpot, et al.
(author)
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Spin Centers in Vanadium-Doped Cs2NaInCl6 Halide Double Perovskites
- 2024
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In: ACS Materials Letters. - : AMER CHEMICAL SOC. - 2639-4979. ; 6:2, s. 566-571
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Journal article (peer-reviewed)abstract
- We provide direct evidence for a spin-active V4+ defect center, likely in the form of a VO2+ complex, predominantly introduced in single crystals of vanadium-doped Cs2NaInCl6 halide double perovskites grown by the solution-processed hydrothermal method. The defect has C-4v point group symmetry, exhibiting an electron paramagnetic resonance (EPR) spectrum arising from an effective electron spin of S = 1/2 and a nuclear spin of I = 7/2 (corresponding to V-51 with nearly 100% natural abundance). The determined electron g-factor and hyperfine parameter values are g(perpendicular to)= 1.973, g(parallel to) = 1.945, A(perpendicular to) = 180 MHz, and A(parallel to) = 504 MHz, with the principal axis z along a < 001 > crystallographic axis. The controlled growth of V-doped Cs2NaInCl6 in an oxygen-free environment is shown to suppress the V4+ EPR signal. The defect model is suggested to have a VOCl5 octahedral coordination, where one of the nearest-neighbor Cl- of V is replaced by O2-, with octahedral compression along the V-O axis. This VO complex formation competes with the isolated V3+ substitution of In3+, which in turn provides a means for the charge-state tuning of V ions. This finding calls for a better understanding and control of defect formation in solution-grown halide double perovskites, which is critical for optimizing and tailoring material design for solution-processable optoelectronics and spintronics.
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| 4. |
- Mopoung, Kunpot, et al.
(author)
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Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors
- 2024
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In: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 128:12, s. 5313-5320
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Journal article (peer-reviewed)abstract
- Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs-2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the B-I site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic B-I site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.
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