| 1. |
- Atthapak, C., et al.
(författare)
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Effect of atomic configuration and spin-orbit coupling on thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 solid solutions
- 2021
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 23:24, s. 13535-13543
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Tidskriftsartikel (refereegranskat)abstract
- Through a combination of density functional theory calculations and cluster-expansion formalism, the effect of the configuration of the transition metal atoms and spin-orbit coupling on the thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 is investigated. Our investigation reveals that, in spite of exhibiting a weak ordering tendency of Mo and W atoms at 0 K, monolayer 2H-Mo1-xWxS2 is thermodynamically stable as a single-phase random solid solution across the entire composition range at temperatures higher than 45 K. The spin-orbit coupling effect, induced mainly by W atoms, is found to have a minimal impact on the mixing thermodynamics of Mo and W atoms in monolayer 2H-Mo1-xWxS2; however, it significantly induces change in the electronic bandgap of the monolayer solid solution. We find that the band-gap energies of ordered and disordered solid solutions of monolayer 2H-Mo1-xWxS2 do not follow Vegards law. In addition, the degree of the SOC-induced change in band-gap energy of monolayer 2H-Mo1-xWxS2 solid solutions not only depends on the Mo and W contents, but for a given alloy composition it is also affected by the configuration of the Mo and W atoms. This poses a challenge of fine-tuning the bandgap of monolayer 2H-Mo1-xWxS2 in practice just by varying the contents of Mo and W.
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| 2. |
- Ektarawong, A., et al.
(författare)
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Boron vacancy-driven thermodynamic stabilization and improved mechanical properties of AlB2-type tantalum diborides as revealed by first-principles calculations
- 2023
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Ingår i: Journal of Physics. - : IOP Publishing Ltd. - 2515-7639. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- Thermodynamic stability as well as structural, electronic, and elastic properties of boron-deficient AlB2-type tantalum diborides, which is designated as alpha-TaB2-x, due to the presence of vacancies at its boron sublattice are studied via first-principles calculations. The results reveal that alpha-TaB2-x, where 0.167 less than or similar to x less than or similar to 0.25, is thermodynamically stable even at absolute zero. On the other hand, the shear and Youngs moduli as well as the hardness of stable alpha-TaB2-x are predicted to be superior as compared to those of alpha-TaB2. The changes in the relative stability and also the elastic properties of alpha-TaB2-x with respect to those of alpha-TaB2 can be explained by the competitive effect between the decrease in the number of electrons filling in the antibonding states of alpha-TaB2 and the increase in the number of broken bonds around the vacancies, both induced by the increase in the concentration of boron vacancies. A good agreement between our calculated lattice parameters, elastic moduli and hardness of alpha-TaB2-x and the experimentally measured data of as-synthesized AlB2-type tantalum diborides with the claimed composition of TaB similar to 2, available in the literature, suggests that, instead of being a line compound with a stoichiometric composition of TaB2, AlB2-type tantalum diboride is readily boron-deficient, and its stable composition in equilibrium may be ranging at least from TaB similar to 1.833 to TaB similar to 1.75. Furthermore, the substitution of vacancies for boron atoms in alpha-TaB2 is responsible for destabilization of WB2-type tantalum diboride and orthorhombic Ta2B3, predicted in the previous theoretical studies to be thermodynamically stable in the Ta-B system, and it thus enables the interpretation of why the two compounds have never been realized in actual experiments.
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| 3. |
- Kanchanavatee, N., et al.
(författare)
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Phase transitions and suppression of magnetoresistance in WTe2-xSex system
- 2022
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Ingår i: Journal of Physics. - : IOP Publishing Ltd. - 0953-8984 .- 1361-648X. ; 34:43
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Tidskriftsartikel (refereegranskat)abstract
- X-ray diffraction, Raman spectroscopy, and electrical resistivity measurements on polycrystalline WTe2-xSex (0 <= x <= 0.8) reveal a T-d - 1T structural phase transition and suppression of magnetoresistance at x = 0.2. These phenomena are consistent with the pressure phase diagram of WTe2. However, chemical pressure due to substitution of smaller Se ion cannot generate pressure required for the phase transition. Strain induced by sample inhomogeneity is believed to be a trigger to the behaviors. In agreement with previous predictions and reports, a mixed phase of 1T and 2H structures was also detected in Se-rich samples. Coincidentally at x = 0.2, electrical resistivity analysis suggests a phase transition from a metallic phase to a nonmetallic phase that is possibly a topological-insulating phase.
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| 4. |
- Pakornchote, T., et al.
(författare)
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Phase stabilities and vibrational analysis of hydrogenated diamondized bilayer graphenes: A first principles investigation
- 2019
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Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 146, s. 468-475
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Tidskriftsartikel (refereegranskat)abstract
- The phase stabilities as well as some intrinsic properties of hydrogenated diamondized bilayer graphenes, 2-dimensional materials adopting the crystal structure of diamond and of lonsdaleite, are investigated using a first-principles approach. Our simulations demonstrate that hydrogenated diamondized bilayer graphenes are thermodynamically stable with respect to bilayer graphene and hydrogen molecule even at 0 GPa, and additionally they are found to withstand the physical change in structure up to at least 1000 K, ensuring their dynamical and thermal stabilities. The studied hydrogenated diamondized bilayer graphenes are predicted not only to behave as direct and wide band gap semiconductors, but also to have a remarkably high resistance to in-plane plastic deformation induced by indentation as implied by their high in-plane elastic constants comparable to those of diamond and of lonsdaleite. The mechanical stability of the materials is confirmed though the fulfilment of the Born stability criteria. Detailed analysis of phonon vibrational frequencies of hydrogenated diamondized bilayer graphenes reveals possible Raman active and IR active modes, which are found to be distinctly different from those of hydrogenated diamond-like amorphous carbon and defective graphene and thus could be used as a fingerprint for future experimental characterization of the materials. (c) 2019 Elsevier Ltd. All rights reserved.
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| 5. |
- Ektarawong, A., et al.
(författare)
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A comparison of the mixing thermodynamics of the antifluorite-structured Mg2Si1-xGex, Mg2Sn1-xGex and Mg2Si1-xSnx alloys from first principles
- 2021
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Ingår i: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 185
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Tidskriftsartikel (refereegranskat)abstract
- The mixing thermodynamics of the antifluorite-structured Mg2Si1-xGex is investigated using the first-principles calculations. We find that Mg2Si and Mg2Ge readily mix with each other leading to formation of a single-phase random solid solutions of Mg2Si1-xGex across the entire composition range from the temperature of about 50 K and above. At 0 K, Mg2Si1-xGex exhibits a weak energy preference toward local phase segregation into Mg2Si and Mg2Ge without forming any ordered patterns of Si and Ge atoms. Through a comparison with the mixing thermodynamics of Mg2Sn with Mg2Si or Mg2Ge, a small lattice misfit between Mg2Si and Mg2Ge of less than 1 % is responsible for the formation of stable Mg2Si1-xGex random solid solutions at such a low temperature. Besides their thermodynamic stability, our prediction reveals that the random solid solutions of Mg2Si1-xGex are dynamically and mechanically stable. These findings justify the uses of structural models of Mg2Si1-xGex, assuming a random distribution of Si and Ge atoms in the previous theoretical studies, and also provide an insight into the complete solubility of Mg2Ge in Mg2Si and vice versa at all temperature where the atomic diffusion is activated.
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| 6. |
- Ektarawong, A., et al.
(författare)
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Effect of thermally excited lattice vibrations on the thermodynamic stability of tungsten ditellurides WTe2 under high pressure: A first-principles investigation
- 2021
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Ingår i: Computational materials science. - : ELSEVIER. - 0927-0256 .- 1879-0801. ; 186
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Tidskriftsartikel (refereegranskat)abstract
- The thermodynamic stability as a function of pressure and temperature of three WTe2 polytypes, i. e., Td, 1T , and 2H phases, is examined using first-principles calculations, where the temperature-dependent contributions, arising particularly from the lattice vibrations, are derived from the quasiharmonic approximation. We find that the critical transition pressure to the 2H phase increases almost linearly with increasing temperature. Through the inspection of the phonon density of states, 2H-WTe2 is dynamically stable not only at high pressure, but also at zero pressure, indicating possible existence of 2H-WTe2 also at ambient conditions. On the other side, our results demonstrate that, at relevant temperature and pressure, the thermodynamic stability of 1T-WTe2 is comparable to that of Td-WTe2, and further analysis reveals substantial similarities in terms of structural behavior between Td-WTe2 and 1T-WTe2. These findings suggest not only that the two polytypes are likely to coexist in practical samples of WTe2 due probably to grains/regions either with tiny difference in stress, for example, or with different growth history, but also that the designation of 1T , having been regularly used to described the compound under pressure in the literature, might actually be a mixture of Td and 1T polytypes, whose WTe2 layers laterally slide via low transition barrier induced probably by the applied pressure, resulting in the splitting of (101) and (113) peaks as observed in the diffraction experiments.
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| 7. |
- Ektarawong, A., et al.
(författare)
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Role of spin-orbit coupling in the alloying behavior of multilayer Bi1-xSbx solid solutions revealed by a first-principles cluster expansion
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:13
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Tidskriftsartikel (refereegranskat)abstract
- We employ a first-principles cluster-expansion method in combination with canonical Monte Carlo simulations to study the effect of spin-orbit coupling on the alloying behavior of multilayer Bi1-xSbx. Our simulations reveal that spin-orbit coupling plays an essential role in determining the configurational thermodynamics of Bi and Sb atoms. Without the presence of spin-orbit coupling, Bi1-xSbx is predicted to exhibit at low-temperature chemical ordering of Bi and Sb atoms, leading to formation of an ordered structure at x approximate to 0.5. Interestingly, the spin-orbit-coupling effect intrinsically induced by the existence of Bi and Sb results in the disappearance of chemical ordering of the constituent elements within an immiscible region existing at T < 370 K, and consequently Bi1-xSbx displays merely a tendency toward local segregation of Bi and Sb atoms, resulting in coexistence of Bi-rich and Sb-rich Bi1-xSbx solid solutions without the formation of any ordered structure of Bi1-xSbx as predicted in the absence of spin-orbit coupling. These findings distinctly highlight an influence of spin-orbit coupling on the alloying behavior of Bi1-xSbx and probably other alloys composed of heavy elements, where the spin-orbit-coupling effect is supposed to be robust.
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| 8. |
- Kim, D. Y., et al.
(författare)
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Phase stability and superconductivity of strontium under pressure
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 101:5, s. 052604-
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Tidskriftsartikel (refereegranskat)abstract
- We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.
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| 9. |
- Kotmool, K., et al.
(författare)
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High pressure-induced distortion in face-centered cubic phase of thallium
- 2016
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 113:40, s. 11143-11147
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Tidskriftsartikel (refereegranskat)abstract
- The complex and unusual high-pressure phase transition of III-A (i.e. Al, Ga, and In) metals have been investigated in the last several decades because of their interesting periodic table position between the elements having metallic and covalent bonding. Our present first principles-based electronic structure calculations and experimental investigation have revealed the unusual distortion in face-centered cubic (f.c.c.) phase of the heavy element thallium (Tl) induced by the high pressure. We have predicted body-centered tetragonal (b.c.t) phase at 83 GPa using an evolutionary algorithm coupled with ab initio calculations, and this prediction has been confirmed with a slightly distorted parameter (√2 x a - c)/c lowered by 1% using an angle-dispersive X-ray diffraction technique. The density functional theory (DFT)-based calculations suggest that s-p mixing states and the valence-core overlapping of 6s and 5d states play the most important roles for the phase transitions along the pathway h.c.p → f.c.c. → b.c.t.
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| 10. |
- Kotmool, K., et al.
(författare)
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High-Pressure Structural Transformation Pathway and Electronic Properties of AgGaTe2 : Ab Initio Evolutionary Structural Searching
- 2022
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:33, s. 14236-14244
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Tidskriftsartikel (refereegranskat)abstract
- We have used systematic ab initio evolutionary structural searching to uncover the high-pressure transformation pathway of a promising thermoelectric material, AgGaTe2. The global structures of the ternary Ag-Ga-Te system have been predicted up to 100 GPa. The known chalcopyrite phase at ambient pressure is validated by the searching method. The B3-like structure with the space group (s.g.) of P((4) over bar)m(2) exhibits a metastable one at a low-pressure range. The first structural phase transition is calculated at about 4 GPa, processing the I((4) over bar) (2)d phase to a B1-like phase (s.g. Pmma). Other predicted structures, Pmn2(1) and Pm phases, are potentially coexisting phases up to 30 GPa because of the slightly different enthalpy. This finding reasonably explains the ambiguous results in the previous experiments. The high-pressure phase beyond 30 GPa is proposed to be a short-range alloy of bcc-Te and B2-AgGa rather than a cation-disordered B2-like phase. The band gap of the I((4) over bar)(2)d phase is increased with increasing pressure, while the metastable P((4) over bar)(2)m(2) phase is a narrow band gap semiconductor. The electron-phonon coupling of the metallic phases of ternary IB-IIIA-VIA(2) compounds is derived for the first time in AgGaTe2. They exhibit superconductors with a maximum Tc of 2.4 K in the Pmma phase at 6 GPa. The findings of this work not only provide a clear explanation of the high-pressure transformation pathway of AgGaTe2 but also suggest promising electronic properties guiding further applications, especially in a thermometric device, of this material under high pressure.
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