| 1. |
- 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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| 2. |
- Koči, Love, et al.
(författare)
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Melting of Na at high pressure from ab initio calculations
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:13
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Tidskriftsartikel (refereegranskat)abstract
- The alkali metals continue to attract attention due to their enigmatic melting behavior at high pressure. In this study, ab initio molecular dynamics (AIMD) calculations for Na using the projector augmented waves method and the generalized gradient approximation as exchange-correlation energy were performed. The melting temperature (T-m) results from the AIMD at extreme conditions confirm the recent findings of a T-m maximum and show a decrease of similar to 200 K between 45 and 120 GPa. Furthermore, from elastic constant calculations using the exact muffin-tin orbitals method, a Lindemann model curve was calculated. The model reproduces the trend of the experimental results of Gregoryanz [Phys. Rev. Lett. 94, 185502 (2005)] who reported an extraordinarily big melting temperature drop of similar to 700 K between 31 and 118 GPa.
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| 3. |
- Manotum, R., et al.
(författare)
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Effect of pressure on the structure stability, electronic structure and band gap engineering in Zn16O1S15
- 2018
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Ingår i: Computational Condensed Matter. - : Elsevier B.V.. - 2352-2143. ; 17
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Tidskriftsartikel (refereegranskat)abstract
- Crystal structures and high pressure structural phase transitions of Zn16O1S15 have been investigated using density functional theory calculation. The two candidate high pressure structures namely Wurtzite and Zincblende were examined for theirs stability and properties up to 20 GPa. The co-exist phase of both structure which occurred during the different film growth conditions was fully explained. Phonon dispersion and the Born criteria reveal that Zincblende is only stable up to 10 GPa. Besides, Wurtzite structure yield no imaginary phonon frequencies and also satisfy the elastic constants sufficiency condition up to 20 GPa which indicated that the co-exist phase would eventually become the single Wurtzite structure above 10 GPa. The electronic structure and PDOS were also fully investigated using HSE06. The multiple band gap energy and mid O-3 state between fundamental ZnS band gap was revealed for the first time. The pressure effect on their electronic structure has been investigated for possible applications in adjustable optoelectronic device.
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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. |
- Phusittrakool, A., et al.
(författare)
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High pressure structural phase transitions in Sr from ab initio calculations
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:17
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Tidskriftsartikel (refereegranskat)abstract
- The pressure-induced phase transition in strontium, including the recent experimental observation of high-pressure phase Sr-IV, was investigated theoretically using the projector augmented wave method. We also concentrated on the experimentally observed beta-tin structure as this structure has never been found in any other alkaline-earth metals. The fcc-bcc phase transition was reproduced, but Sr-IV was found to be energetically more stable than the beta-tin structure at all pressures. By considering enthalpy, the sequence of the phase transitions along the pressure range is fcc -> bcc -> Sr-IV. With some detailed examination, we concluded that the beta-tin structure is either a metastable state or a coexisting phase, and further experimental investigation is suggested.
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| 6. |
- Pinsook, U., et al.
(författare)
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Internal vibrations of the Li(NH3)(4)(+) complex analyzed from ab initio, density functional theory, and the classical spring network model
- 2008
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:24, s. 5323-5326
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Tidskriftsartikel (refereegranskat)abstract
- We report our theoretical findings regarding internal vibrations of the Li(NH3)(4)(+) complex which have been studied using three different methods, namely, a classical spring network model, density functional theory, and ab initio Hartree-Fock plus Moller-Plesset correlation energy correction truncated at second-order. The equilibrium Li center dot center dot center dot N and N center dot center dot center dot N distances are found to be 2.12 and 3.47 angstrom, respectively, in good agreement with the experimental data. The theoretically determined vibrational frequencies of the lowest modes are in good agreement with those extracted from inelastic X-ray scattering measurements. From group theory considerations, the internal vibrations of Li(NH3)(4)(+) complexes resemble those of a tetrahedral object. Further experimental investigation is suggested.
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| 7. |
- Srepusharawoot, P., et al.
(författare)
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Ab Initio Study of Molecular Hydrogen Adsorption in Covalent Organic Framework-1
- 2009
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 113:19, s. 8498-8504
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption energies of hydrogen molecules at all possible adsorption sites of covalent organic framework-1 (COF-1) are studied by density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2). The most favorable adsorption sites from our DFT results are on the top of an oxygen atom for the B3O3 ring and on the top of the center of the C-C bond for the benzene ring when a single H-2 is interacting with the COF-1. The adsorption energy trend obtained from the DFT calculations is found to be in good agreement with the MP2 binding energy trend. The binding preferences are slightly changed when high hydrogen loading is considered. H-2 molecules prefer to be trapped on the top of the carbon atoms of the benzene ring and also on the top of the oxygen atoms of the B3O3 ring. These findings are confirmed by hydrogen center-of-mass distribution results obtained using molecular dynamics simulations. Moreover, our DFT results reveal that the hydrogen adsorption energies are boosted when we increase the number of hydrogen loadings due to attractive H-2-H-2 interactions. In addition, the nonavailability of the remaining adsorption sites in the COF-1 leads to a reduction of the H-2 mobility.
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| 8. |
- Srepusharawoot, P., et al.
(författare)
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Evidence of a medium-range ordered phase and mechanical instabilities in strontium under high pressure
- 2012
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Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 152:13, s. 1172-1175
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Tidskriftsartikel (refereegranskat)abstract
- We provided the first theoretical evidence for a medium-range ordered phase in high pressure strontium from the first-principles calculations. At the absolute zero temperature, the enthalpy-pressure relation shows that the bcc and hcp are energetically more favorable than the other experimentally observed phases between 24 and 27 GPa. In the present work, we concentrate on the bcc phase because we found a link to a medium-range ordered phase. Our results reveal that the bcc phonon dispersion at the N and H points starts softening at around 24.1 GPa. The ab initio molecular dynamics at 300 K and 27 GPa showed that the bcc is quickly transformed into a lower energy structure with R3c symmetry and distorted basis. The simulated diffraction patterns showed that the R3c structure has only a single major peak at low angle. The R3c peak locates near the first peak of the bcc structure. This is the evidence of the so-called medium-range ordered phase. This structure is a strong candidate for the unsolved S-phase reported by experiments. (C) 2012 Elsevier Ltd. All rights reserved.
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| 9. |
- Sukmas, W., et al.
(författare)
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Data-driven analysis of the rotational energy landscapes of an organic cation in a substituted alloy perovskite
- 2021
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Ingår i: Materials Advances. - : Royal Society of Chemistry. - 2633-5409. ; 2:7, s. 2366-2372
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Tidskriftsartikel (refereegranskat)abstract
- Lead-free hybrid organic-inorganic perovskites have recently emerged as excellent materials particularly in highly potential yet low-cost photovoltaic technologies. Calculations have previously suggested that CH3NH3BiSeI2 can be used as an alternative material for the highly studied CH3NH3PbI3 due to its eco-friendliness and comparable performance. Herein, with the aid of Euler angles, the interplay between the organic CH3NH3 (MA) cation and the inorganic BiSeI2 framework, obtained from first-principles calculations, is thoroughly scrutinised by means of the multidimensional total energy landscape. The highest peak of 17.9 meV per atom, protruding from the average plateau of 9 meV per atom within the four-dimensional topography, is equivalent to 208 K, the temperature at which the MA cations freely reorient. Moreover, the complexity of the angle-energy relationship is mitigated by exploiting a high-fidelity simulation based on deep learning. The deep artificial neural network of five hidden layers with 500 neurons, each fed by ten descriptors-three Euler angles and seven various bond lengths-predicts the total energies with an accuracy within the root mean square error of 0.39 ± 0.03 meV per atom for the test dataset. This novel statistical model in turn offers a tantalising promise to provide an accurate prediction of this materials energies, while diminishing the need for costly first-principles calculations. © The Royal Society of Chemistry.
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