Papers by Dr.thiti Bovornratanaraks
Computational Condensed Matter, 2018
Crystal structures and high pressure structural phase transitions of Zn 16 O 1 S 15 have been inv... more Crystal structures and high pressure structural phase transitions of Zn 16 O 1 S 15 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 difference 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.
Carbon, 2019
We applied laser-heating in diamond anvil cells (LHDAC) to synthesize a hydrogenated single-layer... more We applied laser-heating in diamond anvil cells (LHDAC) to synthesize a hydrogenated single-layer graphene (SLG) and to explore the pathway toward graphane (fully hydrogenated SLG). We employed Raman spectroscopy to investigate SLG on a Cu substrate that was compressed up to 8 GPa and 20 GPa with 2.2% and 4.6% compressive strain, respectively, followed by laser-heating. After laser-heating, G and 2D peaks exhibit a redshift, and then form a hysteresis loop during decompression. This phenomenon can be due to either of two mechanisms, or both; the formation of CeH chemical bonds in massive hydrogenated SLG, and a reduction of the frictional stress between SLG and Cu substrate causing a relaxation of SLG lattice toward its free-standing equilibrium structure. The correlation between G and 2D peaks also changes significantly after laser-heating at 8 GPa, resembling the correlation measured in hole-doping experiments. Finally, residual hydrogen remains bonded to the graphene layer after decompression to ambient pressure, and the amount of hydrogen increases as a function of pressure at which the sample was laser-heated.
Nature of electronic topological transition and superconductivity in bismuth under high pressure from ab initio random structure searching
Computational Materials Science, 2021
Abstract We have predicted the hexagonal close-packed (hcp) structure of bismuth (Bi) using ab in... more Abstract We have predicted the hexagonal close-packed (hcp) structure of bismuth (Bi) using ab initio random structure searching (AIRSS) at extreme conditions. The calculation, which included spin–orbit coupling, shows that the hcp structure is thermodynamically and dynamically stable at high pressure. The electronic band structure calculations suggest the downshifting of the flat band through compression due to Lifshitz transitions. The Fermi surface shape of hcp Bi produces the metallicity in this material. The electron localization function reveals a weak bonding of Bi. The solutions of electronic topological transition and a soft-mode of phonon dispersion provide the possibility for prediction and reduction of the superconducting transition temperature.
Final Report on Structure and Properties of Materials Under High Pressure
旭硝子財団助成研究成果報告, 2008
Thesis (Ph. D.)--University of Edinburgh, 2001.
RSC Advances
Lanthanum hydride compound LaH3 become stabilized by yttrium substitution under the influence of ... more Lanthanum hydride compound LaH3 become stabilized by yttrium substitution under the influence of moderate pressure.
Recent experiments have shown that CeH9 and (Ce,La)H9 can be synthesized under high pressure betw... more Recent experiments have shown that CeH9 and (Ce,La)H9 can be synthesized under high pressure between 90-170GPa and become a superconductor with a high value of superconducting critical temperature (Tc) between 100-200K. In this work, we performed a theoretical study of a (Ce,La)H9 compound where the Ce:La ratio is equal to 1. We used the density functional theory and the ab initio molecular dynamics (AIMD) method. From the phonon dispersion, there exist some unstable modes around the K-point phonons. Then, we performed AIMD simulation at around 203K and found that the compound becomes stable. The superconducting spectral function can be calculated. We found that λ is as high as 3.0 at 200GPa. By using Allen-Dynes-modified McMillan equation in the strong coupling regime, we found that Tc = 87K at 200GPa.
Scientific Reports, 2022
We use the first principle calculation to investigate the intrinsic magnetism of graphitic carbon... more We use the first principle calculation to investigate the intrinsic magnetism of graphitic carbon nitrides (GCNs). By preserving three-fold symmetry, the GCN building blocks have been built out of different combinations between 6 components which are C atom, N atom, s-triazine, heptazine, heptazine with C atom at the center, and benzimidazole-like component. That results in 20 phases where 11 phases have been previously reported, and 9 phases are newly derived. The partial density of states and charge density have been analyzed through 20 phases to understand the origin of the presence and absence of intrinsic magnetism in GCNs. The intrinsic magnetism will be present not only because the GCNs comprising of radical components but also the $$\pi$$ π -conjugated states are not the valence maximum to break the delocalization of unpaired electrons. The building blocks are also employed to study alloys between g-$$\hbox {C}_3\hbox {N}_4$$ C 3 N 4 and g-$$\hbox {C}_4\hbox {N}_3$$ C 4 N 3 ...
Effects of Mg substitution on electronic properties of CaF2
Journal of Physics: Conference Series, 2021
We investigated the phase transitions of CaF2, using the state-of-the-art density functional theo... more We investigated the phase transitions of CaF2, using the state-of-the-art density functional theory. The calculations show that the fluorite structure, with the space group Fm 3 ¯ m , transforms into the cotunnite structure, with the space group Pnma, at 8 GPa. The electronic band structure of the Pnma-CaF2 structure can be classified as an insulating phase with a wide bandgap, and the bandgap decreases under pressure above 70 GPa. Also, we proposed the Ca/Mg-substituted fluorides MgxCa1-xF2, using the cluster expansion technique which is commonly used to study alloys. We found that the Mg0.25Ca0.75F2 compound with a Pm structure is stable at 70 GPa. The results of the electronic band structure reveal that the substitution of Mg enhances the bandgaps of the Mg0.25Ca0.75F2 compounds under pressure above 70 GPa, compared with those of CaF2.
Stability and electronic structure of magnesium hydride and magnesium deuteride under high pressure
Journal of Physics: Conference Series, 2021
Metal polyhydrides have attracted considerable attention because some of them become a metal unde... more Metal polyhydrides have attracted considerable attention because some of them become a metal under high pressure, and some undergo a phase transition into a superconductor. Some superconducting metal polyhydrides have recently been discovered with a high value of critical temperature (Tc) under pressure. In this research, we calculated the structures of MgH2, MgH3 and MgD3 under pressure between 0-300 GPa in order to determine the formation enthalpy and electronic property of their structures under high pressure by using density functional theory (DFT) based on the Quantum Espresso code. We found that the band structures reveal the metallic character of the compounds under high pressure. The energy band structures of MgHx and MgDx are exactly the same. However, their phonon dispersions are different due to the so-called isotope effect. We determined the composition stability by using the convex hull of Mg, H and the compounds. We found that MgH3 becomes thermodynamically more stable...
Lead-free hybrid organic–inorganic perovskites have recently emerged as excellent materials parti... more 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 highfidelity simulation based on deep learning. The deep artificial neural network of five...
Scientific Reports, 2020
A microscopic viewpoint of structure and dipolar configurations in hybrid organic–inorganic perov... more A microscopic viewpoint of structure and dipolar configurations in hybrid organic–inorganic perovskites is crucial to understanding their stability and phase transitions. The necessity of incorporating dispersion interactions in the state-of-the-art density functional theory for the $$CH_3NH_3PbI_3$$ C H 3 N H 3 P b I 3 perovskite (MAPI) is demonstrated in this work. Some of the vdW methods were selected to evaluate the corresponding energetics properties of the cubic MAPI with various azimuthally rotated MA organic cation orientations. The highest energy barrier obtained from PBEsol reaches 18.6 meV/MA-ion, which is equivalent to 216 K, the temperature above which the MA cations randomly reorient. Energy profiles calculated by vdW incorporated functionals, on the other hand, exhibit various distinct patterns. The well-developed vdW-DF-cx functional was selected, thanks to its competence, to evaluate the total energies of different MA dipolar configurations in $$2\times 2\times 2$$ ...
Superconductivity of superhydride CeH10 under high pressure
Materials Research Express, 2020
A large class of metal superhydrides was found to be a conventional BCS superconductor under high... more A large class of metal superhydrides was found to be a conventional BCS superconductor under high pressures. In this work, we focused on cerium decahydride, CeH10. Ce is a member of the so–called lability belt in the periodic table, where the physical properties can be largely affected by pressure. It was reported and we confirmed that CeH10 can be formed with the Fm-3m structure, where a cerium atom is embedded in a H32 clathrate cage. Our phonon calculations show that it is dynamically stable at around 300 GPa onwards. We examined the evolution under pressures of the phonons, the electronic states, the Fermi surface, and the electron localization function (ELF). There exists a small van Hove singularity (vHs), and it gradually moves to below the Fermi surface as pressure increases. This behaviour associating with the reducing value of the electron-phonon coupling strength (λ), causes the superconductivity transition temperature (Tc) to gradually reduce under pressures. The maximum...
Scientific Reports, 2018
Effects of electronic nonlocality in density functional theory study of structural and energetic ... more Effects of electronic nonlocality in density functional theory study of structural and energetic properties of a pseudocubic CH 3 NH 3 PbI 3 are investigated by considering coherent rotation around C-N axis of a CH 3 NH 3 cation. A number of truly non-local and semi-local exchange correlation density functionals are examined by comparing calculated structural parameters with experimental results. The vdW-DF-cx which takes into account the non-local van der Waals correlation and consistent exchange shows the best overall performance for density functional theory study of this system. Remarkable distinctions between results from vdW-DF-cx and those from PBEsol exchange correlation functionals are observed and indicate the need of including the non-local interaction in the study of this system, especially its dynamical properties. The obtained rotational barriers are 18.56 meV/formula and 27.71 meV/ formula which correspond to rotational frequencies of 3.71 THz and 2.60 THz for vdW-DF-cx and PBEsol calculations, respectively. Interestingly, the maximally localised Wannier function analysis shows the hydrogen bonding assisted covalent character of two iodide anions at a moderate rotational angle which can lead to I 2 formation and then material degradation.
Journal of Electronic Materials, 2019
Bismuth telluride (Bi 2 Te 3) thin films have been deposited onto polyimide sheet substrates by d... more Bismuth telluride (Bi 2 Te 3) thin films have been deposited onto polyimide sheet substrates by direct-current (DC) magnetron sputtering at different powers and their microstructure, composition, and electrical and thermoelectrical properties studied. The experimental results indicated that the sputtering power was the key parameter determining their thermoelectric properties. X-ray diffraction analysis confirmed the highly (015) preferred orientation of the films. The Te content and grain size depended on the sputtering power. The power factor of Bi 2 Te 3 deposited by DC sputtering at power of 60 W was comparable to that obtained for highly (00l) Bi 2 Te 3 thin film.
Journal of Alloys and Compounds, 2017
Structural, electronic, optical and mechanical properties of Zn, Si, Sn and S substitutions on In... more Structural, electronic, optical and mechanical properties of Zn, Si, Sn and S substitutions on InP supercell under pressure in zinc blende (ZB) and rock salt (RS) phases are presented using first-principles method. Cohesive energy and enthalpy difference are observed, and found that the order of possible spontaneous process in experimental growth, which introduced from enthalpy difference, is (In,Zn) P > In(P,S) > In(Si,P) > In(Sn,P). The lower enthalpy difference in RS structure indicates that the spontaneous process of impurity substitution can be occurred in RS more than in ZB. Phase transition from ZB to RS reduces the strain on crystal lattice by the increasing of chemical bond length. The chemical bonding of Zn-P in ZB is the strongest sharing electrons when compared with other compounds, Zn-P>Si-In>S-In>Sn-In. The dielectric performance of InP is reduced by the alloying effect, and it transforms to the conductor performance as high frequencies. Order of photo-absorption coefficient in range of visible light with the impurities is Sn>Si>S>Zn, and it reduces under high-pressure. Mechanical stability of InP alloys was observed, and satisfied the Born stability criteria. The impurities reduce Shear modulus of pure InP. Poisson's ratio of InP alloys in RS exhibit small deformation and high isotropy, corresponding to high-symmetry cubic phase. B/G ratio indicates that ductility of InP alloys is reduced, when it transformed to RS. The ductility of InP is induced by the alloying effect due to the B/G ratio increasing.
The Journal of Physical Chemistry C, 2016
Employing a systematic first-principles investigation with crystal structure searching based on a... more Employing a systematic first-principles investigation with crystal structure searching based on an evolutionary algorithm, we have uncovered the novel phase (P4 2 /nmc) of OsB 4 with a novel superhardness and semiconducting state. In this investigation, metal-to-semiconductor phase transition is predicted at only a few gigapascals above ambient pressure, i.e. 11 GPa. As a result, the P4 2 /nmc phase should potentially become a metastable phase at ambient pressure. The Vickers (polycrystalline) hardness 1
Acta Crystallographica Section A Foundations of Crystallography, 2008
E-mail t.yamanaka@ kce.biglobe.ne.jp Many AB2O4 spinels transform to post-spinel structures witho... more E-mail t.yamanaka@ kce.biglobe.ne.jp Many AB2O4 spinels transform to post-spinel structures without decomposition. Three structures of CaMn2O4 (Pmab), CaFe2O4 (Pnam) and CaTi2O4 (Bbmm) have been reported as post-spinel phases. Structure studies were conducted under high pressure using SR. CaMn2O4 transforms to CaTi2O4 type structure at about 30GPa. MnO6 octahedron is distorted by the Jahn-Teller effect induced from Mn 3+ (3d4) in the octahedral site. X-ray emission study at APS has been executed to discloses high-low spin transition of CaMn2O4 up to 100GPa. The spin transition gradually proceeds from 50GPa and then mixed spin states at 80GPa. Low-spin state is confirmed at 85GPa. A new phase is found by powder diffraction at 82 GPa. This structure (Pnam) is produced via martensic transformation by displacing atoms in every three layers perpendicular to the c-axis. Four independent sites of Mn atoms yield the different spin transition pressures. PbTiO3 tetragonal perovskite structure (P4mm) at ambient conditions transforms cubic (Pm3m) at about 12GPa, which induces the transition from ferroelectric to paraelectric. Electron density distribution analysis is performed by maximum entropy method (MEM) using single-crystal diffraction intensities up to 12GPa taken at PF and up to 35GPa by two-circle diffractometer with IP at APS. Pressure dependence of the static dipole moment is elucidated by charge density analysis. MEM provides the split atom model of Ti and oxygen atoms in the direction to the c-axis and in the cubic phase no more splitting was observed. The split is generated from the statistical distribution of atoms or domain structures. The split becomes gradually smaller with increasing pressure. This is an intrinsic phenomenon in PbTiO3. Polarization in PbTiO3 is partly canceled.
Solid State Communications, 2015
Carbon dioxide exhibits a richness of high pressure polymorphs ranging from typical molecular sol... more Carbon dioxide exhibits a richness of high pressure polymorphs ranging from typical molecular solids to fully extended covalent solids, which, in turn, makes it a very appealing topic of fundamental research in condensed matter physics and simultaneously provides with valuable insights into the routes of developing possibly novel materials with advanced properties. This marked for the first time that the single crystal X-ray diffraction (XRD) and Brillouin scattering spectroscopy of CO2-I had been performed under high temperature and pressure. Densities, acoustic velocities and elastic moduli of CO2-I were obtained along 300 K, 400 K and 580 K isotherms up to the phase transition boundaries. CO2-I transforms to phase III and phase IV at room temperature (at 12.19 GPa) and 580 K (at 10.83 GPa), respectively. It was observed that high temperature suppresses pressure-induced stress in single crystal CO2-I. All elastic constants and thermal elasticity parameters of CO2-I were obtained and analyzed using finite-strain theory and thermal equation of state modeling. The C11, C12 and KS increase almost linearly with pressure, while shear moduli C44 and G exhibit a downward trend with pressure, showing a noticeable reduction at higher temperature. Elastic anisotropy A is practically independent of pressure along each isotherms and increases from 1.75 to 1.9.
Physical Review B, 2000
A previously unknown type of elemental structure has been discovered recently in barium, phase IV... more A previously unknown type of elemental structure has been discovered recently in barium, phase IV. The structure is composed of a ''host'' and two or more ''guest'' components made up of chains that lie in channels in the host, and form structures that are incommensurate with the host. This implies a segregation into different electronic species. The same structure has now been found in phase V of Sr above 45 GPa.
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Papers by Dr.thiti Bovornratanaraks