First-principles Predictions of Structures and Piezoelectric Properties of PbTiO3 Single Crystal

김민찬,Sang Goo Lee,Cheeyoung Joh,Hee Seon Seo 한국전기전자재료학회 2016 Transactions on Electrical and Electronic Material Vol.17 No.1

Using the various exchange-correlation functionals, such as LDA, GGA-PBE, GGA-PBEsol and GGA-AM05 functionals, first principle studies were conducted to determine the structures of paraelectric and ferroelectric PbTiO3. Based on the structures determined by the various functionals, the piezoelectric properties of PbTiO3 are predicted under the density-functional perturbation theory (DFPT). The present prediction with the various GGA functionals are closer to the experimental findings compared to the LDA values. The present DFT calculations using the GGA-PBEsol functional estimate the experimental data more reasonably than the conventional LDA and GGA fucntionals. The GGA-AM05 functional also predicts the experimental data as well as the GGA-PBEsol. The piezoelectric tensor calculated with PBEsol is relatively insensitive to pressure.

제일원리계산을 이용한 자성 연구

임성현(S. H. Rhim),홍순철(Soon Cheol Hong) 한국자기학회 2017 韓國磁氣學會誌 Vol.27 No.5

This review aims a brief and short-handed introduction to nonexperts, where most contents are based on talk given in Summer School 2017 in Pohang. Detailed derivations and formalism are avoided unless it is necessary to present. Historical evolution of density functional theory (DFT) is provided to grasp the main motivation of first-principles calculations in a more broad perspective. By doing so, a list of acronyms is shown, which in many cases become an obstacle to newcomers. Furthermore, a classification of DFT packages according to vendors, methods, and others is provided for better understanding. Next, the practical employment of ab initio calculations in magnetism is introduced, which includes spintronics, for more specifically, spin and anomalous Hall effect (SHE and AHE), magnetocrystalline anisotropy (MCA), magneto-optical Kerr effect (MOKE). Also, ingredients for DFT calculations - human, package, and infrastructure (cluster or supercomputing center) are briefly outlined. To do so, we expect long-term improvement and investment in our field, which in turn help other expertises.

First-principle study on catalytic activity of functionalized Ti3C2 MXene as cathode catalyst for Li–O2 batteries

Yang Yingying,Chen Jian,Gao Qiqian,Feng Yu,Xing Fei,Yao Man 한국물리학회 2022 Current Applied Physics Vol.34 No.-

Two-dimensional layered Ti3C2, one representative MXene, is notable as promising cathode catalyst for rechargeable lithium-oxygen (Li–O2) batteries. Using first-principles calculations, we construct cathode electrochemical interface catalytic model to simulate the structural evolution during discharging and charging processes, and the calculated ORR, OER and TOT overpotentials are used to quantitatively assess the catalytic activity of Ti3C2 MXene with and without O, F and OH functional groups. Interestingly, we find that the catalytic activity follows such a trend: Ti3C2O2>Ti3C2F2>Ti3C2(OH)2>Ti3C2, which suggests that O-terminated Ti3C2 MXene has great advantages and potentiality for catalyzing ORR and OER in Li–O2 batteries. This is caused by Ti3C2O2 surface shows stronger oxidation capability toward O22 compared to Ti3C2F2, Ti3C2(OH)2 and Ti3C2. The present study may provide a guideline to accelerate ORR and OER reactions of Ti3C2 MXene as cathode catalyst in Li–O2 batteries, with O-terminated group being taken into consideration.

Studies of the Electronic and the Optical Properties of the Ordered Ternary Alloys X0.5Y0.5O2(X, Y = Si, Ge, Sn) by Using First-principles Calculations

Song-You Wang,Zhao-Hua Geng,Gang Wang,Song Wu,Liang-Yao Chen,Yu Jia 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.4

In this paper, the electronic and the optical properties of oxides for group-IV elements in an inverse Ag2O structure, including Si, Ge, and Sn, and their ternary alloys, were studied with the density function theory of first principles. The results show that they have a high-density phase and a high dielectric constant. Moreover, the lattice constants of Si0.5Ge0.5O2 and SiO2 are really close to those of Si(100) surfaces, suggesting the possibility of forming a defect-free superlattice. With changing element, the calculated results show that the lattice constants of the ternary alloys tend to be bigger while the energy gaps show a different trend. The optical properties of these compounds as functions of physical quantities, such as the density of states, the complex dielectric function,and the static dielectric constant, are given to support the potential applications of the compounds in the future.

First-principles calculations on ferroelectricity and lattice dynamics of Type-II multiferroic SmMn2O5

Dai Jian-Qing,Yuan Jin 한국물리학회 2021 Current Applied Physics Vol.29 No.-

In this work, we report on the structural, electronic, and ferroelectric properties of SmMn2O5 by using firstprinciples density functional theory plus on-site Coulomb interaction (DFT + U) calculations. A thorough analysis was preformed to reveal the competing characteristics of different high-temperature (T) phases and the polarization mechanism in the low-T multiferroic phase. We show that the structural characteristics of the high-T phases have a strong influence on the low-T multiferroicity. In addition to the spin-induced lattice distortion that reduces substantially the purely electronic ferroelectricity, the dominant polarization mechanism in low-T SmMn2O5 still originates from the electronic polarization. By performing mode decomposition of the Hellmann– Feynman forces and the lattice distortion induced by the q = (0.5, 0, 0) magnetic order, we find that the Raman-active Ag mode characterized by the Mn4+O6 octahedron distortion and synergistic displacement of Mn3+ and Sm ions is of primary importance, while the infrared (IR)-active B2u mode plays a secondary role. These findings provide a theoretical foundation for future studies concerning the enhanced magnetoelectric effects of SmMn2O5 due to its pure exchange–striction mechanism.

제일원리 계산을 활용한 전기화학 촉매 연구

김동연 한국전기전자재료학회 2021 전기전자재료학회논문지 Vol.34 No.6

As the recent climate problems are getting worse year after year, the demands for clean energy materials have highly increased in modern society. However, the candidate material classes for clean energy expand rapidly and the outcomes are too complex to be interpreted at laboratory scale (e.g., multicomponent materials). In order to overcome these issues, the firstprinciples calculations are becoming attractive in the field of material science. The calculations can be performed rapidly using virtual environments without physical limitations in a vast candidate pool, and theory can address the origin of activity through the calculations of electronic structure of materials, even if the structure of material is too complex. Therefore, in terms of the latest trends, we report academic progress related to the first-principles calculations for design of efficient electrocatalysts. The basic background for theory and specific research examples are reported together with the perspective on the design of novel materials using first-principles calculations.

First-Principles Study on the Thermal Stability of LiNiO₂ Materials Coated by Amorphous Al₂O₃ with Atomic Layer Thickness

Kang, Joonhee,Han, Byungchan American Chemical Society 2015 ACS APPLIED MATERIALS & INTERFACES Vol.7 No.21

<P>Using first-principles calculations, we study how to enhance thermal stability of high Ni compositional cathodes in Li-ion battery application. Using the archetype material LiNiO<SUB>2</SUB> (LNO), we identify that ultrathin coating of Al<SUB>2</SUB>O<SUB>3</SUB> (0001) on LNO(012) surface, which is the Li de-/intercalation channel, substantially improves the instability problem. Density functional theory calculations indicate that the Al<SUB>2</SUB>O<SUB>3</SUB> deposits show phase transition from the corundum-type crystalline (c-Al<SUB>2</SUB>O<SUB>3</SUB>) to amorphous (a-Al<SUB>2</SUB>O<SUB>3</SUB>) structures as the number of coating layers reaches three. Ab initio molecular dynamic simulations on the LNO(012) surface coated by a-Al<SUB>2</SUB>O<SUB>3</SUB> (about 0.88 nm) with three atomic layers oxygen gas evolution is strongly suppressed at <I>T</I> = 400 K. We find that the underlying mechanism is the strong contacting force at the interface between LNO(012) and Al<SUB>2</SUB>O<SUB>3</SUB> deposits, which, in turn, originated from highly ionic chemical bonding of Al and O at the interface. Furthermore, we identify that thermodynamic stability of the a-Al<SUB>2</SUB>O<SUB>3</SUB> is even more enhanced with Li in the layer, implying that the protection for the LNO(012) surface by the coating layer is meaningful over the charging process. Our approach contributes to the design of innovative cathode materials with not only high-energy capacity but also long-term thermal and electrochemical stability applicable for a variety of electrochemical energy devices including Li-ion batteries.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2015/aamick.2015.7.issue-21/acsami.5b02572/production/images/medium/am-2015-025723_0001.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am5b02572'>ACS Electronic Supporting Info</A></P>

First-principles study of lithium-ion diffusion in β-Li3PS4 for solid-state electrolytes

임명수,지승훈 한국물리학회 2018 Current Applied Physics Vol.18 No.5

Understanding the role of partially occupied sites in Li-based superionic conductors is key to improving performance of solid-state electrolyte materials. We study the optimized structure of crystalline β-Li3PS4 and the Liion diffusion using first-principles calculations and the nudged elastic band method. Considering diffusion paths through both interstitial and vacancy exchanges, we calculate the migration energies of Li ions. We find that the phonon-mode softening and concurrent inversion symmetry breaking leads to a more stable structure with low symmetry. Atomic distortion from the phonon softening provides diffusion paths for Li ions with less migration energies than the ones in high-symmetry structures. Our results show that diffusion of Li ion is highly anisotropic through the armchair- or zigzag-shaped channels along the b-axis that contain Li-ion sites with fractional occupation.

First-principles study on charge transport mechanism of lithium sulfide (Li2S) in lithium-sulfur batteries

김도훈,강기석 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.0

The lithium-sulfur chemistry is regarded as a promising candidate for next-generation battery systems because of its high specific energy (1675 mAhg-1). Although issues such as the low cycle stability and power capability of the system remain to be addressed, extensive research has been performed experimentally to resolve these problems. Attaining a fundamental understanding of the reaction mechanism and its reaction product would further spur the development of lithium-sulfur batteries. Here, we investigated the charge transport mechanism of lithium sulfide (Li2S), a discharge product of conventional lithium-sulfur batteries using first-principles calculations. Our calculations indicate that the major charge transport is governed by the lithium-ion vacancies among various possible charge carriers. Furthermore, the large bandgap and low concentration of electron polarons indicates that the electronic conduction negligibly contributes to the charge transport mechanism in Li2S.

Electronic Structures and Thermoelectric Properties of Layered Chalcogenide PbBi4Te7 from First Principles

Tran Van Quang,김미영 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.3

The electronic structures and the thermoelectric (TE) properties of the ternary chalcogenide PbBi4Te7 are investigated by using first-principles calculations within the density functional theory and the solutions of semi-classical Boltzmann equation. Employing the screened-exchange local density approximation, we found that PbBi4Te7 to be a narrow-gap semiconductor with an indirect band gap of 0.11 eV. The combination of light and heavy valence bands near the band edge gives rise to large Seebeck coefficients, S, for p-type doping, which is found to be improved by 11% from that of Bi2Te3 at room temperature (RT). Moreover, in contrast to conventional Bi2Te3 where the value of S decreases rapidly with temperatures higher than RT, the values of S increases with temperature reaching up to 350 μVK−1 at 500 K indicating that PbBi4Te7 is a promising TE material with operating temperatures above RT. Our result also reveals that the intrinsic layered structure results in a prominent anisotropy in the TE coefficients, implying that the TE performance can be optimized by using the transport direction, as well as the type and the level of doping.