Full Structural Relaxation Research Articles

Theoretical Modeling of High Spin to Low Spin Transition and Structural Stability Under Pressure in CaFeO3

Evolution of magnetic and spectral properties and structure stability of CaFeO3 are investigated in frames of the GGA + U method. It is shown that the GGA + U method is able to reproduce the transition from high spin to low spin under pressure obtained experimentally. At ambient pressure monoclinic structure has lower energy which agrees with experiment. Full structural relaxation shows that orthorhombic structure in unstable above 30 GPa and undergoes a distortion toward monoclinic structure.

Hexagonal-to-base-centered-orthorhombic 4Q charge density wave order in kagome metals KV3Sb5, RbV3Sb5, and CsV3Sb5

I search for the ground state structures of the kagome metals KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$ using first principles calculations. Group-theoretical analysis shows that there are seventeen different distortions that are possible due to the phonon instabilities at the $M$ $(\frac{1}{2},0,0)$ and $L$ $(\frac{1}{2},0,\frac{1}{2})$ points in the Brilouin zone of the parent $P6/mmm$ phase of these materials. I generated these structures for the three compounds and performed full structural relaxations that minimize the atomic forces and lattice stresses. I find that the $Fmmm$ phase with the order parameter $M_1^+$ $(a,0,0)$ $+$ $L_2^-$ $(0,b,b)$ has the lowest energy among these possibilities in all three compounds. However, the $Fmmm$ exhibits a dynamical instability at its $Z$ $(0,0,1)$ point, which corresponds to the $A$ $(0,0,\frac{1}{2})$ point in the parent $P6/mmm$ phase. Condensation of this instability leads to a base-centered orthorhombic structure with the space group $Cmcm$ and $4Q$ order parameter $M_1^+$ $(a,0,0)$ $+$ $L_2^-$ $(0,b,b)$ $+$ $A_6^+$ $(\frac{1}{2}c,\frac{-\sqrt{3}}{2}c)$.

Emergence of intrinsically isolated flat bands and their topology in fully relaxed twisted multilayer graphene

We study the electronic structure and band topology of fully relaxed twisted multi-layer graphene (TMLG). Isolated flat bands emerge in TMLG with the number of layers [$M+N$ with $M$ the layer number of the bottom few-layer graphene (FLG)] up to 10 and with various stacking orders, and most of them are on the hole side. The touched bands of FLGs around the Fermi level are split by the moir\'{e} coupling through the electron-hole asymmetry in low-energy bands of FLGs and by the vertical hopping between next-nearest layers. The full structural relaxation leads to global gaps that completely isolate a flat band. For TMLG with given $M$ and $N$, the highest magnitude of Chern numbers ($|C|$) of the separable flat bands reaches $M+N-1$ and can be hosted by certain isolated bands. The $|C|=9$ occurs in the isolated flat valence band of several configurations with 10 layers. Such high $|C|$ originates from the lifting of the band-state degeneracy in the weak regime of moir\'{e} coupling or from the topological phase transitions induced by the strong moir\'{e} coupling. Moreover, large orbital magnetic moments arise in isolated flat bands with high $|C|$ and depend on the structural configurations of TMLG.

Orthorhombic-to-monoclinic transition in Ta2NiSe5 due to a zone-center optical phonon instability

I study dynamical instabilities in Ta$_2$NiSe$_5$ using density functional theory based calculations. The calculated phonon dispersions show two unstable optical branches. All the acoustic branches are stable, which shows that an elastic instability is not the primary cause of the experimentally observed orthorhombic-to-monoclinic structural transition in this material. The largest instability of the optical branches occurs at the zone center, consistent with the experimental observation that the size of the unit cell does not multiply across the phase transition. The unstable modes have the irreps $B_{1g}$ and $B_{2g}$. Full structural relaxations minimizing both the forces and stresses find that the monoclinic $C2/c$ structure corresponding to the $B_{2g}$ instability has the lowest energy. Electronic structure calculations show that this low-symmetry structure has a sizable band gap. This suggest that a $B_{2g}$ zone-center optical phonon instability is the primary cause of the phase transition. An observation of a softening of a $B_{2g}$ zone-center phonon mode as the transition is approached from above would confirm the mechanism proposed here. If none of the $B_{2g}$ modes present in the material soften, this would imply that the transition is caused by electronic or elastic instability.

Microscopic theory of onset of decaging and bond-breaking activated dynamics in ultradense fluids with strong short-range attractions.

We theoretically study thermally activated "in cage" elementary dynamical processes that precede full structural relaxation in ultradense particle liquids interacting via strong short-range attractive forces. The analysis is based on a microscopic theory formulated at the particle trajectory level built on the dynamic free energy concept and an explicit treatment of how attractive forces control the formation and lifetime of physical bonds. Mean time scales for bond breaking, the early stage of cage escape, and non-Fickian displacement by a fixed amount are analyzed in the repulsive glass, bonded repulsive (attractive) glass, fluid, and dense gel regimes. The theory predicts a strong length-scale-dependent growth of these time scales with attractive force strength at fixed packing fraction, a much weaker slowing down with density at fixed attraction strength, and a strong decoupling of the shorter bond-breaking time with the other two time scales that are controlled mainly by perturbed steric caging. All results are in good accord with simulations, and additional testable predictions are made. The classic statistical mechanical projection approximation of replacing all bare attractive and repulsive forces with a single effective force determined by pair structure incurs major errors for describing processes associated with thermally activated escape from transiently localized states.

Magnetic resonance as a local probe for kagom\xe9 magnetism in Barlowite Cu4(OH)6FBr

Temperature- and field-dependent 1H-, 19F-, and 79,81Br-NMR measurements together with zero - field 79,81Br-NQR measurements on polycrystalline samples of barlowite, Cu4(OH)6FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the 79,81Br-NMR spin-lattice relaxation rates 1/T1 indicate a phase transition at TN simeq 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below TN. Moreover, 1/T1T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR- lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of vQ ≃ 28.5 MHz and 24.7 MHz for 79Br- and 81Br-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to η ≃ 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above TN. This broadening and the relative large η value suggests a symmetry reduction at the Br- site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large η. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.

Pt/Au nanoalloy supported on alumina and chlorided alumina: DFT and experimental analysis

Density functional theory (DFT) was used to explore the adsorption of Pt/Au nanoalloy onto a pure and chlorided γ-Al2O3(110) surface, which has been applied in numerous catalytic reactions. First, we considered the adsorption properties of Pt clusters (n ≤ 5) onto the Al2O3(110) surface to determine the most stable Pt cluster on alumina surface in reforming processes. After full structural relaxations of Pt clusters at various configurations on alumina, our computed results expressed that the minimum binding energy (−5.67 eV) is accrued for Pt4 cluster and the distance between the nearest Pt atom in the cluster to the alumina surface is equal to 1.13 Å. Then, we investigated the binding energies, geometries, and electronic properties of adsorbed Aun clusters (n ≤ 6) on the γ-Al2O3(110) surface. Our studied showed that Au5 was the most thermodynamically stable structure on γ-Al2O3. Finally, we inspected these properties for adsorbed Au clusters onto the Pt4-decorated alumina (Aun/Pt4-alumina) system. The binding energy of the Au4/Pt4-alumina system was −5.01 eV, and the distance between Au4 cluster and Pt4-alumina was 1.33 Å. The Au4/Pt4alumina system was found to be the most stable nanometer-sized catalyst design. At last, our first-principles calculations predicted that the best position of embedment Cl on the Au4/Pt4-alumina.

DFT study of Au adsorption on pure and Pt-decorated γ-alumina (110) surface

In this study, the adsorption properties of a Pt-decorated γ-Al2O3 (110) surface (Pt-alumina) were investigated to determine the favorable sublayer for Au adsorption using first principles calculations. After full structural relaxation of various configurations, our calculated results indicated that the lowest binding energy and the distance between the Pt atom and the nearest atoms in the alumina (O atom) were −1.87eV and 2.36Å, respectively. Finally, the adsorption energy, geometry, and electronic structure for the adsorption of an Au atom onto pure (Au-alumina) and Pt-decorated alumina (Au/Pt-alumina) were examined. The binding energy for the Au/Pt-alumina system was −3.06eV, which is more than that for the Au-alumina system (−1.40eV). Therefore, the results from our first principles calculations predict that the Au atom adsorptive capability on Pt-alumina is better than that of pure alumina.

Energetics of the AK13 semilocal Kohn-Sham exchange energy functional

The recent nonempirical semilocal exchange functional of Armiento and K\"ummel [Phys. Rev. Lett. 111, 036402 (2013)], AK13, incorporates a number of features reproduced by higher-order theory. The AK13 potential behaves analogously with the discontinuous jump associated with the derivative discontinuity at integer particle numbers. Recent works have established that AK13 gives a qualitatively improved orbital description compared to other semilocal methods, and reproduces a band structure closer to higher-order theory. However, its energies and energetics are inaccurate. The present work further investigates the deficiency in energetics. In addition to AK13 results, we find that applying the local-density approximation (LDA) non-self-consistently on the converged AK13 density gives very reasonable energetics with equilibrium lattice constants and bulk moduli well described across 13 systems. We also confirm that the attractive orbital features of AK13 are retained even after full structural relaxation. Hence, the deficient energetics cannot be a result of the AK13 orbitals having adversely affected the quality of the electron density compared to that of usual semilocal functionals; an improved orbital description and good energetics are not in opposition. This is also confirmed by direct calculation of the principal component of the electric field gradient. In addition, we prove that the non-self-consistent scheme is equivalent to using a single external-potential-dependent functional in an otherwise consistent, nonvariational Kohn-Sham density-functional theory (KS DFT) scheme. Furthermore, our results also demonstrate that, while an internally consistent KS functional is presently missing, non-self-consistent LDA on AK13 orbitals works as a practical nonempirical computational scheme to predict geometries, bulk moduli, while retaining the band structure features of AK13 at the computational cost of semi-local DFT.

Bio-Inspired Aggregation Control of Carbon Nanotubes for Ultra-Strong Composites.

High performance nanocomposites require well dispersion and high alignment of the nanometer-sized components, at a high mass or volume fraction as well. However, the road towards such composite structure is severely hindered due to the easy aggregation of these nanometer-sized components. Here we demonstrate a big step to approach the ideal composite structure for carbon nanotube (CNT) where all the CNTs were highly packed, aligned, and unaggregated, with the impregnated polymers acting as interfacial adhesions and mortars to build up the composite structure. The strategy was based on a bio-inspired aggregation control to limit the CNT aggregation to be sub 20–50 nm, a dimension determined by the CNT growth. After being stretched with full structural relaxation in a multi-step way, the CNT/polymer (bismaleimide) composite yielded super-high tensile strengths up to 6.27–6.94 GPa, more than 100% higher than those of carbon fiber/epoxy composites, and toughnesses up to 117–192 MPa. We anticipate that the present study can be generalized for developing multifunctional and smart nanocomposites where all the surfaces of nanometer-sized components can take part in shear transfer of mechanical, thermal, and electrical signals.

Atomic oxygen adsorption on 3.125 at.% Ga stabilized δ-Pu (1 1 1) surface

All-electron density functional theory was used to investigate atomic oxygen adsorption on a gallium stabilized δ-plutonium (111) surface. High symmetry on-surface and interstitial adsorption sites, along with local environment (as determined by the absence or presence of gallium) were explored. The calculations comprised full structural relaxations. Spin–orbit-coupling was also taken into account to assess the complexity of absorbate–substrate interactions. We observed that O adsorbate prefers to bind strongly to a gallium deficient environment, with the most stable site being the threefold hollow fcc site and associated chemisorption energy of −5.06eV. The binding energies were least favored when gallium is a nearest neighbor to the O adsorbate, suggesting that the presence of gallium in a plutonium matrix tends to slow down the oxide layer growth. Although the oxygen coordination is the highest in the interstitial sites, the adsorption energy is less favored compared to on-surface adsorption, implying that the diffusion of oxygen from the surface layer into the subsurface layers is an activated process. The adsorption process induced non-trivial deformations of the surface. Additionally, some delocalization of the plutonium 5f and 6d partial electron density of states (PDOS) at the Fermi energy was observed. Further analysis in the PDOS indicated that gallium tends to suppress hybridization between the plutonium 5f and oxygen 2p orbitals, while the 6d orbitals hybridized with oxygen 2p orbitals.

Full and ideal mixing behavior between Zr–Wd (K2ZrSi3O9) and Ti–Wd (K2TiSi3O9): evidences from mineral chemistry, X-ray diffraction pattern and Raman spectrum

A series of wadeite solid solutions (Wdss) along the join K2ZrSi3O9 (Zr–wadeite, Zr–Wd)–K2TiSi3O9 (Ti–wadeite, Ti–Wd) was synthesized at 2 GPa and 1,200–1,400 °C for 24 h with a piston-cylinder apparatus. The chemical composition data demonstrated that Ti and Zr readily substitute for each other, and a full series of Wdss exists between the Zr–Wd and Ti–Wd end-members at the P–T conditions of our synthesizing experiments. The volume–composition relationship at ambient P–T conditions did not show any significant deviation from the Vegard’s law, indicating a nearly ideal mixing between the Zr–Wd and Ti–Wd. All the Raman peaks observed at ambient P–T conditions were linearly correlated with the composition, suggesting an almost full structural relaxation for the Wdss. These findings suggest that the complete mixing behavior along the join K2ZrSi3O9–K2TiSi3O9, as demonstrated by our high-P experiments, may extend to a much wider range of P–T conditions. Consequently, the coexistence of the Zr–Wd and Ti–Wd in the lamproites from the Leucite Hills, Wyoming, may be due to a chemical composition change of the magma rather than a crystal structural limit.

Structural, Electronic and Optical Properties of ZnMnIn2Te4 Chalcopyrite: A DFT + U Study

Our density functional theory (DFT) based first principle calculation shows that structural, electronic and optical properties of ZnMnIn2Te4 chalcopyrite type semiconductor are improved by DFT + U functional compared to LDA. These calculations are carried out using plane wave basis and atomic pseudopotentials as implemented in Quantum ESPRESSO. The lattice parameters (‘a’, ‘c’) and tetragonal distortion (η = c/2a) are calculated by energy minimization procedure. The full structural relaxation is carried out within DFT + U. Density of states (DOS) shows that it is n-type semiconductor. The calculated bond lengths show that the system undergoes anion displacement. Optical properties are studied using TB-LMTO method with the input of structural parameters as found from plane wave method using DFT + U.

Mechanism of the aerobic oxidation of methanol to formic acid on Au8−: A DFT study

AbstractThe mechanism of the aerobic oxidation of methanol to formic acid catalyzed by Au8− has been systematically investigated using density functional theory with the M06 functional. The reaction pathways were examined by taking into account the full structural relaxation of the Au8−. Stepwise and concerted reaction mechanisms are proposed. The stepwise mechanism is initiated by the hydrogen abstraction of a methoxy species by a superoxo‐like anion on the gold cluster, resulting in the formation of formaldehyde. Subsequently, the formaldehyde is activated by the hydroxyl group of a hydroperoxyl‐like complex, leading to the formation of a hemiacetal intermediate. The formation of formic acid in the final step is achieved by hydrogen abstraction of the hemiacetal intermediate by atomic oxygen attached to the gold cluster. Our calculations indicate that the first step of the stepwise mechanism, that is, hydrogen abstraction of the methoxy species, is the rate‐determining step. Another possible reaction pathway involving a single‐step hydrogen abstraction, a concerted mechanism, is also discussed. This mechanism may also be responsible for the reasonable catalytic activity of aerobic oxidation of methanol on Au8− because of the low activation energy barrier. © 2012 Wiley Periodicals, Inc.

Structural relaxation and its influence on the elastic properties and notch toughness of Mg–Zn–Ca bulk metallic glass

Structural relaxation and its influence on the elastic properties and mechanical properties of Mg–Zn–Ca bulk metallic glass (BMG) were investigated. It was found that relaxation can occur spontaneously even at room temperature ( T R) because of the high homologous temperature ( T R/ T g > 0.7) of the BMG. A linear dependence between enthalpy change (Δ H t) and free volume change (Δ v f) for the annealed Mg 66Zn 30Ca 4 BMG over a wide temperature range was discovered. The structural relaxation of the Mg 66Zn 30Ca 4 BMG resulted in increased shear modulus μ (3.4%) and decreased Poisson's ratio ν (1.4%). However, the tested BMG samples exhibited no obvious degradation of notch toughness ( K Q) even after full structural relaxation. The K Q values are 9.0 ± 1.7 MPam 1/2 and 7.1 ± 1.8 MPam 1/2 for the freshly prepared samples (annealed at T R for 1.5 h) and fully relaxed samples (annealed at 373 K for 4 h), respectively. These results indicate that the mechanical properties of the Mg 66Zn 30Ca 4 BMG are not very sensitive to its structural relaxation over a wide temperature and time range. These findings may facilitate the clinical applications of Mg 66Zn 30Ca 4 BMG as a promising implant material for future clinical applications.

Anisotropic exchange interaction between nonmagnetic europium cations in Eu2O3

The electronic structure of the cubic and (high pressure) hexagonal phases of Eu${}_{2}$O${}_{3}$ have been investigated by means of full potential linearized augmented plane wave calculations, within the $\mathit{LDA}+U$ method. A full structural relaxation was performed for both phases, and the transition pressure was determined. The band structure shows the correct insulating character with gap values comparable to the experimental ones. The contact charge density was also calculated for Eu${}_{2}$O${}_{3}$ (and other Eu compounds) and found to be in good agreement with experimental values. For the cubic phase, the comparison between ferromagnetic and antiferromagnetic calculations shows that the exchange interaction is very weak and is therefore expected to have a negligible effect on the magnetic susceptibility. The calculations performed for the high pressure hexagonal phase, on the other hand, show that there is an antiferromagnetic exchange interaction between nearest-neighbor Eu ions, which should have a sizable effect on the susceptibility. Our results allow us to evaluate the existing theories.

Noncollinear Magnetism Calculation of Iron Clusters with Spin-Orbit Coupling

We study iron clusters containing 2 ∼ 13 atoms by ab initio calculations with both collinear and noncollinear magnetic methods. Spin-orbit coupling is only available in the noncollinear method. After full structural relaxations, it is found that atom positions derived from the noncollinear method have better stability in all clusters, including those having coparallel spin arrangements. Binding energies of clusters calculated by the noncollinear method are also 17.3 ∼ 19.8 meV/atom lower, which are too large to ignore. By comparing the magnetic properties and electronic structures from the two methods, we believe that the difference has resulted from spin-orbit coupling. We recommend reconsidering the importance of the noncollinear magnetic method with spin-orbit coupling in magnetic systems. Especially in transition metal clusters when atom positions and energy values are important for determining the crucial properties.

High-pressure structural phase transitions and mechanical properties of calcite rock

In the present work, a state of the art first principles theory is used to examine the structural and mechanical properties of calcium carbonates CaCO3. Our calculations allowed full structural relaxation, which permits an appropriate evaluation of material properties at ambient conditions as well as under hydrostatic pressure. Compared to experimental measurements the calculated ground state properties show a suitable agreement. By performing a structural phase stability analysis, we were able to predict both first and second order phase transitions that calcium carbonates minerals undertake under hydrostatic pressure. The first one occurs between the calcite and aragonite phases at 3.3GPa and the second one between the aragonite and post-aragonite phases at ∼40GPa. The previous value agree very well with experimental one (40GPa) reported by Ono et al. In order to verify the reliability of such phase transitions, we study the mineral high pressure stability by means of mechanical properties behaviour. Both transversal wave velocity and elastic moduli show an unexpected decrease at phase transition pressure range.

Interplay between Magnetic and Orbital Ordering in the Strongly Correlated Cobalt Oxide: A DFT + U Study

The strongly correlated CoO is investigated by means of DFT + U calculations to elucidate the origin of the cubic-to-monoclinic distortion observed below TN for that system. An effective Ueff value between 4 and 5 eV is required to reproduce, in the meanwhile, the monoclinic symmetry, the band gap, and the type-II antiferromagnetic ordering of CoO. The extraction of the exchange coupling constants combined with full structural relaxations allows the interpretation of the low-temperature structure of CoO. An interplay between magnetic and orbital ordering is shown to be responsible for the peculiar structural behavior of CoO. For Ueff 5 eV, leading to a tetragonal structure.

Electronic and optical properties of carbon nanotubes under pure bending

The high aspect ratio of carbon nanotubes makes them prone to bending. To know how bending affects the tubes is therefore crucial for tube identification and for electrical component design. Very few studies, however, have investigated tubes under small bending well below the buckling limit, because of technical problems due to broken translational symmetry. In this Letter a cost-effective and exact modeling of singe-walled nanotubes under such small bending is enabled by revised periodic boundary conditions, combined with density-functional tight-binding. The resulting, bending-induced changes in electronic and optical properties fall in clear chirality-dependent trend families. While the correct trends require full structural relaxation, they can be understood by one general argument. To know these trends fills a fundamental gap in our understanding of the properties of carbon nanotubes.