From ultrasoft pseudopotentials to the projector augmented-wave method
TL;DR: In this paper, the formal relationship between US Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived and the Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional.
Abstract: The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Bl\"ochl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules $({\mathrm{H}}_{2}{,\mathrm{}\mathrm{H}}_{2}{\mathrm{O},\mathrm{}\mathrm{Li}}_{2}{,\mathrm{}\mathrm{N}}_{2}{,\mathrm{}\mathrm{F}}_{2}{,\mathrm{}\mathrm{BF}}_{3}{,\mathrm{}\mathrm{SiF}}_{4})$ and several bulk systems (diamond, Si, V, Li, Ca, ${\mathrm{CaF}}_{2},$ Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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TL;DR: This work demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL, but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.
Abstract: Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97 Eu0.03 Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm-2 ) are particularly effective at reducing κL . When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.
394 citations
TL;DR: In this article, a joint experimental and computational systematic exploration of the driving forces that govern encapsulation of active ingredients (solvent, starting material dehydration, drug/material ratio, immersion time, and several consecutive impregnations) and its kinetics of delivery (structure, polarity,...) was performed using a series of porous biocompatible metal-organic frameworks (MOFs) that bear different topologies, connectivities, and chemical compositions.
Abstract: A joint experimental and computational systematic exploration of the driving forces that govern (i) encapsulation of active ingredients (solvent, starting material dehydration, drug/material ratio, immersion time, and several consecutive impregnations) and (i) its kinetics of delivery (structure, polarity, ...) was performed using a series of porous biocompatible metal–organic frameworks (MOFs) that bear different topologies, connectivities, and chemical compositions. The liporeductor cosmetic caffeine was selected as the active molecule. Its encapsulation is a challenge for the cosmetic industry due to its high tendency to crystallize leading to poor loadings (<5 wt %) and uncontrolled releases with a subsequent low efficiency. It was evidenced that caffeine entrapping reaches exceptional payloads up to 50 wt %, while progressive release of this cosmetic agent upon immersion in the simulated physiological media (phosphate buffer solution pH = 7.4 or distilled water pH = 6.3, 37 °C) occurred mainly depend...
394 citations
TL;DR: This theoretical study demonstrates that chemical short-range order is thermodynamically favored in HEAs and can be tuned to affect the mechanical behavior of these alloys.
Abstract: High-entropy alloys (HEAs) are an intriguing new class of metallic materials due to their unique mechanical behavior. Achieving a detailed understanding of structure-property relationships in these materials has been challenged by the compositional disorder that underlies their unique mechanical behavior. Accordingly, in this work, we employ first-principles calculations to investigate the nature of local chemical order and establish its relationship to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi medium-entropy solid-solution alloys, whose combination of strength, ductility, and toughness properties approaches the best on record. We find that the average intrinsic and extrinsic SFE are both highly tunable, with values ranging from -43 to 30 mJ⋅m-2 and from -28 to 66 mJ⋅m-2, respectively, as the degree of local chemical order increases. The state of local ordering also strongly correlates with the energy difference between the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases, which affects the occurrence of transformation-induced plasticity. This theoretical study demonstrates that chemical short-range order is thermodynamically favored in HEAs and can be tuned to affect the mechanical behavior of these alloys. It thus addresses the pressing need to establish robust processing-structure-property relationships to guide the science-based design of new HEAs with targeted mechanical behavior.
393 citations
Cites methods from "From ultrasoft pseudopotentials to ..."
...functional theory, with a plane wave cut-off energy of 420 eV. Brillouin zone integrations were performed using Monkhorst–Pack meshes with a 2×2×2 grid (43). Projector-augmented-wave (PAW) potentials (42,44) were employed with the Perdew-Burke-Ernzerhof (PBE) generalized-gradient approximation (GGA) for the exchange-correlation functional (45). In the DFT simulations the magnetic moments were initialized...
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TL;DR: It is demonstrated that the terminations modulate the crystal structures through valence electron density redistribution of the atoms, particularly surface Ti (Ti2) in the monosheets, and both Raman and infrared-active vibrational modes are predicted and conclusively assigned.
Abstract: We present a comparative study on the static and dynamical properties of bare Ti3C2 and T-terminated Ti3C2T2 (T = O, F, OH) monosheets using density functional theory calculations. First, the crystal structures are optimized to be of trigonal configurations (Pm1), which are thermodynamically and dynamically stable. It is demonstrated that the terminations modulate the crystal structures through valence electron density redistribution of the atoms, particularly surface Ti (Ti2) in the monosheets. Second, lattice dynamical properties including phonon dispersion and partial density of states (PDOS) are investigated. Phonon PDOS analysis shows a clear collaborative feature in the vibrations, reflecting the covalent nature of corresponding bonds in the monosheets. In the bare Ti3C2 monosheet, there is a phonon band gap between 400 and 500 cm−1, while it disappears in Ti3C2O2 and Ti3C2(OH)2 as the vibrations associated with the terminal atoms (O and OH) bridge the gap. Third, both Raman (Eg and A1g) and infrared-active (Eu and A2u) vibrational modes are predicted and conclusively assigned. A comparative study indicates that the terminal atoms remarkably influence the vibrational frequencies. Generally, the terminal atoms weaken the vibrations in which surface Ti atoms are involved while strengthening the out-of-plane vibration of C atoms. Temperature-dependent micro Raman measurements agree with the theoretical prediction if the complexity in the experimentally obtained lamellae for the Raman study is taken into account.
392 citations
TL;DR: Site preferences of dopant Eu2+ on the locations of K+, Ba2+, and Ca2+ in the mixed cation phosphate K2BaCa(PO4)2 (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence.
Abstract: Site preferences of dopant Eu2+ on the locations of K+, Ba2+, and Ca2+ in the mixed cation phosphate K2BaCa(PO4)2 (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu2+ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu2+ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu2+ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs.
390 citations
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Book•
31 Dec 1993
TL;DR: The linearized augmented planewave (LAPW) method has emerged as the standard by which density functional calculations for transition metal and rare-earth containing materials are judged.
Abstract: With its extreme accuracy and reasonable computational efficiency, the linearized augmented planewave (LAPW) method has emerged as the standard by which density functional calculations for transition metal and rare-earth containing materials are judged. This volume presents a thorough and self-conta
1,150 citations