There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Simple rules for the understanding of Heusler compounds

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Alloy negative electrodes for Li-ion batteries.

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Post-Li batteries based on Na, K, Ca, and Mg offer compelling alternatives to Li ones, whose resources are scarce and unevenly distributed in the earth’s crust. However, the development of these forthcoming batteries is currently thwarted by the lack of cost-effective, nontoxic, and highly efficient anode materials. To tackle this challenge, we employed a comprehensive structure search using the minima hopping method, followed by density functional theory (DFT) calculations. Our search led to an as-yet-unobserved metallic C7N monolayer with a Haeckelite structure. This unique structure features a network of sp2-nitrogen-containing heptagon and pentagon rings that are arranged in a way that enforces metallic characteristics. As such, C7N outperforms widely known 2D anode materials such as graphene, MoS2, and black phosphorus because of its superior storage capacity, lower diffusion barriers, and higher open-circuit voltages. It achieves a remarkable storage capacity of 1366 mA h g–1 for Na/K and an impressive 2730 mA h g–1 for Ca. Detailed analyses of charge, elastic constants, and molecular dynamics simulations demonstrate that the C7N possesses a strong yet flexible covalent network with a volume change of 2–4% during full charge and discharge cycles, ensuring long-term stability and reliability. The robust network of C7N also allows it to maintain a flat and thermally stable surface at full metal coverage and high temperatures. These findings open up avenues for exploring the Haeckelite carbon-nitride family as a promising candidate for next-generation battery technologies. 

First-Principles Study of 2D Haeckelite C7N as a High-Capacity Anode for Post-Lithium-Ion Batteries

Developing low-cost, eco-friendly, and viable photocatalysts for visible-light-driven water splitting is essential for generating clean hydrogen energy to assuage the global energy crisis. Using first-principle calculations, we show that newly synthesized Rb3NaSn3Se8, K2Na2Sn3S8, and their solid solutions spontaneously split water under visible-light illumination. Based on our chemical and bond analysis, the oxidation state IV of Sn, which has no 5s2 lone pair anymore, plays a key role in the photocatalytic activity enhancement. Strong covalent bonding formed among Sn4+ and S/Se improves the structural stability, optics, and water reduction, while weak-ionically bonded alkali-metal cations facilitate the water oxidation reaction on the surface, known as the bottleneck of the water-splitting reaction. The Sn4+ empties the 5s states, places it in the conduction band, and widens the band gap up to 2.6 eV, beyond that of Sn2+ compounds. Moreover, Sn4+ uses its sp3 and sp3d hybridization to build robust covalent polyanions that allow the construction of full-range solid solutions. As such, the high-entropy engineering reduces the band gap of K2Na2Sn3S8–xSex (x = 0–8), switches its band’s nature from indirect to direct, and adjusts the conduction band minimum. Here, our findings provide a roadmap for developing nontoxic, earth-abundant photocatalysts. 

Sn(IV) Polyanionic Materials as Efficient Visible-Light-Driven Water-Splitting Photocatalysts

Using 4-Aminobenzonitrile (4-ABN) as ligand and different anions, five cadmium coordination polymers of the formulas [Cd(4-ABN)I2]n (1), [Cd(4-ABN)Br2]n (2), [Cd(4-ABN)2(NO3)2]n (3), [Cd(4-ABN)2(OAc)2]n (4) and {[Cd(4-ABN)2(H2O)2](ClO4)2}n (5) are synthesized and structurally...

Anion-induced structural diversity in cadmium coordination polymers of 4-aminobenzonitrile: a structural and DFT study

We present a systematic study of the high-pressure FeSe phase performed by means of the first-principle electronic structure calculations. Basing on available experimental information about the unit cell geometry we calculate the band structure and characterize the related properties during their pressure driven evolution. The electronic structure including the hybrid functional B3LYP or the Hubbard parameter U for the iron d states lead to the correct semiconducting ground state for the hexagonal stoichiometric FeSe within the broad pressure range (up to 30 GPa).

S. Shahab Naghavi

Papers

First-Principles Study of 2D Haeckelite C7N as a High-Capacity Anode for Post-Lithium-Ion Batteries

Sn(IV) Polyanionic Materials as Efficient Visible-Light-Driven Water-Splitting Photocatalysts

Anion-induced structural diversity in cadmium coordination polymers of 4-aminobenzonitrile: a structural and DFT study

The role of correlations in the high-pressure phase of FeSe