Electronic Structure and Lattice dynamics of NaFeAs

The similarity of the electronic structures of NaFeAs and other Fe pnictides has been demonstrated on the basis of first-principle calculations. The global double-degeneracy of electronic bands along X-M and R-A direction indicates the instability of Fe pnictides and is explained on the basis of a tight-binding model. The de Haas-van Alphen parameters for the Fermi surface (FS) of NaFeAs have been calculated. A $\mathbf{Q}_{M}=(1/2,1/2,0)$ spin density wave (SDW) instead of a charge density wave (CDW) ground state is predicted based on the calculated generalized susceptibility $\chi(\mathbf{q})$ and a criterion derived from a restricted Hatree-Fock model. The strongest electron-phonon (e-p) coupling has been found to involve only As, Na z-direction vibration with linear-response calculations. A possible enhancement mechanism for e-p coupling due to correlation is suggested

Electronic Structure and Lithium Diffusion in LiAl2(OH)6Cl Studied by First Principle Calculations

First-principles calculations based on the density functional theory (DFT) were carried out to study the atomic structure and electronic structure of LiAl2(OH)6Cl, the only material in the layered double hydroxide family in which delithiation was found to occur. Ab initio molecular dynamics (AIMD) simulations were used to explore the evolution of the structure of LiAl2(OH)6Cl during a thermally induced delithiation process. The simulations show that this process occurs due to the drastic dynamics of Li+ at temperatures higher than ~450 K, in which the [Al2(OH)6] host layers remain stable up to 1100 K. The calculated large value of the Li+ diffusion coefficient D, ~ 3.13 × 10 − 5 c m 2 / s , at 500 K and the high stability of the [Al2(OH)6] framework suggest a potential technical application of the partially-delithiated Li1-xAl2(OH)6Cl1-x (0 < x < 1) as a superionic conductor at high temperatures

Physical Properties of Molecules and Condensed Materials Governed by Onsite Repulsion, Spin-Orbit Coupling and Polarizability of Their Constituent Atoms

The onsite repulsion, spin–orbit coupling and polarizability of elements and their ions play important roles in controlling the physical properties of molecules and condensed materials. In celebration of the 150th birthday of the periodic table this year, we briefly review how these parameters affect the physical properties and are interrelated

Oxychalcogenides building blocks toward nonlinear optical properties

International audienc

The Preparation of a Challenging Superconductor Nb₃Al by Exploiting Nano Effect

The Nb3Al superconductor with excellent physical and working properties is one of the most promising materials in high-magnetic-field applications. However, it is difficult to prepare high-quality Nb3Al with a desired superconducting transition temperature (Tc) because of its narrow phase formation area at high temperatures (>1940 °C). This work reports a method to prepare stoichiometric Nb3Al powder samples at a relatively low temperature (1400 °C) by exploiting the nano effect of Nb particles with pretreatment of Nb powder under H2/Ar atmosphere. The obtained Nb3Al samples exhibit high Tc’s of ~16.8K. Based on density functional theory (DFT) calculations and statistical mechanics analysis, the crucial role of quantum effect in leading to the success of the preparation method was studied. A new measure of surface energy (MSE) of a model particle is introduced to study its size and face dependence. A rapid convergence of the MSE with respect to the size indicates a quick approach to the solid limit, while the face dependence of MSE reveals a liquid-like behavior. The surface effect and quantum fluctuation of the Nbn clusters explain the success of the preparation method

Negative Second Harmonic Response of Sn4+ in the Fresnoite Oxysulfide Ba2SnSSi2O7

International audienceOxychalcogenides are promising candidates for the design of IR nonlinear optical materials. Here, we prepared the first oxysulfide in the polar Fresnoite mineral type, Ba2SnSSi2O7 , and show that it has rare tin square pyramids, SnO4S, with apical Sn-S bond. These units and the Si2O7 groups are corner-shared to form the SnSSi2O7 layer with their Sn-S and apical Si-O bonds pointed along the polar c axis. Second harmonic generation measurements reveal that Ba2SnSSi2O7 is an IR NLO compound. Formally, Ba2SnSSi2O7 results from the mineral Fresnoite, Ba2TiOSi2O7 , by replacing the TiO5 square pyramid with the SnO4S square pyramid. This substitution increases the apical/the equatorial bond ratio of the square pyramid, namely, Ti-Oap /Ti-Oeq = 1.66/2.00 = 0.83 in TiO5 , and Sn-S/Sn-O = 2.03/2.31 = 0.88 in SnO4S. This change has a major impact on the cleavage along the stacking polar c-axis and on the second harmonic generation response, which decreases from Ba2TiOSi2O7 to Ba2SnSSi2O7 by a factor greater than two. The atom response theory analyses based on the density functional theory calculations reveal a remarkable difference between Ba2TiOSi2O7 and Ba2SnSSi2O7 ; the second harmonic generation is positive for the Ti4+ ion of Ba2TiOSi2O7 , but negative for the Sn4+ ion of Ba2SnSSi2O7. A Tauc plot analysis for Ba2SnSSi2O7 assuming indirect and direct transitions led to the optical band gaps of 2.4 and 2.7 eV, respectively