Paper & Patents

In this article, we propose an antisite defect tolerant ternary Heusler alloy, Co2MnSb, which is highly efficient for spintronic applications. Using ab-initio Density Functional Theory (DFT), we study the effect of experimentally observed intrinsic point defect (antisite defect) and lattice constant (Lc) on the halfmetallic characteristics, mechanical stability and magnetic properties of Co2MnSb. Ab-initio simulation predicts halfmetallic ferromagnetic characteristics with a high value of total magnetic moment, 6.00 (5.92) μB/f.u. and large Curie temperature (TC), 1109 K (1094 K) at relaxed (experimental) Lc. Halfmetallic characteristics and mechanical stability are sensitive to Lc variation. Experimentally, it has been observed that intrinsic defects in Heusler alloys always degrades the halfmetallic characteristics and spin polarization. Hence, all the possible binary and ternary kind of antisite defects between the transition metals and the non-magnetic p-block element of Co2MnSb have been simulated upto the disorder concentration (’x’) of 11.1%. Our theoretical analysis reveals that even in presence of antisite disorder, the alloy preserves its halfmetallic characteristics specially at lower disorder concentrations. However, electronic density of states and total magnetic moment are affected significantly in presence of disorder. In some disordered alloys, total magnetic moment exceeds beyond 6.00 μB, indicating towards the higher value of TC with respect to the parent compound. Formation energy of particular disordered alloys compete with the formation energy of parent alloy, makes the compound defect tolerant material. Halfmetallic characteristics, high magnetic moment and large TC make the defect tolerant Co2MnSb alloy highly efficient for spintronic applications.

Double perovskites using dual metal cations are promising candidates for Pb-free perovskites. This study shows that the electronic structures of double perovskites (A2B⁺B³⁺X6) can be significantly modulated by cation ordering changes. The bandgap of Cs2AgBiCl6 can be affected by changing octahedron alignments, and even zero gap states can be realized for the 2-dimensional BiCl6 (AgCl6) configuration. It is presented that different types of B⁺/B³⁺-site orderings in double perovskites could be the origin of bandgap dispersion. Comparative studies on the various compositions show that, among B⁺/B³⁺ cations, Tl/Bi could be promising for the suppression of ordering variation.

Structural stability of Fe2P is investigated in detail using first-principles calculations based on density functional theory. While the orthorhombic C23 phase is found to be energetically more stable, the experiments suggest it to be hexagonal C22 phase. In the present study, we show that in order to obtain the correct ground state structure of Fe2P from the first-principles based methods it is utmost necessary to consider the zero-point effects such as zero-point vibrations and spin fluctuations. This study demonstrates an exceptional case where a bulk material is stabilized by quantum effects, which are usually important in low-dimensional materials. Our results also indicate the possibility of structural quantum phase transition in Fe2P, which should form the basis for further theoretical and experimental efforts.

Oxygen reduction reaction (ORR) is of great interest in various areas, including energy conversion. This paper presents the simple synthesis and characterization of one-dimensional silver halides nanowires (AgClNW and AgBrNW) as an electrocatalyst for ORR in alkaline media, as well as an investigation of the ORR pathway at AgCl. AgClNW and AgBrNW were prepared via a galvanic replacement reaction (GRR) between silver nanowires (AgNW) and a halide precursor. AgClNW exhibited excellent ORR catalytic activity that was comparable to or better than that for commercial Pt (20 wt% Pt loading on Vulcan carbon), demonstrating potential to replace Pt-based catalysts. A scanning electrochemical microscopy (SECM) analysis supports the existence of an associative ORR pathway at AgCl, and first-principles density functional theory (DFT) calculations suggest that the high ORR activity of AgCl is possibly attributed to the up-shifted Ag d-band center energy in AgCl as well as the assistance of adsorbed water molecules.

The theme of this work is to highlight the significance of green plant extracts in the synthesis of nanostructures. In asserting this statement, herein, we report our obtained results on the synthesis of hexagonal CdSe nanorods preferably oriented along (0002) plane through henna leaf extract-mediated reaction along with a discussion about the structural, morphological and optical properties of the synthesized nanorods. The possible mechanism for the synthesis of CdSe nanorods was explored. The formation of nanorods along (0002) plane was confirmed by the relatively high intensity of the (0002) peak in X-ray diffraction pattern. To account for the experimentally realistic condition, we have calculated the surface energies of hexagonal CdSe surface slabs along the low indexed (0002), (10¯10) and (11¯20) plane surfaces using density functional theory approach and the calculated surface energy value for (0002) surface is 802.7 mJ m⁻², which is higher than (11¯20) and (10¯10) surfaces. On realizing the calculated surface energies of these slabs, we determined that the combination of (11¯20) and (10¯10) planes with lower surface energies will lead to the formation of CdSe nanorods growth along (0002) orientation. Finally, we argue that the design of new greener route for the synthesis of novel functional nanomaterials is highly desired.

The effects of 3d transition-metal alloying elements (V, Cr, Mn, Co, Ni, and Cu) on the structural stability, mechanical properties, and electronic structure of Fe3Al alloys have been investigated using first-principles calculations. The site preference of the alloying elements was clearly observed from the formation enthalpy calculations of the Fe3Al–X alloys. With regards to their mechanical properties, it was found that the alloying elements V, Cr, and Ni improve the ductility of the alloys by increasing Pugh's ratio, whereas Mn, Co, and Cu cause the materials to be more brittle. Among the tested alloying elements, V leads to the largest increase of Pugh's ratio as well as significant reduction in the alloy's elastic anisotropy, which implies that Fe3Al–V should have an enhanced ductility. An electronic structure analysis showed that the alloying elements altered the density of states of the alloys. From the projected density of states and from a crystal orbit Hamiltonian population analysis, it could clearly be seen that V leads to the stabilization of Fe3Al–V by reducing the magnetic moment and by relaxing the spin asymmetry of the Fe atoms. This result is in good agreement with that of a previous experiment, which reported that the alloy Fe3Al–V has an enhanced strength. However, Ni has antibonding states at the Fermi level that are substantially filled, which was found to be detrimental to the structural stability of the Ni alloy.

Mechanical, structural, electronic, magnetic and thermoelectric properties of multi functional CoFeMnSb quaternary Heusler alloy have been studied using spin polarized Density Functional and semiclassical Boltzmann transport theory. Ab-initio study reveals halfmetallic ferromagnetic characteristics of the alloy leading to high spin polarization. The calculated value of total magnetic moment (mt = 4.99 μB) almost follow the Slater-Pauling rule which is another prerequisite of halfmetallicity. Study of hydrostatic pressure shows a significant effect on the electronic structure of the alloy however, along with the robust mechanical stability there is no magnetic phase transition seen as such in the whole range of pressure. Due to the occurrence of flat bands near the Fermi level(EF) in the minority spin channel, thermoelectric properties are also studied for both the spin polarized channel using BoltzTraP code under constant relaxation time approximation of the charge carriers. Halfmetallic properties, robust mechanical stability and significantly high value of Seebeck coefficient (∼800μV/K for p-type and ∼−900μV/K for n-type) makes the material promising for spintronic as well as thermoelectric applications.

The role of spin orientation on the reactivity of oxygen reduction reaction (ORR) intermediates (O, OH) on a ferromagnetic electrode surface is studied using constrained density functional theory formalism. We show that the strength of the binding of these reaction intermediates depend on their relative spin orientations with respect to the magnetization of the electrode. This suggests that oxygen-based electrochemical reactions on ferromagnetic catalyst surfaces can be controlled through the applied magnetic field. In the present study, we demonstrate such a possibility through the study of an oxygen reduction reaction on a PdFe (001) surface by introducing a new concept: spin orientation dependent overpotential. Also, we have explained the origin of lower dissociation barrier for the O2 molecule on ferromagnetic surfaces when its spin moment is anti-parallel to the surface magnetization as reported in the recent experiments.