Paper & Patents

First principles studies were performed in order to find out the possibility of inducing half-metallicity in Heusler Compound CoFeMnSb, by means of alloying it with 3d-transition metal elements. Proper alloying element is selected through the calculations of formation energies. These calculations were tested with different concentrations of alloying elements at different atomic sites. Among the selected transition metal elements Sc and Ti are proposed to be excellent alloying elements particularly at Mn site. By using these alloying elements complete half metallic behaviour is obtained in CoFeMn_0.25 Sc_0.75 Sb, CoFeMn_0.75Ti_0.25Sb, CoFeMn_0.625Ti_0.375Sb, CoFeMn_0.50Ti_0.50Sb, CoFeMn_0.25Ti_0.75Sb and CoFeTiSb alloys. Shifting of Co-Fe d-states towards lower energy region leads to zero density of states at Fermi level for the spin minority channel. Alloying effects on the electronic structure and magnetization are discussed in details. Thermodynamical stability of these new alloys are major part of this study. The Curie temperatures of CoFeMn_0.25Sc_0.75Sb and CoFeMn_0.75 Ti_0.25Sb were found to be 324.5 K and 682 K; respectively, showing good candidature for spintronics applications. For understanding the bonding nature of constituent atom of CoFeMnSb, crystal orbital Hamiltonian populations have been analysed.

A detailed study on the inter-metallic alloy, Zr 2 TiAl, has been carried out using first principle electronic structure calculations. We found that a small value of bi-axial strain/stress cause a phase change from anti-ferromagnetic (AFM) to ferromagnetic (FM) with a structural transition from face center cubic (fcc) to body center tetragonal (bct). Calculated electronic band structures show that all strained structures are metallic in nature with Zr-d and Ti-d orbital dominated energy bands near the Fermi level (E F). The stability of FM phase is confirmed with phonon dispersion calculations by using density functional perturbation theory (DFPT). It has been observed that AFM state with both positive and negative bi-axial stress exhibits unstable modes while corresponding FM state shows no such instability. This clearly indicates the existence of large spin phonon coupling in this material.

Chemical reactions are often associated with an energy barrier along the reaction pathway which hinders the spontaneity of the reaction. Changing the energy barrier along the reaction pathway allows one to modulate the performance of a reaction. We present a module, Python Algorithms for Searching Transition stAtes (PASTA), to calculate the energy barrier and locate the transition state of a reaction efficiently. The module is written in python and can perform nudged elastic band, climbing image nudged elastic band and automated nudged elastic band calculations. These methods require the knowledge of the potential energy surface (and its gradient along some direction). This module is written such that it works in conjunction with density functional theory (DFT) codes to obtain this information. Presently it is interfaced with three well known DFT packages: Vienna Ab initio Simulation Package (VASP), Quantum Espresso and Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA). This module is easily extendable and can be interfaced with other DFT, force-field or empirical potential based codes. The uniqueness of the module lies in its user-friendliness. For users with limited computing resources, this module will be an effective tool as it allows to perform the calculations image by image. On the other hand, users with plentiful computing resources (such as users in a high performance computing environment) can perform the calculations for large number of images simultaneously. This module gives users complete flexibility, thereby enabling them to perform calculations on large systems making the best use of the available resources. Program summary: Program Title: PASTA Program files doi: http://dx.doi.org/10.17632/rv7fdm5gkf.1 Licensing provisions: BSD 3-clause Programming language: Python External routines/libraries: numpy, matplotlib Nature of problem: Most of the reactions have an energy barrier on their reaction pathway. This energy barrier affects the progress of the reaction. Solution method: We implement the NEB, CI-NEB and AutoNEB method to locate transition state and estimate the energy barrier. Our module works with density functional theory codes: VASP, SIESTA and Quantum Espresso presently.

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.