Birabar Ranjit Kumar Nanda

The practical application of Li–S batteries is severely limited due to low sulfur utilization, sluggish sulfur redox kinetics, intermediate polysulfide dissolution/shuttling, and subsequent anode degradation. A smart cathode with efficient electrocatalyst and a protected anode is necessary. Herein, hollow carbon (HC) spheres are used as a sulfur host to improve the electrical conductivity and buffer the volume expansion of active materials. Considering the weak interaction between carbon and lithium polysulfides (LiPS), tungsten diboride (WB2) nanoparticles are used as a conductive additive. Both experimental and density functional theory (DFT) comprehensively exhibit that metallic WB2 nanoparticles can firmly anchor the LiPS through B–S bond formation, accelerate their electrocatalytic conversion, and immobilize them. DFT also reveals that boron interacts with LiPS either through molecular or dissociative adsorption depending on its boron layer arrangement in WB2. Further, a freestanding lithiated-poly(4-styrene sulfonate) membrane constructed on lithium, offers a homogeneous Li-ion flux, stable interface, and protection from LiPS. Finally, cells with the HC-S+WB2 cathode and protected anode exhibit improved active material utilization, superior rate performance, and impressive cycling stability, even at high sulfur loading and less quantity of the electrolyte. Further, the pouch cells demonstrate high reversible capacity and an excellent capacity retention.

Artificial confinement of electrons by tailoring the layer thickness has turned out to be a powerful tool to harness control over competing phases in nano-layers of complex oxides. We investigate the effect of dimensionality on transport properties of d-electron–based heavy-fermion metal CaCu3Ru4O12. Transport behavior evolves from metallic to localized regime upon reducing thickness and a metal-insulator transition is observed below 3 nm film thickness for which sheet resistance crosses h/e2 ∼ 25 kΩ, the quantum resistance in 2D. Magnetotransport study reveals a strong interplay between inelastic and spin-orbit scattering lengths upon reducing thickness, which results in weak-antilocalization (WAL) to weak-localization (WL) crossover in magnetoconductance.

Using density functional calculations we explore the possibilities of inducing spin moments in otherwise non-magnetic electronic structure of graphene. Through intercalation of H, N, O and F atoms between two hexagonal stacked graphene layers, we show that unpaired electrons can be generated when the planar coordinates of the functional atoms coincide with the center of the graphene hexagon. The spin-half states are realized at the functional sites for certain values of interlayer separations. For oxygen and fluorine these interlayer separations represent the natural stable phases and for hydrogen and nitrogen they induce instability which can be overcome by applying external pressure. We attribute the formation of spin-half states to the one dimensional confinement potential exerted by the graphene layers on the valence electrons of the functional elements.

Density functional calculations are performed to study the magnetic order of the severely distorted square planar cupric oxide (CuO) and local spin disorder in it in the presence of the transition metal impurities M (=Cr, Mn, Fe, Co and Ni). The distortion in the crystal structure, arisen to reduce the band energy by minimizing the covalent interaction, creates two crisscrossing zigzag spin-1/2 chains. From the spin dimer analysis we find that while the spin chain along (1 0 1) has strong Heisenberg type antiferromagnetic coupling (J ∼ 127 meV), along it exhibits weak, but robust, ferromagnetic coupling (J ∼ 9 meV) mediated by reminiscent p-d covalent interactions. The impurity effect on the magnetic ordering is independent of M and purely orbital driven. If the given spin-state of M is such that the dx2-y2 orbital is spin-polarized, then the original long-range ordering is maintained. However, if dx2-y2 orbital is unoccupied, the absence of corresponding covalent interaction breaks the weak ferromagnetic coupling and a spin-flip takes place at the impurity site leading to breakdown of the long range magnetic ordering.

Anionic alloying (halide mixing) in lead-free halide double perovskites is an effective strategy to tailor the optoelectronic properties including band gap. An important question that needs to be addressed is whether halide ions mix up homogeneously at the atomic scale as has already been inferred in a hybrid halide solid solution. Here, we show from Raman spectral analyses that halide ions (X = Cl-, Br-) preferentially form Br-rich or Cl-rich octahedra in Cs2AgBiBr6-xClx (x = 0 to 6; M = Ag, Bi) double perovskites. Octahedral vibrations show discontinuity in Raman shifts upon alloying, and the observation of octahedral modes from both [MCl6-xBrx]5- and [MBr6-xClx]5- with a shift from the end-member vibrational frequencies confirms the absence of homogeneous mixing (i.e., octahedra [MBr3Cl3]5-) and preferential formation of X-rich octahedra. The lattice parameter and the optical band gap of Cs2AgBiBr6-xClx vary linearly resembling Vegard’s rule, suggesting a macroscopic solid solution behavior while maintaining, at the sublattice level, the preferential X-rich octahedra. This is further corroborated through comprehensive first-principles calculations that the alloyed structure with preferential occupation of halide ions, instead of local phase segregation or homogeneous mixing, tends to be the more stable configuration. An equal number of dissimilar halogen atoms in each octahedron as conventionally assumed is not a stable configuration. The linear variation of the band gap is attributed to the fact that individual Ag-X and Bi-X interactions add up to form the electronic structure, and therefore, the band gap is primarily correlated to the concentration of Cl and Br anions rather than their distribution in the individual octahedra.

We report the transition from robust ferromagnetism to a spin-glass state in nanoparticulate La0.7Sr0.3MnO3 through solid solution with BaTiO3. The field- and temperature-dependent magnetization and the frequency-dependent ac magnetic susceptibility measurements strongly indicate the existence of a spin-glass state in the system, which is further confirmed from memory effect measurements. The breaking of long-range ordering into short-range magnetic domains is further investigated using density-functional calculations. We show that Ti ions remain magnetically inactive due to insufficient electron leakage from La0.7Sr0.3MnO3 to the otherwise unoccupied Ti-d states. This results in the absence of a Mn-Ti-Mn spin exchange interaction and hence the breaking of the long-range ordering. Total-energy calculations suggest that the segregation of nonmagnetic Ti ions leads to the formation of short-range ferromagnetic Mn domains.

Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2-4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk OV are nearly flat, degenerate, and discrete, the defect states due to surface OV are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells.

Density-functional calculations are carried out to understand and tailor the electrochemical profile - diffusivity, band gap, and open-circuit voltage - of transition-metal-doped olivine phosphate: LiFe1-xMxPO4 (M=V, Cr, Mn, Co, and Ni). Diffusion and, hence, the ionic conductivity is studied by calculating the activation barrier Vact experienced by the diffusing Li+ ion. We show that the effect of dopants on diffusion is both site dependent and short ranged, and thereby it paves ways for microscopic control of ionic conductivity via selective dopants in olivine phosphates. Dopants with lower-valence electrons (LVEs) compared to Fe repel the Li+ ion to facilitate its outward diffusion, whereas higher-valence-electron (HVE) dopants attract the Li+ ion to facilitate the inward diffusion. From the electronic structure calculation, we establish that irrespective of the dopant M, except Mn, the band gap is reduced since the M d states always lie within the pure band gap. Atomically localized d states of HVE dopants lie above the Fermi energy and that of LVE lie below it. Half-filled Mn d states undergo a large spin-exchange split to bury the dopant states in the valence and conduction bands of the pristine system, and, in turn, the band gap remains unchanged in LiFe1-xMnxPO4. Baring Mn, the open-circuit voltage increases with HVE dopants and decreases with LVE dopants.

We show how epitaxially grown Cu-Rh and Cu-Ni heterostructures exploit the strain effect, due to the lattice mismatch, and the ligand effect, arising from the electronic interaction between the heterolayers, to achieve improved CO2 electroreduction. In this study we have performed density functional calculations on Cui/Mj/Cu(211) sandwiched surfaces and Cui/M(211) overlayers (where M = Rh or Ni, with varied i and j monolayers). We examined the free energy profiles of the reaction mechanisms for CO2 reduction to CO and CH4. We find that in Cu1/M1/Cu(211), in which the Cu monolayer experiences only a pure ligand effect, the influence of Ni is weaker than Rh and it decreases the overpotential for CO2 reduction by ∼10-20 mV. A larger decrease (33-64 mV) in the overpotential is predicted for other sandwiched surfaces: Cu1/Ni2/Cu(211), Cu2/Rh1/Cu(211), and Cu2/Rh2/Cu(211) in which the ligand effect is weaker. In the Cu1/M(211) overlayer, Cu is affected by both the strain and ligand effects, of which the latter dominates. As the number of Cu monolayers increases from one to three, the strain effect becomes dominant in the Cu overlayers. We demonstrate that the tensile strain on Cu in Cu2-3/Rh(211) overlayers causes a significant decrease (by 86 mV) in the overpotential for CO2 electroreduction, while the compressive strain in Cu2-3/Ni(211) overlayers has an opposite effect. Furthermore, Cu2/Rh2/Cu(211) and Cu2-3/Rh(211) will also exhibit an increase in exchange current density, i.e., electrocatalytic activity for CO2 reduction. This is accompanied by a retention of selectivity for CO and CH4 over hydrogen evolution. (Graph Presented).

The 5d transition metal oxides, in particular iridates, host novel electronic and magnetic phases due to the interplay between onsite Coulomb repulsion (U) and spin-orbit coupling (SOC). The reduced dimensionality brings another degree of freedom to increase the functionality of these systems. By taking the example of ultrathin films of SrIrO3, theoretically we demonstrate that confinement led localization can introduce large magnetic anisotropy energy (MAE) in the range of 2-7 meV/Ir, which is one to two order higher than that of the traditional MAE compounds formed out of transition metals and their multilayers. Furthermore, in the weak correlation limit, tailored terminations can yield multiple Dirac states across a large energy window of 2 eV around the Fermi energy, which is rare phenomena in correlated oxides and upon experimental realization it will give rise to unique transport properties with excitation and doping.