Birabar Ranjit Kumar Nanda

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.

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).

We have studied the electronic structure and magnetism of the spin chain compounds Ca3ZnMnO6 and Ca3ZnCoO6 using density functional theory with generalised gradient approximation (GGA). In agreement with experiment our calculations reveal that high spin (HS) state for Mn4+ ion and low spin (LS) state for Co4+ ion stabilize the magnetic structure of the respective compounds. The magnetic exchange paths, calculated using Nth order muffin-tin orbital downfolding method, shows dominant intra-chain exchange interaction between the magnetic ions (Mn, Co) is antiferromagnetic for Ca3ZnMnO6 and ferromagnetic for Ca3ZnCoO6. The magnetic order of both the compounds is in accordance with the Goodenough-Kanamori-Anderson rules and is consistent with the experimental results. Finally we have investigated the importance of spin-orbit coupling (SOC) in these compounds. While SOC practically has no effect for the Mn system, it is strong enough to favor the spin quantization along the chain direction for the Co system in the LS state.

Oxygen plays a critical role in strongly correlated transition metal oxides as crystal field effect is one of the key factors that determine the degree of localization of the valence d/f states. Based on the localization, a set of conventional mechanisms such as Mott-Hubbard, Charge-Transfer and Slater were formulated to explain the antiferromagnetic and insulating (AFI) phenomena in many of these correlated systems. From the case study on LiFePO 4, through density-functional calculations, we demonstrate that none of these mechanisms are strictly applicable to explain the AFI behavior when the transition metal oxides have polyanions such as (PO 4) 3â'. The symmetry-lowering of the metal-oxygen complex, to stabilize the polyanion, creates an asymmetric crystal field for d/f states. In LiFePO 4 this field creates completely non-degenerate Fe-d states which, with negligible p-d and d-d covalent interactions, become atomically localized to ensure a gap at the Fermi level. Due to large exchange splitting, high spin state is favored and an antiferromagnetic configuration is stabilized. For the prototype LiFePO 4, independent electron approximation is good enough to obtain the AFI ground state. Inclusion of additional correlation measures like Hubbard U simply amplifies the gap and therefore LiFePO 4 can be preferably called as weakly coupled Mott insulator.

Polycrystalline BaTi1-xSnxO3 samples (x = 0.06, 0.07, 0.08, 0.09, 0.10, and 0.11) were synthesized by the solid state technique. The samples exhibit the tetragonal phase at 300 K. In addition, the samples x = 0.06, 0.07, 0.08, and 0.09 also show the orthorhombic phase with enhanced phase fractions upon poling. However, the % orthorhombic phase fractions show an increase up to x = 0.07 and a decrease with an increase in x. The dielectric studies indicate that TC (cubic to tetragonal phase transition) shifts toward lower temperature where the samples x = 0.10 and 0.11 show the tetragonal phase at 300 K. The samples exhibit the maximum remnant polarization and piezoelectric coefficient for x = 0.08. But the bandgap for the x = 0.07 sample shows the value of 2.61 eV before poling and 2.95 eV after poling. A giant photovoltaic (PV) response is seen in the samples with the open-circuit voltage (VOC) as large as 16 V (for x = 0.07). VOC shows a decreasing trend with an increase in the Sn content after x = 0.07, and it did not follow the trend in polarization and the bandgap. The observed results are correlated with the structural symmetry of the compound, and they are validated by the band-structure calculations. The experimental and theoretical studies indicate that the sample with the orthorhombic phase is preferable for the enhanced photovoltaic response in comparison to the tetragonal phase. These studies show a new way to achieve a large photovoltaic response so as to design the system for several device applications such as UV detectors and microactuators.

The electrocatalytic properties of Cu, Ni, and Cu0.75Ni0.25 alloy are investigated for CO and CO2 reduction to methane by density functional calculations. We show that, as the Ni content increases in Cu(1-x)Nix (211) surfaces (x = 0, 0.25, and 1), the binding energies (Es) of the adsorbates involved in the reaction mechanism decrease. Linear scaling relations are known to exist between Es of adsorbates binding via a C (O) atom over pure transition metal surfaces. However, we find that alloying Cu and Ni has the potential for breaking these relations for certain pairs of adsorbates. The decrease in the repulsive Coulombic interaction between the adsorbate and the charges induced on the Cu-Ni alloy surface explains the adsorption site preference. The E shift with respect to pure Cu is larger for species binding through C than O. Various trends exhibited by the binding energies are understood by analyzing the chemical bonding through local density of states and charge density isosurfaces of the bare and adsorbed surfaces. The free energy profile for CO and CO2 reduction to CH4 on the alloy surface is a mix of its behavior on Cu and Ni (211). Our calculations predict that CH4 generation directly from CO reduction on Cu0.75Ni0.25 (211) can occur at an earlier applied potential than required for Cu and Ni (211) surfaces. However, it will be the opposite case for CO2 reduction to CH4.

From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO3/SrTiO3. Depending on the strengths of the various competing interactions, viz., the electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface. © 2011 American Physical Society.