J. J. Pulikkotil

Mg3Sb2-based Zintl compound: a non-toxic, inexpensive and abundant thermoelectric material for power generation

The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the “phonon-glass electron crystal” (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg3Sb2 and its derivative of isoelectronically Bi doped Bi; Mg3Sb2−xBix (0 ≤ x ≤ 0.4) alloys in Mg3Sb2. Single phase p-type Mg3Sb2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg3Sb2. However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 × 1020/cm3 for x ≥0.25. The substantial increase in ZT in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

Cooperative effects of lattice and spin-orbit coupling on the electronic structure of orthorhombic SrIrO3

Orthorhombic SrIrO3 subjected to strain shows tunable transport properties. With underlying symmetry remaining invariant, these properties are associated with IrO6 octahedral tilting. Adopting first-principles methods, the effects of crystal field, spin-orbit coupling (SOC), and Coulomb correlations, on comparable interaction length scales, are discussed. While tilting induces a t(2g) - e(g) crystal-field splitting and band narrowing, SOC induces a partial splitting of the J(eff) bands rendering SrIrO3 a semi-metallic ground state. The SOC enhanced hybridization of Ir-O orbitals serves as an explanation as to why the critical Hubbard correlation strength increases with increasing SOC strength in SrIrO3 to induce an insulating phase.

Energetics and electronic structure of La/Sr disorder at the interface of SrTiO3/LaTiO3 heterostructure

In the artificially tailored heterostructures of certain materials, a polar discontinuity across the interface introduces a large energy cost. The total energy of such systems may be reduced either by electronic reconstruction leading to the interface phases or by simple atomic reconstruction by inter-site cation mixing. While most of the experiments and theoretical calculations assume an abrupt interface, in this work, we consider the La/Sr inter-site disorder across the interface of SrTiO3/LaTiO3 heterostructures and study its energetics and electronic structure properties. The calculations find that inter-site mixing of La/Sr atoms across the interface also reduces the total energy. However, the extent of such disorder is found to be dramatically minimized by allowing the interfacial atoms to relax fully and that for such systems, the changes in the electronic structure are negligible.

Retentivity of spin state transitions in LaCoO3 with chemical disorder

By means of structural refinements, magnetic susceptibility measurements and density functional theory based calculations; we study the physical properties of LaCoO3 with Al substitutions. Despite Al substitutions, which result in a reduction of unit-cell volume, increased chemical disorder, and local octahedral distortions, we find that the thermally mediated low-spin and high-spin states in LaCoO3 survive against the odds. The observed relative shift of the low-spin and high-spin representative peaks in the magnetic susceptibility data is determined to be of electronic origin via a change in the material’s chemical bonding properties. Our study reveals a strong interplay between bonding and disorder for the temperature dependent spin states of LaCoO3.

Post-perovskite CaIrO3: a conventional Slater type antiferromagnetic insulator

To resolve the controversy of whether or not the origin of an electronic gap in antiferromagnetic post-perovskite (pPv) CaIrO3 is due to Coulomb repulsion or spin-orbit coupling, and/or both, we have performed comprehensive full potential density functional theory based calculations. A rather consistent electronic structure, which explains the origin and magnitude of the electronic gap, inter-band d-d transition energies, high thermopower and large magneto-crystalline anisotropy, is obtained with the use of a modified Becke-Johnson (mBJ) exchange potential. Fundamentally, mBJ calculations correctly capture the strong interplay of the crystal field and long range antiferromagnetic ordering of Ir spins as the mechanism that drives pPv-CaIrO3 to an insulating state. Based on our findings, we propose that pPv-CaIrO3 is a conventional Slater type antiferromagnetic insulator.

Mg9Si5: a potential non-toxic thermoelectric material for mid-temperature applications

Mid-temperature thermoelectric applications include waste heat recovery from automobile exhausts, various industrial process and solar thermoelectrics. The current systems that have been extensively investigated for mid-temperature applications are mostly tellurides. However, the low crustal abundance of tellurium contributes significantly to their price volatility which impedes their usage in commercial technology. Looking from the perspective of material cost effectiveness in terms of crustal abundance and environmental sustainability, we investigate the thermoelectric properties of Mg9Si5 systems, the constituents of which are geo-abundant and non-toxic. By means of first principles calculations, we find Mg9Si5 to be a potential mid-temperature thermoelectric material, with an operational temperature in the range of ≃400–600 K, depending on the exact carrier concentration. We also discuss the enhanced viability of substitution in Mg9Si5, a case that seems to be restricted in Mg2Si because of its chemically balanced Zintl nature.

Effect of pressure on itinerant magnetism and spin disorder in cubic FeGe

The results of ab initio calculations of the pressure dependence of Fe magnetism in cubic FeGe are presented. We find that when the pressure-volume scale is set by means of generalized gradient approximation total energies and magnetism is described by means of the local density approximation, the critical pressure at which the magnetic phase transition occurs is estimated at ≈18 GPa, which is in good agreement with experiments. Using the disordered local moment method we find a localized to itinerant model cross-over of Fe magnetism in cubic FeGe, as a function of volume. Moreover, our calculations also suggest subtle signatures of longitudinal spin fluctuations in cubic FeGe, and that the stiffness parameter softens with increasing pressure. We associate the retention of metallicity in FeGe under pressure with the spin-disorder scattering. The effect of spin-orbit coupling on the electronic structure is also discussed.