Oxygen deficient perovskites: effect of structure on electrical conductivity, magnetism and electrocatalytic activity.

The present thesis deals with the synthesis and study of the physico-chemical properties of perovskite based oxide materials. Several novel oxygen deficient perovskites (ODP) have been synthesized by conventional solid state synthesis method. The novel compounds are CaSrFe2O6-δ, CaSrFeCoO6-δ, Ca2Fe1.5Ga0.5O5, CaSrFeGaO5 and BaSrFe2O5. Their magnetic, charge transport and electrocatalytic properties have been studied. Structural effect on electrical conductivity, magnetic and electrocatalytic properties have been studied in some series of ODPs. CaSrFe2O6-δ, CaSrFeCoO6-δ, Ca2Fe1.5Ga0.5O5 and CaSrFeGaO5 have brownmillerite type orthorhombic structures with layered structure having alternate tetrahedral and octahedral layers which are connected to one another by corner sharing. These are vacancy ordered compounds. BaSrFe2O5 is vacancy disordered compound with cubic structure. Most of the studied materials exhibited G-type long range antiferromagnetic arrangement of magnetic moments. During the study of charge transport property, compounds with structural order in a particular series show relatively less conductivity at room temperature and semiconductive nature and transition to metallic conductivity during temperature dependent conductivity measurement. Vacancy disordered compounds show relatively higher conductivity at room temperature and show mixed (semiconductive and metallic) conductivity during temperature dependent conductivity measurement. The study of electrocatalytic properties revealed the relation with the conductivity and the structural order. The electrocatalytic activity toward oxygen evolution reaction is highly efficient if the material is highly conductive or highly ordered

Effect of Reverse Micelles Size on the Electron Transfer Reaction within the Ion Pair of Co (III)/Fe (II) Complexes

The electron transfer process between pentammineaquacobalt (III) and hexacyanoferrate (II), [Co(NH3)5H2O]3+/Fe(CN)6]4− ion pair was investigated in water/dioctyl sodium sulfosuccinate (AOT)/Isooctane reverse micelles. The study observed that the electron transfer rate depends on the size of the reverse micelles. The concentrations of Fe (II) ions were varied in different-sized (Wo) reverse micelles of Wo = [H2O]/[AOT] = 10 to 30, but the concentration of Co (III) ions was kept constant. The rate of electron transfer in the ion pair [Co(NH3)5H2O]3+/[Fe(CN)6]4− increased with decreasing size (Wo) of reverse micelles. The smallest reverse micelles Wo = 10 demonstrated the fastest electron transfer rate, and the biggest Wo = 30 reverse micelles showed the slowest electron transfer rate. The change of reaction environment and the location of the reactants in the reverse micelles due to confinement are considered the factors responsible for the results

Effect of Reverse Micelles Size on the Electron Transfer Reaction within the Ion Pair of Co (III)/Fe (II) Complexes

The electron transfer process between pentammineaquacobalt (III) and hexacyanoferrate (II), [Co(NH3)5H2O]3+/Fe(CN)6]4− ion pair was investigated in water/dioctyl sodium sulfosuccinate (AOT)/Isooctane reverse micelles. The study observed that the electron transfer rate depends on the size of the reverse micelles. The concentrations of Fe (II) ions were varied in different-sized (Wo) reverse micelles of Wo = [H2O]/[AOT] = 10 to 30, but the concentration of Co (III) ions was kept constant. The rate of electron transfer in the ion pair [Co(NH3)5H2O]3+/[Fe(CN)6]4− increased with decreasing size (Wo) of reverse micelles. The smallest reverse micelles Wo = 10 demonstrated the fastest electron transfer rate, and the biggest Wo = 30 reverse micelles showed the slowest electron transfer rate. The change of reaction environment and the location of the reactants in the reverse micelles due to confinement are considered the factors responsible for the results

Investigation of Grain, Grain Boundary, and Interface Contributions on the Impedance of Ca₂FeO₅

Conductivity properties such as the impedance contributions of grain, grain boundary, and electrode–material interface of brownmillerite-type Ca2Fe2O5 are studied using alternate current (AC) impedance at different temperatures over a wide range of frequencies. The compound was synthesized at 1000 °C by a solid-state reaction. Powder X-ray diffraction confirmed the pure and single-phase formation. The correlation of the electrical properties with the microstructure of the compound was studied by an AC impedance spectroscopic technique at different temperatures (25–300 °C), which demonstrated the contribution of both the grain (bulk) and grain boundary to the impedance. The frequency-dependent electrical conductivity was used to study the conductivity mechanism. The electric impedance and the frequency at different temperatures supported the suggested conduction mechanism

Comparative Thermal Insulation Nature of Ca2FeMnO6−δ and Sr2FeMnO6−δ

In this study, we investigate the utility of Ca _2 FeMnO _6- _δ and Sr _2 FeMnO _6- _δ as materials with low thermal conductivity, finding potential applications in thermoelectrics, electronics, solar devices, and gas turbines for land and aerospace use. These compounds, characterized as oxygen-deficient perovskites, feature distinct vacancy arrangements. Ca _2 FeMnO _6- _δ adopts a brownmillerite-type orthorhombic structure with ordered vacancy arrangement, while Sr _2 FeMnO _6- _δ adopts a perovskite cubic structure with disordered vacancy distribution. Notably, both compounds exhibit remarkably low thermal conductivity, measuring below 0.50 Wm ^−1 K ^−1 . This places them among the materials with the lowest thermal conductivity reported for perovskites. The observed low thermal conductivity is attributed to oxygen vacancies and phonon scattering. Interestingly as SEM images show the smaller grain size, our findings suggest that creating vacancies and lowering the grain size or increasing the grain boundaries play a crucial role in achieving such low thermal conductivity values. This characteristic enhances the potential of these materials for applications where efficient heat dissipation, safety, and equipment longevity are paramount