Structural Characterization Of Complex Oxides And Sulfates Towards the Design of Photocatalytic And Sodium Ion Conducting Materials.

Abstract

The thesis entitled "Structural Characterization of Complex Oxides and Sulfates Towards the Design of Photocatalytic and Sodium Ion Conducting Materials" consists of _ve chapters. Chapter 1 gives a brief introductory note which outlines the various synthesis procedures, characterization techniques and a description of properties like photocatalysis and ionic conductivity. Chapter 2 discusses the solution combustion synthesis of Bi2Zr2O7 using urea, tartaric Acid and glycine as fuels. Only the samples prepared using urea and tartaric acid result in pure compounds and are further characterized by X-ray di_raction studies depicting a disordered uorite type structures. Careful Rietveld re_nements bring out subtle structural di_erences in these two samples as well, a feature which is demonstrated for the _rst time among samples prepared from two di_erent fuels. Di_erence Fourier maps con_rms the structures, and the catalytic behaviour is shown to correlate to these subtle changes in oxygen occupancy. The band gap determined from UV-Vis spectroscopic results conform to the structural di_erences of the compound. Photocatalytic degradation of cation dyes suggest that the compound prepared using urea shows better photocatalytic activity and is comparable to the commercial Degussa P-25. Chapter 3 describes the e_ect of Bi doping on photocatalytic activity of CeO2 (band gap is in the UV range) is evaluated with BixCe1-xO2ð€€€_ (x = 0.2, 0.4, 0.6) using solution combustion method using glycine as fuel. These compounds have a band gap in the visible range and the structures are established by Rietveld re_nements clearly establishing that the oxygen vacancies increase with increasing bismuth substitution. Featureless di_erence Fourier maps con_rm the structures and photodegradation experiments on a cationic and an anionic dye clearly establishing that the photocatalytic activity increases with increase in bismuth content leading to increased oxygen vacancies. Chapter 4 describes synthesis, crystal structure, phase transition and ionic conductivity in a family of vantho_te mineral Na6Mn(SO4)4. Single crystal of Na6Mn(SO4)4 are grown from aqueous solution by slow evaporation method at 80°C, crystal grown are analyzed by single crystal X-ray di_raction which depict monoclinic system with space group P21/c at room temperature. Ionic conductivity measurements are carried out by using impedance spectroscopy, and conductivity value is found to be 2.01x10ð€€€5 Scmð€€€1 at 490°C and 7.4x10ð€€€3 Scmð€€€1 at 505°C. Two order magnitude change in conductivity value on a temperature window of 15°C con_rms a _rst order nature of phase transition. Further, conductivity of the mineral reached of 3.9x10ð€€€2 Scmð€€€1 at 600°C which establishes the superionic nature of the mineral.In addition, the nature of phase transition was examined by using thermal analysis such asDSC, DTA and variable temperature powder X-ray di_raction technique. The PXRD after the phase transition at 550°C was also indexed, pro_le _tted with orthorhombic space group. Chapter 5 presents the crystal growth and in situ structural studies of di and tetra hydrate of vantho_te mineral Na6M(SO4)4 (M = Ni and Co). As discussed in Chapter 4, these crystals are grown in aqueous solution by slow evaporation method at 80°C in an oven. Interestingly, di and tetra hydrate of Na6M(SO4)4 (M = Ni and Co) are grow concomitantly. Single crystal X-ray di_raction measurements reveal the structure to be triclinic with space group P_1. Further di-hydrates of Na6M(SO4)4 (M = Ni and Co) are isostructural, and the tetrahydrates also follow the same trend. Thermal Gravimetric analyses and in situ powder di_raction studies were carried out to characterize the step-wise dehydration process in these materials.