Studies on the synthesis, crystal growth, structural and physical properties of naxcoo2 and fe-as based compounds

Abstract

The discovery of first high-Tc cuprate superconductor in 1986 has boosted the research in strongly correlated materials. The motivation for understanding the mechanism of superconductivity has increased much in order to engineer novel superconducting materials with high transition temperatures. Hence, the investigation of other families (non-cuprates) of superconductors has been of great interest that might also help to understand the mechanism of superconductivity. The program of my doctoral thesis was two-fold: (i) studies on the synthesis and crystal growth of layered sodium cobalt oxide (NaxCoO2) and investigations on their thermal, structural and physical properties; (ii) synthesis of newly discovered K doped MFe2As2 (M = Ba and Sr) and Ru doped BaFe2As2 superconductors by novel methods and to study their structural and superconducting properties. Both the systems are of layered structure like high Tc cuprates and d electrons play a crucial role on the conduction band. The main difference is that the NaxCoO2 system has the triangular lattice symmetry and Fe-pnictide compounds have the tetragonal coordination when compared to the square planar coordination in cuprates. The first chapter provides a brief introduction about superconductivity. The second chapter presents the review on NaxCoO2 and Fe-pnictide superconductors. Also various experimental techniques used for the sample characterisation are given in this chapter. The third chapter comprises of the synthesis, deintercalation/interaction studies and characterization of the polycrystalline NaxCoO2. Synthesis of NaxCoO2 compounds with precise x value is difficult due to volatile nature of Na. In the present work, NaxCoO2 (x = 0.75, 0.80 and 0.90) samples were synthesized by modified quenching method. Samples with lower x values (x = 0.30 and 0.40) were prepared by bromination method. The melting behavior of NaxCoO2 was analyzed by TG-DTA measurements. All the samples were identified as single phase by means of powder newlineXRD studies.