Investigation of effect of elemental substitution in layered iron-based and related superconductors on nanoscale structural disorder

This thesis work provides a study of effect of direct elemental substitution to the active layer on nanoscale structural disorder and physical properties in layered Iron-based Superconductors (FeSCs). In this work we have studied Mn, Co and Ni substitution to the Fe site (active layer) on LaFeAsO (1111 type), BaFe2As2 (122 type), SrFe2As2 (122 type) respectively and Te substitution to the Se site (active layer) on FeSe by means of local structure and physical properties. La(Fe1-xMnx)AsO results reveal that the FeAs-layer thickness, measured by the As-height (hAs) from the Fe-Fe plane, is correlated with the Néel temperature. Ba(Fe1-xCox)2As2 (x=0.06) superconductor the Fe-Fe sublattice reveals a sharp anomaly across Tc, indicated by a significant drop in mean square relative displacements. SrFe2-xNixAs2 Fe–As bondlength have a tendency to decrease with increasing Ni concentration. On the other hand mean square relative displacements (σ2) of Fe–As bondlengths display a gradual decrease with increasing Ni concentration which can be a indicator of a emerging configurational ordering. In FeSe1-xTex The Fe 3d-Se 4p/Te 5p hybridization is found to decrease with Te substitution, accompanied by an increase in unoccupied Se 4p states and a decrease in unoccupied Te 5p states.