Physics of transition metal chalcogenides based on deeper investigation of their phase-diagram and crystal growth mechanism

Abstract

This thesis aims to create a chemical approach for fabricating controlled Fe-Se system nanoparticles and comprehending the formation process. Chapter 1 of this thesis gives an overview of the history and progress of essential formation processes for better understanding and controlling the Fe-Se system properties. Chapter 2 investigates the theoretical crystal habit of all the phases of the Fe-Se system using BFDH (pure crystallographic approach) and HP (periodic bond chain vector approach) models. Chapter 3 deals with the Fe-Se system's complete phase transformation sequence using a wet-chemical method, from plotting the phase diagram to optimizing the appropriate conditions required to synthesize individual phases. Finally, the cause for the similarities and dissimilarities in theoretical crystal habits and real-life experimental morphologies is examined. Chapter 4 explains the correlation between the phases and the magnetic properties of the Fe-Se system. The magnetic properties vary enormously with change in Fe:Se ratio due to the change in Fe2+/Fe3+ ratio, crystal field environment around Fe-ions, magnetocrystalline anisotropy, Fe-vacancies, and so forth. It is observed that Fe3Se4 has the most unique and interesting magnetic properties. Chapter 5 deals with the growth directions of Fe3Se4 in different reaction conditions. The entire roadmap is laid-out―starting from the formation of the unit cell to the diffusion and attachment of monomers and the fate of various facet growth. Finally, we have shown how the distinct growth in various facets influences the magnetic properties. In chapter 6, a conscious effort has been made to fabricate the monoclinic M3Se4 compounds (where M can be Fe, Co, or Ni). The influence of transition metal (M) on the magnetic properties is investigated for monoclinic M3Se4 NPs. The Fe3Se4 is observed to be well-known ferrimagnetic with a Curie temperature of nearly 322 K. However, the other two compounds, Co3Se4 and Ni3Se4, are examined to be paramagnetic over the whole measurement temperature range (5 K to 300 K). Chapter 7 summarizes the work done in this thesis and makes recommendations for future study in many domains.