DESIGN, SYNTHESIS, AND PROPERTIES OF INTERMETALLIC COMPOUNDS

Abstract

Heuslers’, which are one of the most desired classes of multifunctional intermetallic materials, are potentially expected to find their utility in almost every aspect of our life. They are usually defined as ordered alloy of several elements, amongst which at least one of them is a non-metal. They are often loaded with a multitude of wide chemical/physical/mechanical properties, which can also be tailored as per requirements. Till now, they have mainly been studied in the bulk role; however, due to the limited number of studies, their behavior at nano-dimension is still a matter of guess. This thesis aims to design and synthesize ternary intermetallic compounds (with prime focus on Heusler compounds) through a wet chemical approach and presents a comprehensive study of nano-dimensional Heusler compound particles. The thesis starts with a report on the detailed study of Ni3Sn (a DO19 structure type compound) nanoparticle synthesized by the wet-chemical approach. A comprehensive study of the structural properties of the samples synthesized under different conditions is discussed in detail. Playing with surface chemistry, the current study also resulted in the synthesis of highly crystalline Ni3Sn nanoparticles with very different surface morphology. While moving toward Heuslers via doping of third element (3d-transition metal element from Mn to Cu), structural phase stability, along with their catalytic property, was thoroughly scanned. It was observed that the journey from DO19 structure type to Heusler (L21) was possible only with Cu doping, where Ni2CuSn nanoparticles were successfully synthesized with a very unusual raspberry shaped hollow morphology. To the best of our knowledge, preparation of Ni2CuSn has never been reported by chemical methods; here, the wet-chemical doping approach has done the job and justifies the importance of the wet-chemical approach in the context of Heuslers.Moving ahead, this thesis further talks about another Heusler class, namely, ‘half-Heuslers’, and summarises the results related to the experimental and theoretical investigation of an earlier known compound, CoFeSn, which have been revisited by us to further investigate its properties. Here, samples were synthesized by both physical and wet-chemical approaches, and various characterizations were performed to understand their true nature. Motivated by the available literature and our computational studies, we investigated the possibility of polytypism and found clear evidence of polytypism (with two phases, namely, hexagonal and cubic), which indicated that the emerging phase is very much dependent on the synthetic approach employed. Magnetic properties of prepared samples were found to be consistent with the computational studies. Using DFT calculations, a thorough investigation of the cubic phase suggests it to be of a half-metallic character whose robustness is discussed in much detail. Moreover, a pressure-induced study suggested the stability of both phases under the tuned environment. Overall, our study showed that CoFeSn could be prepared in both phases under certain specific conditions. In a nutshell, this thesis provides enough compelling evidence that under certain conditions, the wet-chemical approach can be used for the synthesis of both classes of Heusler compounds and has certainly added new dimensions to the fundamental understanding of Heusler compounds at nano-dimensions.