The AM2X2 (A = alkaline, alkaline earth or rare earth element M = transition metals, and X = pnictogen), 122 type ternary compounds acquire overwhelming attention due to their richness of the correlations and competition between the lattice, electronic, and magnetic degrees of freedom. Properly controlling those correlations can lead to a variety of novel ground states including different crystallographic structures, magnetically ordered states, or high-temperature superconductors. This thesis summarizes experimental work using high pressure and element substitution as tuning parameters in the study of selected 122 systems to understand their diverse structural and magnetic properties and interplay between the magnetism and electronic properties. This thesis contains 8 chapters and Chapter 1, and Chapter 2 present a general overview and introduction. The theoretical and experimental background are discussed in Chapter 3.

Chapters 4 and 5 are devoted to explaining the studies which are performed on SrCo2As2, the first compound discussed in this thesis. The collapsed tetragonal phase is induced with a modest applied pressure at low temperatures. This tetragonal to collapsed tetragonal structural transition is found to be temperature independent as it shows a steep structural transition phase line. The collapsed tetragonal phase may favor the occurrence of magnetism in Co based 122 compounds as observed in CaCo1.86As2. And SrCo2As2 compound already shows a magnetic instability as it displays stripe-type spin fluctuations below T < 5 K. Therefore, it encouraged searching for magnetism in the collapsed phase of SrCo2As2. The detailed investigation studies and results on this issue will be addressed in chapter 5 in this thesis.

Substitution of various atoms in all three sites are possible for 122 compounds, hence the system can be modified in numerous ways. The CaCo1.86As2 and the derived compounds by Fe substitution are the second system discussed in this thesis work. As discussed in Chapter 6, a smooth crossover from collapsed tetragonal phase of CaCo1.86As2 to tetragonal phase of CaFe2As2 is observed with Fe substitution in CaCo1.86As2. The suppression of low temperature A-type magnetic ordered phase is observed with Fe substitution and no new magnetic phase appeared with the suppression of A-type AFM ordering. Both the magnetic moment and the transition temperature are decreased monotonically. The results suggest that hole doping on CaCo1.86As2 show a less dramatic effect on the magnetism. The Chapter 7 summarize the results and discuss future work. Finally, Chapter 8 present a brief summary of some other projects that I collaborated, apart from the projects which I have discussed in this thesis.