This dissertation consists of eight chapters, which include experimental and theoretical studies of novel polar intermetallics and Zintl phases to understand the interrelationships among stoichiometry, structure and properties. Chapter 1 shows general introduction and motivation including a preview of projects. The main part of my doctoral research starts from Chapter 2, and four subsequent chapters are about the investigations of RE(M_1-xM’_x)₂ systems, where RE is rare-earth metals, and M and M’ are Group 12–14 elements. To gain insights into driving forces determining their crystal structures at the given compositions, I exploited two controllable factors one at a time which were the atomic size and the valence electron counts. In Chapter 2: ‘Planar versus Puckered Nets in the Polar Intermetallic Series EuGaTt (Tt = Si, Ge, Sn)’, only the atomic size was changed by substituting tetrels to see the impact of atomic size to the crystal and electronic structures. On the other hand, in Chapter 3 and 4: ‘To What Extent Does the Zintl-Klemm Formalism Work? The Eu(Zn_1—xGe_x) (0 ≤ x ≤ 1) Series’ and ‘Theoretical Interpretation of the Structural Transition along the Eu(Zn_1—xGe_x) (0 ≤ x ≤ 1) Series’, I attempted to reduce the atomic size factor and to focus on valence electron count for the series only by altering atomic compositions. In Chapter 5: ‘Structure-Composition Sensitivities in “Metallic” Zintl Phase: A study of Eu(Ga_1—xTt_x) (Tt = Si, Ge; 0 ≤ x ≤ 1) Series’, I exploited two controllable factors, which are atomic size and valence electron counts, simultaneously, and compared the results with two previously investigated systems. Other parts of my research are composed of Chapter 6 and 7, which are respectively, ‘On the “Coloring Problem” in YMgZn and Related Phase’ and ‘Phase Width and Site Preference in the EuMg_xGa_4—x Series’:. In these two chapters, I tried to understand the observed phase widths of certain structural types from the perspective of particular chemical bondings. General conclusion of my overall doctoral research is shown in Chapter 8.