This work is motivated by the increasing concern on novel earth-abundant and low-cost semiconductors to meet the growing demand in industry, such as solar cell applications. Since Zn-IV-N2 (IV: Ge, Sn) are widely reported as earth-abundant semiconductors, which allow excellent bandgap energy tuning, these materials are being considered as promising candidates for solar cells. In addition to alloying the group IV elements, an interesting bandgap tuning mechanism through the cation disorder in these materials is postulated by many calculation studies. The cation disorder arises as an interesting question because cation in this material can have different arrangements that lead to two possible crystal structures. One is the wurtzite-type structure, in which cations are disordered, sharing one 2b Wyckoff site, and the other is the β-NaFeO2-type structure, in which cations are ordered in the structure, distributed on two different 4a Wyckoff sites. Change in the crystal structure based on cation disorder naturally affects the material properties essential for solar energy harvesting. Though numerous studies computationally predict the wurtzite-type structure and the β-NaFeO2-type structure for Zn-IV-N2 (IV: Ge, Sn), the compelling experimental validation of the cation disorder effect on the crystal structure transformation is still lacking. The well-crystallised bulks (signal crystal, powder) are ideal for the study of crystal structure and cation disorder based on diffraction methods. However, bulk Zn-IV-N2 are challenging to prepare, particularly the ZnSnN2 due to its shallow formation energy. Therefore, the synthesis of well-crystallised bulk material is one of the challenges in the experimental investigation of crystal structure and cation disorder. Allowing oxygen content in the nitrides (avoiding oxygen in nitrides is a challenge), one could achieve the synthesis of Zn1+xIV1-x(OxN1-x)2 (IV: Ge, Sn) powders, which can crystallise in the wurtzite-type structure and the β-NaFeO2-type structure, similar to Zn-IV-N2 (IV: Ge, Sn). Therefore, this work attempts to study Zn1+xIV1-x(OxN1-x)2 (IV: Ge, Sn) as the exemplary system to understand the similar cation disorder effect in Zn-IV-N2 (IV: Ge, Sn). This thesis optimises the synthesis method for Zn1+xGe1-x(OxN1-x)2. The nature of the reaction mechanisms is studied by correlating chemical composition obtained by X-ray fluorescence spectroscopy (XRF) and hot gas extraction method (HE) with reaction conditions, hence realising a targeted synthesis of phase-pure Zn1+xGe1-x(OxN1-x)2 powders with varying compositions. An evident transformation of crystal structure between the wurtzite-type structure and the β-NaFeO2-type structure is observed. The experimental investigation of cation disorder in Zn1+xGe1-x(OxN1-x)2 is achieved by using X-ray and neutron diffraction. Simultaneous Rietveld refinement was performed to characterise the site occupancy factor to understand the degree of cation disorder. Moreover, the bandgap tuning mechanism is investigated by correlating optical bandgap energy measured by the UV-Vis method, with chemical composition, crystal structure, and cation disorder studies. Furthermore, this thesis introduces a new synthesis method for Zn1+xSn1-x(OxN1-x)2 powder. This method achieves the synthesis of well-crystallised phase-pure Zn1+xSn1-x(OxN1-x)2 with a large composition range under ambient pressure, allowing the investigation of crystal structure and cation disorder based on PXRD. The crystal structure of Zn1+xSn1-x(OxN1-x)2 is evaluated by the Rietveld refinement of the XRD data. Moreover, the optical bandgap energy analysis was performed using the UV-Vis method. This work suggests an indirect band transition of the wurtzite-type ZnSnN2 by combing the crystal structure study and the bandgap energy analysis. Altogether, this work draws insight into the relationship among crystal structure, cation disorder and optical bandgap energy for Zn1+xIV1-x(OxN1-x)2 (IV: Ge, Sn). The results facilitate developing the high-efficient solar cell based on these novel materials in the future.
