Lead-free halide double perovskites (elpasolites) have emerged as promising alternatives to lead halide perovskites due to the toxicity of lead for various optoelectronic applications, including photodetectors, light-emitting diodes, photocatalysts, and spintronic devices.

This research focuses on fully inorganic chloro-complex compounds with the general formula Cs₂Ag_xNa_1-xInCl₆ (x=0…1) as host materials doped with transition metal ions V³⁺ or Cr³⁺. These systems are of particular interest due to their broadband near-infrared luminescence properties and potential applications in optoelectronic devices. Additionally, this work investigates isostructural transition metal-based double perovskite compounds Cs₂(Ag/Na)FeCl₆ that exhibit antiferromagnetic ordering, making them promising candidates for spintronics and quantum information technology. To probe the atomistic structure of these materials, this thesis employs multiple advanced characterization techniques. Synchrotron- based X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), extended X-ray absorption fine structure (EXAFS), solid-state nuclear magnetic resonance (ssNMR), electron paramagnetic resonance (EPR), and optical spectroscopy were used to examine the dispersion, oxidation state, and local symmetry of dopants and host constituents at the nanoscale. The findings reveal that transition metal ion incorporation may deviate from ideal substitution, presenting challenges for certain applications while simultaneously enabling novel functionalities.

Overall, this work contributes to the fundamental understanding of structure-property relationships in lead-free halide double perovskites and establish a foundation for the rational design of next-generation optoelectronic and spintronic materials with enhanced performance and reduced environmental impact.