Local Structure Investigation of CsPbI₃ and Ba₃CuB′₂O₉ (B′ = Nb, Ta) Perovskites by X-Ray Absorption Spectroscopy

This PhD thesis investigates the local structural properties of ABX3 and A3BB’2X9 perovskite materials by means of X-Ray Absorption Spectroscopy, with the aim of elucidating the role of atomic-scale disorder, lattice distortions, and chemical heterogeneity in determining phase stability and functional behavior. Particular emphasis is placed on systems where conventional diffraction techniques provide only limited insight due to structural softness, short-range disorder, or cation mixing. The first chapter of this thesis will go through a historical and state of the art section to introduce the perovskite materials and the evolution of the research, together with a crystallographic section to introduces some of the key concept to understand the perovskite structure. The second part of the thesis establishes the theoretical and methodological framework of X-Ray Absorption Spectroscopy, covering both the near-edge (XANES) and extended (EXAFS) regions. The physical origin of the absorption process is discussed in terms of many-body effects, core-hole interactions, and single- and multiple-scattering mechanisms, whose correct treatment will be shown to be essential for obtaining physically meaningful structural parameters in complex materials. The experimental part focuses on two representative classes of perovskite systems. The first is the all-inorganic halide perovskite CsPbI3, a material of high relevance for optoelectronic applications but intrinsically unstable in its optically active black phases. XAS measurements at the Pb L3-edge reveal how different processing conditions affect the local structure during the δ → γ phase transition, focusing on the before-after transition comparing specifically the results for δ and γ phases. Further, the halide perovskite system is modified by adding Dimethylammonium iodide (DMAI) that stays as a ”dopant” in the perovskite. While oxygen exposure promotes a more homogeneous local environment through external stabilization mechanisms, the incorporation of DMAI is shown to induce an intrinsic stabilization by suppressing local structural disorder and regularizing the PbI6 octahedra, leading to a higher-symmetry and more stable perovskite phase. The second class of systems investigated includes the Cu-based double perovskites Ba3CuNb2O9 (BCNO) and Ba3CuTa2O9 (BCTO), where chemical disorder and Jahn–Teller distortions play a central role in shaping the magnetic ground state. By combining XAS measurements at the Cu and Nb/Ta K-edges, the local environments of the transition-metal cations are independently resolved. The analysis highlights the coexistence of strongly distorted CuO6 octahedra and more regular NbO6/TaO6 units, as well as the crucial role of multiple-scattering paths in revealing short-range chemical order invisible to average crystallographic probes. Overall, this work demonstrates the power of X-ray Absorption Spectroscopy as a unifying local probe to connect atomic-scale structure with macroscopic properties in perovskite materials, providing insights that are relevant for both fundamental understanding and materials design.