خواص الكتروني و ساختاري تركيبات f -الكتروني و پروسكايت‌هاي اكسيدي

This research aims to explore the structural and electronic characteristics of lanthanide and actinide compounds, and oxide perovskites. At the beginning of this research, we delved into the structural and electronic properties of the tetragonal structure of the lead titanate (PbTiO3) compound, a perovskite oxide that has garnered significant interest in the scientific community. Following our comprehensive calculations of its elastic properties, we made a noteworthy conclusion: the PbTiO3 compound, in its tetragonal form, exhibits a negative minimum Poisson’s ratio. Materials possessing a negative Poisson’s ratio are of great interest due to their unique mechanical properties, finding applications in various industries, notably aerospace and defense. Such materials exhibit the counterintuitive behavior of becoming wider when stretched and thinner when compressed, offering potential for innovative design and utility. The findings from our study of PbTiO3 thus hold considerable importance, potentially contributing to advancements in these technologically advanced sectors. To eva‎luate the elastic properties of these materials, we have utilized the IR-elast computational package. Our research reveals that the elastic properties of the tetragonal structure of the antiferromagnetic phase of UX2(X = P, As, Sb) compounds, as a representative of actinide compounds, have not yet been explored. Given the significance of these calculations, our research focuses on determining the elastic constants and their associated properties in the antiferromagnetic phase of UX2 compounds. This approach enables a comprehensive understanding of the mechanical behavior of these compounds, contributing valuable knowledge to the field of materials science. Despite the amount of research that exists in the study of the electronic properties of antiferromagnetic phase compounds UX2(X = P, As, Sb), we did not find a theoretical study that examines the electron properties of these compounds in their paramagnetic phase. To address this, our study initially examines these properties in the paramagnetic phase of UX2 compounds at room temperature. We employed a combination of Density Functional Theory (DFT) and Embedded Dynamical Mean Field Theory (eDMFT) for this analysis, utilizing WIEN2k and eDMFT codes for DFT and eDMFT sections, respectively. The study then extends to assess the impact of ligand atom types under zero pressure and the influence of hydrostatic pressure on the electron behavior in the 5f orbitals of uranium atoms in the UX2 compounds at room temperature. The final part of the research investigates the reasons behind variations in electron behavior in the 5f orbitals of uranium, considering both the substitution of X = P, As, Sb atoms at zero pressure and the effects of applied pressure. This is achieved by analyzing the fluctuations of valence electrons in the 5f orbitals of the UX2 compounds. At the end of this research, we also have demonstrated, through the use of DFT+SP+SOC+U methodology, that the cubic ferromagnetic GdAl2 compound, a lanthanide with a pure spin orbital 4f, exhibits unique characteristics. Firstly, this compound does not require re-optimization of the effective Hubbard parameter (Ueff) under varying pressure conditions. Secondly, its magnetic moment (MM) remains stable and is not influenced by changes in Ueff or hydrostatic pressure (P). Furthermore, our study finds that while the density of states and the peak-to-peak energy gap Upp between the valence and conduction density of states of the GdAl2 compound, like MM, are unaffected by changes in pressure, they are, in contrast to MM, sensitive to variations in Ueff. This indicates that GdAl2 maintains its magnetic moment and electronic structure robustly, showing less sensitivity to external pressures. This resilience is further supported by our calculations of the elastic constants and their related properties in this compound.

6