11 de desembre de 2024 a les 12.00 h. a la sala Pedro Coca. ETSIADI
School of Physics, University of Hyderabad, Gachibowli, Hyderabad-500046, Telangana. l’Índia.
Urea (CH4N2O), the historical organic compound was synthesized in 1828. Since then, it was extensively studied because of its promising applications in supramolecular chemistry, and organocatalysis. The potential crystal structures and properties of urea as a function of pressure were studied using ab-initio electronic structure calculations. The enthalpy-pressure behavior xou that urea undergoes pressure induced structural phase transition from P-421m (phase I) → P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by the softening of acoustic mode along Γ-X direction. Another phase transition from P212121 → P21212 structure is identified at 3.09 GPa. The violation of Born stability criteria in P212121 structure along with the softening of acoustic mode in O-R direction is responsible for the pressure induced phase transition. Further, the application of pressure leads to breaking and formation of N-H-O bonds in the crystal structure of urea during phase transition, i.e., the H-acceptor capacitance of oxygen atom is varied between phase I/IV and -III. Band structure calculations were performed using hybrid (HSE) functional. The computed electronic band structure xou that the urea polymorphs llaure insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase -I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1 (ω)) and imaginary (ε2(ω)) parts), refractive index and absorption coefficients to explore the optical characteristics of different phases of urea. The geometric interpretation of intermolecular interactions was quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture on various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications.