First-Principles Investigation of Excited-State Lattice Dynamics and Mechanical Properties in Diamond.

The study of the excited-state properties of diamond is crucial for understanding its electronic structure and surface physicochemical properties, providing theoretical support for its applications in optoelectronic devices, quantum technologies, and catalysis. This research employs Density Functional Theory (DFT) with the fixed electron occupation method to simulate the electron excitation. Using the Generalized Gradient Approximation (GGA) within DFT, we systematically investigated the excited-state characteristics of diamond by simulating the transfer of a fraction of electrons from the Highest Occupied Crystal Orbital (HOCO) to the Lowest Unoccupied Crystal Orbital (LUCO). Theoretical calculations indicate that with increasing electron excitation levels, the diamond crystal structure transitions from cubic to tetragonal, accompanied by a gradual decrease in the bandgap. Mechanical property analysis reveals that both Young's modulus and shear modulus decrease with increasing excitation rate, while the bulk modulus remains nearly constant. These findings indicate a significant impact of electronic excitation on the mechanical stability of diamond. Phonon dispersion curves exhibit reduced degeneracy in high-frequency optical branches and a marked decrease in crystal symmetry upon excitation. This study not only advances the understanding of diamond's excited-state properties but also offers valuable theoretical insights into its structural evolution and performance tuning under such extreme conditions.