Étude ab_initio des propriétés structurales, élastiques et électroniques de l’oxyde de magnésium MgO

Abstract

This thesis is devoted to the systematic study of the structural, elastic, and electronic properties of magnesium oxide (MgO) under hydrostatic pressure. Ab initio calculations were performed using the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the WIEN2k code, based on density functional theory (DFT). The GGA-PBE approximations and the mBJ potential were used for exchange and correlation. The first chapter introduces general principles, the crystal structure, and applications of MgO. The second chapter details the theoretical foundations of DFT and the formalism of the FP-LAPW method. The third chapter presents and discusses the numerical results obtained. Structural analysis confirms that phase B1 (NaCl) is the most stable at ambient pressure (a0 = 4.258 Å, B0 = 148.3 GPa), in excellent agreement with experimental results, while the phase transition from structure B1 to B2 (CsCl) is predicted at approximately 511 GPa. Mechanically, the elastic constants C11, C12, and C44 increase under stress, confirming the stiffening of the material, and highlight a change in anisotropy at 20 GPa. Electronically, MgO exhibits a direct energy gap at 0 GPa. The GGA functional underestimates this gap (4.47 eV), while the mBJ correction significantly improves it (6.56 eV). Under pressure, this gap widens, and a transition to an indirect gap is predicted at approximately 195 GPa