Atomistic simulation of magnesia, alumina, spinels and yttria

Abstract

Magnesia, alumina, spinels and yttria are of considerable interest for their wide applications in various technological fields including electrochemistry, catalysis or microelectronics, aerospace, automotive, personnel protection, nuclear materials, chemicals, biomaterials, host material in rare-earth-doped lasers, etc [5-18, 36-39]. To increase the proper use of these minerals in various scientific applications it requires further understanding of the crystal structural, electronic, and optical properties in a systematic way. First-principles density functional theory (DFT) and molecular dynamics (MD) techniques have become popular as they allow us to retain an accurate description of electronic cohesion of the atoms/ions even including the dynamics associated with elevated temperatures [56, 57]. In this thesis first-principles DFT within the local density approximation (LDA) and generalized gradient approximation (GGA-PBE) was applied to study the crystal structural and electronic properties of MgO, Al2O3 (α-, θ-, and γ-Al2O3), MgAl2O4 and Y2O3. The electronic and optical properties of MgO and Al2O3 (α-, θ-, and γ-phases) are improved by state-of-art GW0 approach over DFT. For the γ-Al2O3, various existing models from the literature were examined. The distribution and interactions between cation and vacancies with all possible vacancy configurations were explored homogeneously. The present study concluded that the spinel type hexagonal γ-Al2O3 with Al vacancies at the octahedral sites is more stable rather than other models. The composition-dependent structure and properties of the Al2O3 rich spinels in MgAl2O4 to γ-Al2O3 solid solution have been studied in this thesis. A formula [Mg(1-x)Alx]Tet[Al(2-x/3)Vx/3]OhO4 (x = 0 to 1) was developed with distribution rules of cations, vacancies and anions in the defected structure in a systematic way. The structural chemistry and electronic properties of MgO and γ-Al2O3 surfaces were studied in a systematic way in this thesis. The present study showed good indication of reliability of the method to study for complex surfaces. The six different structures of Y2O3 (i.e., cubic, monoclinic, hexagonal, F-Er2S3-type monoclinic, α-Al2O3-type hexagonal, and fcc phases) have been discussed in detail with their chemical bonding and energetics. The calculations show that C-Y2O3 is the ground state phase. The overall results in this thesis will help to resolve many properties of magnesia, alumina, spinels and yttria regarding the interpretation of numerous applications in various aspects.