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Ab‐initio Calculation of Yttrium Substitutional Impurities in alpha‐Al 2 O 3
Author(s) -
Ching WaiYim,
Xu YongNain,
Rühle Manfred
Publication year - 1997
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1997.tb03250.x
Subject(s) - yttrium , ab initio , atom (system on chip) , materials science , chemistry , atomic physics , condensed matter physics , crystallography , oxide , physics , organic chemistry , computer science , metallurgy , embedded system
Yttrium has a very low solubility in bulk α‐Al 2 O 3 and a great propensity for segregation to the grain boundaries in polycrystalline Al 2 O 3 . The segregation of yttrium influ‐ences many properties of the material. To understand the nature of the chemical bonding of yttrium within Al 2 O 3 , we have, as a first step, conducted an ab‐initio self‐consistent calculation for a yttrium impurity atom replacing an aluminum atom in Al 2 O 3 . We have used the first‐principles orthogonalized linear combination of atomic orbitals (OLCAO) method and a supercell with 120 atoms in the hexagonal lattice. The relaxation of the nearby atoms, be‐cause of the presence of yttrium, is studied via total energy calculation within the local density approximation. The nearest‐neighbor oxygen atoms of yttrium move outward by 8% and the next‐nearest‐neighbor aluminum atoms move inward by 5% of the respective separations in the undistorted lattice. A substitutional energy of 4.79 eV is obtained. The yttrium impurity introduces three defect states in the gap near the conduction band edge, thus natu‐rally explaining the “donor effect.” Two of the defect states are degenerate, and all three are derived mainly from the Y4d orbitals with very different local symmetries. The effec‐tive‐charge and bond‐order calculations show a substantial covalent bonding character between yttrium and the re‐laxed oxygen and aluminum atoms in the host.