Phase stability, electronic structure, and optical properties of indium oxide polytypes

Structural phase stability, electronic structure, optical properties, and high-pressure behavior of polytypes of In2 O3 in three space group symmetries I 21 3, Ia 3̄, and R 3̄ are studied by first-principles density-functional calculations. From structural optimization based on total energy calculations, lattice and positional parameters have been established, which are found to be in good agreement with the corresponding experimental data except for I 21 3, where the symmetry analysis for optimized structure indicates that it arrived at the Ia 3̄ phase. In2 O3 of space group symmetry Ia 3̄ is found to undergo a pressure-induced phase transition to the R 3̄ phase at pressures around 3.8 GPa. From the analysis of band structure coming out from the calculations within the local density and generalized gradient approximations, it is found that In2 O3 of space group symmetry I 21 3 and R 3̄ are indirect band gap semiconductors, while the other phase of space group Ia 3̄ is having direct band gap. The calculated carrier effective masses for all these three phases are compared with available experimental and theoretical values. From charge-density and electron localization function analysis, it is found that these phases have dominant ionic bonding with noticeable covalent interaction between indium and oxygen. The magnitudes of the absorption and reflection coefficients for In2 O3 with space groups Ia 3̄ and R 3̄ are small in the energy range 0-5 eV, indicating that these phases can be regarded and classified as transparent.