Specific features of electronic structures and optical susceptibilities of molybdenum oxide

Orthorhombic molybdenum trioxide, alpha-MoO3, is comprehensively investigated using density function theory based on the all-electron full potential linear augmented plane wave (FPLAPW) method as implemented in the WIEN2k code within four types of exchange correlation potentials, namely, the local density approximation (CA-LDA), generalized gradient approximation (PBE-GGA), Engel-Vosko generalized gradient approximation (EVGGA) and the modified Becke-Johnson potential (mBJ). The conduction band minimum (CBM) is situated at Gamma point of Brillouin zone (BZ), whereas the valence band maximum (VBM) is located at T point of BZ. Calculation demonstrated that alpha-MoO3 is an indirect band gap insulator. The calculated electronic band structure and the total density of states confirm that mBJ brings the calculated energy band gap (2.81 eV) closer to the experimental one (3.03, 3.10 eV). The electronic space charge density distribution of alpha-MoO3 is explored in two crystallographic planes, namely, (0 0 1) and (1 0 1), to scrutinize the origin of chemical bonds. It is found that the majority of the charges are accumulated on the O site and the distribution of electronic charge is spherical. The optical properties are calculated for three tensor components along the polarization directions [1 0 0], [0 1 0] and [0 0 1] with respect to the crystalline axes. It is found that the regions confined between 6.5 and 8.0 eV and 10.0 and 13.5 eV are considered as lossless regions. The calculated optical properties support our observation from the calculated electronic band structure and the density of states, which shows that LDA, GGA and EVGGA underestimate the energy band gap, while mBJ succeeds by a large amount in bringing the calculated energy band gap closer to the experimental one.