First-principles study on Jahn-Teller effect in Fe2+-bearing wadsleyite

Abstract:

Electronic structure of Fe²⁺-bearing wadsleyite b-(Mg,Fe²⁺)SiO₄ has been investigated by first-principles calculations based on the density function theory. Crystal structure of wadsleyite is characterized by edge-sharing octahedra. The single chains of M1 and M2 octahedra running parallel to b-axis are cross-linked by the double columns of M3 octahedra parallel to a-axis. In the wadsleyite structure, Fe²⁺ cation orders preferentially into the M1 and M3 sites. Electron configuration analysis of the Fe²⁺ reveals that the d_xy, d_yz, d_zx orbitals (lower energy orbital t_2g) of Fe²⁺ are occupied by 0.430, 1.931, 1.923 electrons, while the d_x2-y2 and d_z2 orbitals (higher energy orbital e_g) are filled with 1.809 and 0.434 electrons, respectively. The asymmetric electron occupations of the t_2g and e_g can lead to the Jahn-Teller distortion around the octahedrally coordinated Fe²⁺. The molecular orbital calculation shows that the electron orbitals in the Fe²⁺-bearing wadsleyite are completely localized at the Fe3d orbitals and O2p orbital in the M3 sites, which causes the repulsive interactions between the Fe3d orbitals and O2p orbital. The strong repulsion is responsible for the Jahn-Teller distortion in the structure. In contrast, little localization of electron orbitals is observable in the Fe²⁺-free wadsleyite. Therefore, the presence of Fe²⁺ appears to destabilize the wadsleyite structure. This is the reason that the stability of wadsleyite is limited to the Mg-rich portion of the (Mg,Fe)₂SiO₄ binary system. Since coordination polyhedra of Fe³⁺ exhibit no Jahn–Teller distortions, on the other hand, a continuous substitution of Fe³⁺ can occur in wadsleyite via a coupled substitution of 2Fe³⁺ = Si⁴⁺ + Fe²⁺ without structural change, which suggests that the Jahn–Teller effect of Fe²⁺ is critically important for the phase stability of wadsleyite.