Local vs Nonlocal States in FeTiO₃ Probed with 1s2pRIXS: Implications for Photochemistry

Abstract

Metal–metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron–titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core–hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

1s2p RIXS Calculations for 3d Transition Metal Ions in Octahedral Symmetry

“…In both cases, the excited 1s electron can go into either the t 2g orbitals or the e g orbitals. The drastic difference is that for 3d 5 , all spin-up orbitals are full in the total symmetric IRREP 6 A 1g (O h ).…”

“…The ground state 5 E g splits due to 3d SOC into several symmetries from which the term A 1g is the lowest in energy; hence, the ground state is 5 …”

“…The 1s2p RIXS plane of Mn(III)acac [43] is similar to the calculated plane in Figure 36. 5 Ground State System, for Example, Fe 3+ . In this part, we present the elementary differences for the case of an Fe 3+ ion (3d 5 ) (Mn 2+ analogue).…”

“…[44]. The tetrahedral and octahedral Fe 2+ oxides/minerals have been studied and reproduced by multiplet calculations [5,17,46]. Guo et al [47] studied low spin Fe 2+ cyanide complexes.…”

“…In the case of inversion symmetry, local 4p3d mixing is forbidden by symmetry, but a nonlocal mixing of the local 4p orbitals with the 3d orbitals of the neighbouring ligands (→nonlocal) can alter the pre-edge structure and produce additionally the so-called nonlocal peaks [3][4][5][6][7][8] (e.g., TiO 2 [3] and CrO 2 [9]). …”