Electronic Structure of Spinel-Type MgTi₂O₄: Valence Change at Surface and Effect of Fe Substitution for Mg

Bulk and surface electronic states of MgTi₂O₄ have been investigated by means of x-ray absorption spectroscopy (XAS), hard x-ray photoemission spectroscopy (HAXPES), and theoretical calculations. The Ti 2p XAS spectra exhibit multiplet structures derived from the bulk Ti³⁺ and surface Ti⁴⁺ features. In the surface sensitive O 1s XAS of the total electron yield mode, the magnitude of ligand field splitting is enhanced due to existence of surface Ti⁴⁺ species. The bulk-sensitive O 1s XAS spectrum taken in the total fluorescence yield mode is well reproduced by unoccupied part of O 2p density of states obtained by band structure calculations with on-site Coulomb interaction U for Ti 3d. In addition, occupied part of the calculated Ti 3d density of states agrees with the hard x-ray photoemission spectrum indicating Mottness of Ti 3d electrons. In Mg_1−xFe_xTi₂O₄, the bulk sensitive Fe 2p XAS is consistent with high spin Fe²⁺ indicating isovalent substitution for Mg while the surface sensitive Fe 2p XAS includes high spin Fe³⁺. The multiplet structures in the Ti 2p and Fe 2p XAS spectra show that the 3d electrons in the bulk Ti³⁺ and Fe²⁺ are basically localized. On the other hand, a low energy shoulder of the Ti 2p HAXPES peak, which can be assigned to Ti²⁺ or screening effect, suggests charge fluctuation due to proximity to the metal–insulator transition.