Quantifying covalency and metallicity in correlated compounds undergoing metal-insulator transitions

The tunability of bonding character in transition-metal compounds controls phase transitions and their fascinating properties such as high-temperature superconductivity, colossal magnetoresistance, spin-charge ordering, etc. However, separating out and quantifying the roles of covalency and metallicity derived from the same set of transition-metal d and ligand p electrons remains a fundamental challenge. In this study, we use bulk-sensitive photoelectron spectroscopy and configuration-interaction calculations for quantifying the covalency and metallicity in correlated compounds. The method is applied to study the first-order temperature- (T-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates Tl₂Ru₂O ₇ and Hg₂Ru₂O₇. Core-level spectroscopy shows drastic T-dependent modifications which are well explained by including ligand-screening and metallic-screening channels. The core-level metallic-origin features get quenched upon gap formation in valence band spectra, while ionic and covalent components remain intact across the MIT. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveal three energy scales: (a) 4d electronic changes occur on the largest (∼eV) energy scale, (b) the band-gap energies/charge gaps (E_g∼160-200 meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature T_MIT (∼10 meV), which is also the spin gap energy of Tl₂Ru₂O₇ and the magnetic-ordering temperature of Hg₂Ru₂O ₇. The method is general for doping- and T-induced transitions and is valid for V₂O₃, CrN, La_1-xSr _xMnO₃, La_2-xSr_xCuO₄, etc. The obtained transition-metal-ligand (d-p) bonding energies (V∼45-90 kcal/mol) are consistent with thermochemical data, and with energies of typical heteronuclear covalent bonds such as C-H, C-O, C-N, etc. In contrast, the metallic-screening energies of correlated compounds form a weaker class (V ^*∼10-40 kcal/mol) but are still stronger than van der Waals and hydrogen bonding. The results identify and quantify the roles of covalency and metallicity in 3d and 4d correlated compounds undergoing metal-insulator transitions.