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

Abstract

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 Tl${}_{2}$Ru${}_{2}$O${}_{7}$ and Hg${}_{2}$Ru${}_{2}$O${}_{7}$. 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 ($\ensuremath{\sim}$eV) energy scale, (b) the band-gap energies/charge gaps (${E}_{g}\ensuremath{\sim}160--200$ meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature ${T}_{\text{MIT}}$ ($\ensuremath{\sim}$10 meV), which is also the spin gap energy of Tl${}_{2}$Ru${}_{2}$O${}_{7}$ and the magnetic-ordering temperature of Hg${}_{2}$Ru${}_{2}$O${}_{7}$. The method is general for doping- and $T$-induced transitions and is valid for V${}_{2}$O${}_{3}$, CrN, La${}_{1\ensuremath{-}x}$Sr${}_{x}$MnO${}_{3}$, La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$, etc. The obtained transition-metal--ligand ($d--p$) bonding energies ($V\ensuremath{\sim}45$--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 (${V}^{*}\ensuremath{\sim}10$--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.