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 and ligand 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- (-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates TlRuO and HgRuO. Core-level spectroscopy shows drastic -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) electronic changes occur on the largest (eV) energy scale, (b) the band-gap energies/charge gaps ( meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature (10 meV), which is also the spin gap energy of TlRuO and the magnetic-ordering temperature of HgRuO. The method is general for doping- and -induced transitions and is valid for VO, CrN, LaSrMnO, LaSrCuO, etc. The obtained transition-metal–ligand () bonding energies (–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 (–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 and correlated compounds undergoing metal-insulator transitions.
2 More- Received 28 October 2012
DOI:https://doi.org/10.1103/PhysRevB.87.045108
©2013 American Physical Society