Hydride ions (H-) in solvents are chemically active anions with strong electron-donating ability and are used as reducing agents in organic chemistry. Here, we evaluate the energy level of 1s-electrons in H- accommodated in solid lanthanum hydrides, LaHx (2 <= x <= 3), by photoemission (ultraviolet photoelectron and photoelectron yield spectros-copies) measurements and density functional theory calculations. We show that a very shallow valance band maximum with an ionization potential of 3.8 eV is attained in LaH3 and that the primary cause is attributed to the small electronegativity of hydrogen and the significant bonding-antibonding interaction between neighboring H(-)s with a close separation originating from the H-stuffed fluorite-related structure. These results encourage the challenge for p-type conduction in hydride semiconductors and provide a clue to the chemical understanding of polyhydride superconductors.
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