Extremely Shallow Valence Band in Lanthanum Trihydride.

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, LaH x (2 ≤ x ≤ 3), by photoemission (ultraviolet photoelectron and photoelectron yield spectroscopies) 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 LaH 3 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.