Cubic Fluorite-Type CaH2 with a Small Bandgap

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 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.

Aliovalent Calcium Doping of Yttrium Oxyhydride Thin Films and Implications for Photochromism

To develop an understanding of the photochromic effect in rare-earth metal oxyhydride thin films (REH_3-2x O _x , here RE = Y), we explore the aliovalent doping of the RE cation. We prepared Ca-doped yttrium oxyhydride thin films ((Ca _z Y_1-z )H _x O _y ) by reactive magnetron cosputtering with Ca doping concentrations between 0 and 36 at. %. All of the films are semiconductors with a constant optical band gap for Ca content below 15%, while the band gap expands for compositions above 15%. Ca doping affects the photochromic properties, resulting in (1) a lower photochromic contrast, likely due to a lower H^- concentration, and (2) a faster bleaching speed, caused by a higher pre-exponential factor. Overall, these results point to the importance of the H^- concentration for the formation of a "darkened" phase and the local rearrangement of these H^- for the kinetics of the process.

Solid solution for catalytic ammonia synthesis from nitrogen and hydrogen gases at 50 °C

The lack of efficient catalysts for ammonia synthesis from N₂ and H₂ gases at the lower temperature of ca. 50 °C has been a problem not only for the Haber–Bosch process, but also for ammonia production toward zero CO₂ emissions. Here, we report a new approach for low temperature ammonia synthesis that uses a stable electron-donating heterogeneous catalyst, cubic CaFH, a solid solution of CaF₂ and CaH₂ formed at low temperatures. The catalyst produced ammonia from N₂ and H₂ gases at 50 °C with an extremely small activation energy of 20 kJ mol⁻¹, which is less than half that for conventional catalysts reported. The catalytic performance can be attributed to the weak ionic bonds between Ca²⁺ and H⁻ ions in the solid solution and the facile release of hydrogen atoms from H⁻ sites.

Ammonia synthesis via the Haber–Bosch process typically takes place at an elevated temperature in order to achieve a reasonable rate. Here the authors report on a CaFH solid solution with low activation energy for catalytic ammonia synthesis at lower temperatures.