HoHO: A Paramagnetic Air-Resistant Ionic Hydride with Ordered Anions

The substitution of hydrogen for oxygen atoms in metal oxides provides opportunities for influencing the solid-state properties. Such hydride oxides (or oxyhydrides) are potential functional materials and scarce. Here, we present the synthesis and characterization of holmium hydride oxide with the stoichiometric composition HoHO. It was prepared by the reaction of Ho2O3 with either HoH3 or CaH2 as a powder of light-yellow color in sunlight and pink color in artificial light (Alexandrite effect), which is commonly observed for ionic Ho(III) compounds. HoHO crystallizes with an ordered fluorite superstructure (F4̅3m, a = 5.27550(13) Å, half-Heusler LiAlSi type), as evidenced by powder X-ray and neutron powder diffraction on both hydride and deuteride and supported by quantum-mechanical calculations. HoHO is the first representative with considerable ionic bonding for this structure type. The thermal stability and inertness toward air are remarkably high for a hydride because it reacts only above 540 K to form Ho2O3. At 294(1) K and 25(3)% relative humidity, HoHO is stable for at least 3 months. HoHO is paramagnetic with μeff(Ho3+) = 10.41(2) μB without any sign of magnetic ordering down to 2 K.