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

Ternary rare-earth hydride oxides: stability in air and potential use as precursors for the synthesis of materials

Ternary rare-earth hydride oxides (or oxyhydrides) REHO show rather high thermal stability and inertness in air. SmHO remained intact when stored in air for 12 h, while after storage for one year, it completely hydrolysed to form Sm(OH)3. In contrast, YHO and HoHO show only slight decomposition upon longer storage. The cation’s basicity and the air humidity apparently are crucial factors in the air stability of the compounds. Their reactions with various gases were investigated, in order to better understand factors governing the stability in air and to map their potential as precursors in materials synthesis. Both SmHO and YHO reduce CO2 to carbon and form the metastable C-type rare-earth sesquioxides RE 2O3 instead of the thermodynamically stable B-type. YHO reacts with gaseous ammonia to a red powder. By X-ray diffraction, this is identified as yttrium nitride, but the color of the sample suggests it to be an oxygen-poor nitride oxide (oxynitride) phase YN1−x O x . These results underline the potential of rare-earth hydride oxides as precursors for the synthesis of other rare-earth compounds. The stability in air, even at elevated temperatures of some rare-earth hydride oxides such as YHO and HoHO are advantageous for potential applications as functional materials.

Expanding the hydride chemistry: Antiperovskites A3MO4H (A = Rb, Cs; M = Mo, W) introducing the transition oxometalate hydrides

The four compounds A₃MO₄H (A = Rb, Cs; M = Mo, W) are introduced as the first members of the new material class of the transition oxometalate hydrides. The compounds are accessible via a thermal synthesis route with carefully controlled conditions. Their crystal structures were solved by neutron diffraction of the deuterated analogues. Rb₃MoO₄D, Cs₃MoO₄D and Cs₃WO₄D crystallize in the antiperovskite-like K₃SO₄F-structure type, while Rb₃WO₄D adopts a different orthorhombic structure. ²H MAS NMR, Raman spectroscopy and elemental analysis prove the abundance of hydride ions next to oxometalate ions and experimental findings are supported by quantum chemical calculations. The tetragonal phases are direct and wide band gap semiconductors arising from hydride states, whereas Rb₃WO₄H shows a unique, peculiar valence band structure dominated by hydride states.

The synthesis, structures and electronic properties of the first four heteroanionic compounds containing both hydride and transition oxometalate ions are reported.

From SmOF to SmH0.78OF0.22: H/F Substitution in Oxide Fluorides as a Synthesis Route to Heteroanionic Compounds

Mixed anionic hydrides of the rare earths are a fascinating class of compounds as potential functional materials, especially in luminescence, as photochromic thin films and for ion conduction. For exploratory studies, the effectiveness of various synthesis methods must be investigated, which is done here for metathesis reactions. The reaction of Sm₂O₃ with PTFE yields SmOF (P2₁/c, a = 5.60133(19) Å, b = 5.65567(19) Å, c = 5.6282(2) Å, β = 90.169(5)°, V = 178.295(11) ų, and Z = 4) in a new, probably metastable, polymorph of the baddeleyite-type structure. Metathesis reactions of SmOF with LiH, NaH, or CaH₂ led to a samarium hydride oxide fluoride, SmH_xOF_1-x; i.e., incomplete H/F exchange occurs. X-ray diffraction and neutron diffraction on a compound with x = 0.78 obtained via NaH reveal hydride, oxide, and fluoride ions to be partially ordered. SmH_0.78OF_0.22 (Ia3̅, a = 10.947(2) Å, V = 1311.7(4) ų, Z = 32) crystallizes in an anti-Li₃AlN₂-type structure with distorted cubic anion coordination for samarium atoms (site symmetry 3̅ and 2) and distorted tetrahedral arrangement of samarium atoms around the anions (site symmetry 1 and 3). It is a fully structurally characterized hydride oxide fluoride and shows a rare crystal chemical feature─the occupation of a crystallographic site by three different anions (0.188 H + 0.667 O + 0.145 F). Interatomic distances between samarium and hydrogen and samarium and the mixed hydrogen/oxygen/fluorine site range from 2.45 to 2.48 Å and 2.29 to 2.42 Å, respectively, and are similar to those in samarium hydride, samarium oxide, and samarium fluoride. Fluoride extraction by reaction with alkali and alkaline earth hydrides has thus proven to be a useful synthesis route to hydride oxides and also hydride oxide halogenides, which might be further exploited in exploratory research on heteroanionic metal hydrides.

Computational Chemistry-Guided Syntheses and Crystal Structures of the Heavier Lanthanide Hydride Oxides DyHO, ErHO, and LuHO

Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x < 0 or x > 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions.