From SmOF to SmHOF: 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 SmO with PTFE yields SmOF (2/, = 5.60133(19) Å, = 5.65567(19) Å, = 5.6282(2) Å, β = 90.169(5)°, = 178.295(11) Å, and = 4) in a new, probably metastable, polymorph of the baddeleyite-type structure. Metathesis reactions of SmOF with LiH, NaH, or CaH led to a samarium hydride oxide fluoride, SmHOF; i.e., incomplete H/F exchange occurs. X-ray diffraction and neutron diffraction on a compound with = 0.78 obtained via NaH reveal hydride, oxide, and fluoride ions to be partially ordered. SmHOF (3̅, = 10.947(2) Å, = 1311.7(4) Å, = 32) crystallizes in an anti-LiAlN-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.