MHSe: The First Hydride Selenides of the Lanthanides (M = LaNd; GdEr, Lu)

The oxidation of most of the lanthanide dihydrides MH 2 (M = LaNd; GdEr, Lu) with equimolar amounts of selenium results in the formation of the first lanthanide hydride selenides MHSe. The presence of alkali chlorides (e.g., NaCl or CsCl) as fluxes secures complete and fast reactions (7 d) at 700–850°C in sealed, arc‐welded tantalum capsules (protected by evacuated silica vessels) as well as single‐crystalline products (pale bluish‐gray hexagonal columns or platelets). Two different structures were determined from X‐ray single crystal data for the examples of 2HCeHSe (hexagonal, P6 3 /mmc (no. 194), Z = 2, a = 406.36(4), c = 794.81(9) pm, R1 = 0.0365, wR2 = 0.0766) and 1HHoHSe (hexagonal, P 6 m2 (no. 187), Z = 1, a = 381.56(3), c = 387.28(5) pm, R1 = 0.0140, wR2 = 0.0337). According to X‐ray powder data, the hydride selenides MHSe with M = LaNd proved to be isostructural with 2H‐CeHSe, those with M = GdEr and Lu crystallize isotypically with 1HHoHSe just like YHSe. Both structures contain hydrogen in half of the trigonal planar interstices within closest‐packed mono‐layers of the metals. These layers [(M 3+ )(H − ) 3/3 ] 2+ (a,β or b,α) are alternatively sheethed with closest‐packed mono‐layers of Se 2− (C) along [001], and only the stacking sequence decides whether a “stuffed” WC‐ (C(a,β)C  1 H‐MHSe) or a “stuffed” anti ‐NiAs‐type arrangement (C(a,β)C(b,α)C  2HMHSe) emerges.