Single crystals and phase pure samples of oxygen-poor ternary lanthanide oxide selenides with the composition M 10 OSe 14 (M = La-Nd; tetragonal, I4 1 /acd; a = 1592.0-1559.8 pm, c = 2106.5-2062.9 pm) could be obtained by reacting the corresponding metals, selenium and selenium dioxide as oxygen source.

The first two lanthanum selenide chlorides, LaSeCl and La3Se4Cl, both crystallize orthorhombically in space group Pnma (no. 62). They emerged from reactions of LaCl3 with lanthanum and selenium in appropriate molar ratios at 800–900 °C. Single crystals of the yellow LaSeCl adopt the cotunnite‐type structure of PbCl2 with a = 760.63(5), b = 433.52(3) and c = 910.07(6) pm (Z = 4). The tricapped trigonal prisms [LaSe5Cl4]11– share their triangular faces to form chains that run parallel to the [010] direction and are condensed through caps and common edges to give a complete crystal structure in accord with ∞3{LaSe5/5Cl4/4}. Dark‐red La3Se4Cl exhibits the U3S5‐type structure with a = 1271.98(5), b = 855.84(3) and c = 795.05(2) pm (Z = 4). The chloride anions are located at the Wyckoff position 8d together with selenium showing site occupation factors of 0.5 for both anion types. Bicapped trigonal prisms [LaSe7Cl]12– with (La2)3+ are connected through common faces and corners to form a three‐dimensional framework suited to the embedding of chains of fused tricapped trigonal prisms [LaSe5+1Cl2]11– with (La1)3+ running parallel to [010]. The absorption edge energy of approximately 1.75 eV indicates a wide band‐gap semiconductor that is stable towards air, water and some aqueous bases at different concentrations.

Synthesis, Crystal Structures and Properties of the First Lanthanum Selenide Chlorides, LaSeCl and La3Se4Cl. -Single crystals of LaSeCl and La3Se4Cl are prepared on a 100 mg scale from stoichiometric mixtures of La, LaCl3, and Se (evacuated silica ampules, 900°C, 4 d and 800°C, 20 d). The compounds crystallize in the orthorhombic space group Pnma with Z = 4 (single crystal XRD). LaSeCl adopts the cotunnite-type structure. It contains tricapped trigonal [LaSe 5 Cl 4 ] prisms sharing their triangular faces to form chains, which are condensed through caps and common edges. La3Se4Cl shows the U3S5-type structure. It contains bicapped trigonal [LaSe7Cl] prisms connected through common faces and corners to form a three-dimensional framework, which embeds fused tricapped trigonal [LaSe6Cl2] prisms. The compound is a wide band gap semiconductor. -(SCHURZ, C. M.; FRUNDER, S.; SCHLEID*, T.; Eur. J. Inorg. Chem. 2013, 16, 2923-2929, http://dx.