Synthesis, Crystal Structures and Properties of the First Lanthanum Selenide Chlorides, LaSeCl and La3Se4Cl

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. The crystal structures of the first lanthanum selenide chlorides, LaSeCl and La3Se4Cl, are described in terms of condensed bi‐ and tricapped trigonal prisms [LaSenClm]11/12– (n = 5, 5+1, 7; m = 1, 2, 4). A phase‐pure sample of La3Se4Cl displays an optical band gap of 1.75 eV. Both show high stability towards air, water and inorganic bases, but decompose upon contact with acids.