Abstract. Two different polymorphs of the metal nitride halides MNX (M ¼ Ti, Zr, Hf; X ¼ Cl, Br, I) are known to crystallize in layered structures. The two crystal structures differ in the way 2 1 {X[M 2 N 2 ]X} slabs are stacked along the c-axes. Metal atoms and/or organic molecules can be intercalated into the van-der-Waals gap between these layers. After such an electron-doping via intercalation the prototypic band insulators change into superconductors with moderate high critical temperatures T c up to 25.5 K. This review gathers information on synthesis routes, structural characteristics and properties of the prototypic nitride halides and the derivatives after electron-doping with a focus on superconductivity.

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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.

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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.