Topochemical cross-linking of the [Hf2N2] layers with sulfur in α-HfNBr.

α-HfNBr (space group Pmmn) isostructural with FeOCl consists of orthorhombic [Hf2N2] layers sandwiched by bromide layers. By the reaction with Na2S at 750 °C in a sealed glass tube, the α-type [Hf2N2] layers are topochemically cross-linked with sulfur to form α-Hf2N2S with a mutual shift of the layers in the ab plane by (a/2 + b/2) to crystallize in space group Immm and the lattice parameters a = 4.1422(1), b = 3.5058(1), and c = 11.4043(3) A. At a higher reaction temperature of 850 °C, the β-type layered variant β-Hf2N2S with a double honeycomb network is obtained, which adopts the La2O2S structure with space group P3m1 and the lattice parameters a = 3.5805(1) and c = 6.4063(1) A.