Topochemical Synthesis of Ca3CrN3H Involving a Rotational Structural Transformation for Catalytic Ammonia Synthesis

Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties. With these reactions, the overall crystal structure of the host material is generally maintained. Here we report a topochemical synthesis of a hexagonal nitride hydride, h-Ca3 CrN3 H, by heating an orthorhombic nitride, o-Ca3 CrN3 , under hydrogen at 673 K, accompanied by a rotational structural transformation. The hydrogen intercalation modifies the Ca-N rock-salt-like atomic packing in o-Ca3 CrN3 to a face-sharing octahedral chain in h-Ca3 CrN3 H, mimicking a "hinged tessellation" movement. In addition, the h-Ca3 CrN3 H exhibited stable ammonia synthesis activity when used as a catalyst.