Topochemical Synthesis of Ca 3 CrN 3 H 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-Ca 3 CrN 3 H, by heating an orthorhombic nitride, o-Ca 3 CrN 3 , 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-Ca 3 CrN 3 to a face-sharing octahedral chain in h-Ca 3 CrN 3 H, mimicking a "hinged tessellation" movement. In addition, the h-Ca 3 CrN 3 H exhibited stable ammonia synthesis activity when used as a catalyst.