Synthesis and structure of the ternary and quaternary strontium nitride halides, Sr₂N(X, X′) (X, X′=Cl, Br, I)

A number of new, layered nitride mixed halides have been synthesised in the quaternary phase systems Sr-N-Cl-Br and Sr-N-Br-I. The variation in structure with composition has been investigated by powder X-ray and powder neutron diffraction techniques and the structure of strontium nitride iodide, Sr ₂NI, has been determined for the first time (rhombohedral space group R-3m, a=4.0103(1)Å, c=23.1138(2)Å, Z=3). A continuous solid solution exists between Sr₂NCl and Sr₂NBr with intermediate compounds adopting the same anti-α-NaFeO₂ structure (rhombohedral space group R-3m) as the ternary end members. A similar smooth and linear relationship between structure and composition is seen from Sr₂NBr to Sr₂NI and hence cubic close packing of metal-nitrogen layers is adopted regardless of halide, X (X′). While nitride and halide anions occupy distinct crystallographic sites, there is no ordering of the halides in the quaternary materials irrespective of stoichiometry or temperature (between 3 and 673 K).