Preparation and single-crystal structure analysis of Sr2NF

Samples of Sr2NF were prepared and analyzed via single-crystal X-ray diffraction. Two distinct colors (viz. brownish-yellow (BY) and dark red (DR)) of crystalline Sr2NF samples were discovered and found to be isostructural, with slightly different cell dimensions. The structure is essentially that of rocksalt, but with a doubled cubic unit-cell resulting from ordering of N and F atoms. The presence of F interstitials were also detected, and refined as a Frenkel defect. For the BY crystal, the X-ray analysis gave a cell parameter of a=10.6920 (45) ?, and for the DR, a=10.7655(20) ?. Both crystal types have space group Fd-3m (No. 227), with Z=16. X-ray data were collected on a Bruker SMART APEX 4k CCD Single Crystal Diffractometer. Structure solutions were obtained via direct methods, and X-ray refinements were carried out by full-matrix least squares on F2 on all data, to give R1=0.0308 (all data) and wR2=0.0665 for 10 parameters and 96 independent reflections for the BY sample, and R1=0.0404 (all data) and wR2=0.0750 for 10 parameters and 98 independent reflections for the DR sample. The final position assignments were analyzed via bond valence sum calculations.