On the formation mechanism of chromium nitrides: An in situ study

Three different synthetic routes to chromium nitrides were investigated using in situ neutron diffraction (SPODI, FRM-II) and thermal analysis in ammonia: (i) The direct conversion of chromium powder in ammonia, (ii) the ammonolysis of chromium(III) chloride CrCl3, and (iii) the ammonolysis of chromium(III) sulfide Cr2S3. During the direct conversion of chromium first -CrN was formed above 385 degrees C, above 567 degrees C additionally -Cr2N was detected. Finally, a complete conversion of -CrN to -Cr2N was observed at 1080 degrees C. The ammonolysis of chromium(III) compounds (CrCl3 and Cr2S3) revealed a so far in 3d metal nitride synthesis unprecedented redox sequence: The compounds are reduced to chromium(II) chloride CrCl2 and chromium(II, III) sulfide Cr3S4, respectively, prior to the reoxidation of those intermediates to -CrN. In contrast to the ammonolysis of 3d metal(II) halides, for chromium(III) chloride no temporary formation of ammoniates was observed. Since formation of -CrN was detected only after annealing chromium(III) sulfide in flowing ammonia for 4 h to 5 h at maximum temperature, the formation of sufficient amounts of chromium(II, III) sulfide as the rate-determining step for the nitride formation appears likely.