On the Formation Mechanism of Chromium Nitrides: Anin situStudy

Three different synthetic routes to chromium nitrides were investigated usingin situneutron 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 γ‐CrN was formed above 385 °C, above 567 °C additionally β‐Cr2N was detected. Finally, a complete conversion of γ‐CrN to β‐Cr2N was observed at 1080 °C. The ammonolysis of chromium(III) compounds (CrCl3and Cr2S3) revealed a so far in 3d metal nitride synthesis unprecedented redox sequence: The compounds are reduced to chromium(II) chloride CrCl2and chromium(II, III) sulfide Cr3S4, respectively, prior to the reoxidation of those intermediates to γ‐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 γ‐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.