On the Formation Mechanism of Chromium Nitrides: An in situ Study

Abstract

Abstract. 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 CrCl 3 , and (iii) the ammonolysis of chromium(III) sulfide Cr 2 S 3 . During the direct conversion of chromium first γ-CrN was formed above 385°C, above 567°C additionally β-Cr 2 N was detected. Finally, a complete conversion of γ-CrN to β-Cr 2 N was observed at 1080°C. The ammonolysis of chromium(III) compounds (CrCl 3 and Cr 2 S 3 ) revealed a so far in 3d metal nitride synthesis unprecedented redox se-

Synthesis of Metastable Co4N, Co3N, Co2N, and CoO0.74N0.24 from a Single Azide Precursor and Intermediates in CoBr2 Ammonolysis

“…In contrast to the pioneering investigations presented by Juza and Sachße, [4] during the ammonolysis of CoBr 2 no formation of Co 2 N was observed upon application of a low ammonia gas flow (100 ml min -1 ). Instead, the formation of hexagonal Co 3 N occurs, similar to the ammonolysis of FeCl 2 leading to ε-Fe 3 N. [30] Directly after the contact between ammonia and CoBr 2 a swelling of the starting material is observed, which is accompanied by a color change from green to blue and is caused by the exothermal formation of the hexaammoniate [Co(NH 3 ) 6 ]Br 2 . [43,44] The hexaammoniate can be recovered as a single-phase microcrystalline powder.…”

“…However, for the release of four molecules of NH 3 a mass loss of 21.2 % is expected. Deviations to lower measured values in such experiments are well known for the similar hexaammoniates [Mn(NH 3 ) 6 ]Cl 2 [49] and [Fe(NH 3 ) 6 ]Cl 2 [29,30] due to their low decomposition temperatures and thus partial ammonia loss during sample preparation. The loss of four molecules of NH 3 results in the formation of diammoniate Co(NH 3 ) 2 Br 2 , which crystallizes isotypically to Fe(NH 3 ) 2 Br 2 .…”

“…For the first two steps only signals for m/z = 15 (NH + ), 16 (NH 2 + ), and 17 (NH 3 + ) are observed in the mass spectra. In perfect accordance with the observation for FeCl 2 , [30] for CoBr 2 the evapo- [50] indicating an increasing reactivity during the formation of Co 3 N in the ammonolysis reaction. The diammoniate Co(NH 3 ) 2 Br 2 is synthesized in a controlled way by the reaction of CoBr 2 in flowing ammonia at 160°C and cooling under dynamic vacuum.…”

“…[27,28] In the ammonolysis of CoBr 2 an intermediate formation of ammoniates was observed; [4] however, no details on the composition or the structure of those ammoniates are given. Similar to the reaction behavior of FeCl 2 , [29,30] at higher temperatures a reformation of a very fine (often amorphous) powder of CoBr 2 occurs, enabling an enhanced formation of Co 2 N. [4,31] The direct formation of Co 2 N starting from elemental cobalt seems possible if a low partial pressure of hydrogen is ensured. [32] Additionally, a high-pressure-high-temperature synthesis of Co 2 N starting from cobalt and nitrogen was reported at 10 GPa and 1500°C.…”

Chemical Reactions followed by in situ Neutron Powder Diffraction

“…Similarly, the first step in the ammonolysis of CrCl 3 is a reduction to chromium(II) chloride CrCl 2 prior to re-oxidation and formation of γ-CrN. [107] Both reactions sequences were additionally confirmed by thermal analysis experiments in flowing ammonia atmosphere [107] while earlier ex situ observations of those reactions solely suggested a metathesis reaction mechanism.…”