Nitrogen Transfer between Solid Phases in the System Mn-N Detected via in situ Neutron Diffraction

The thermochemical properties of -Mn6N5+x and -Mn2N1-w were investigated by thermal analysis in inert and oxidizing atmospheres. The obtained phase transformation temperatures were used to design an in situ neutron diffraction experiment on the reaction of -Mn and flowing NH3. During this in situ experiment, initially a nitrogen-poor anti-perovskite type phase epsilon-Mn4Ny [0.70(3) y 0.93(3)] was formed, indicating the lowest required nitridation potential necessary for formation of this phase within the binary manganese nitrides. At slightly higher temperatures the - and -type superstructured polymorphs of -Mn2N1-w were observed. In the same temperature region the formation of -Mn3N2 [up to 26(2) wt%] at ambient pressure conditions was noticed. For the first time, a direct evidence for the high-temperature transformation of -Mn3N2 was obtained. At 928 K the nuclear and magnetic superstructures simultaneously collapse and a cubic MnNx phase [0.56(6) x 0.70(3)] with defect rock salt structure was generated. Clear indications for the preferred transfer of N between the different above mentioned solid state nitride phases rather than direct release to the surrounding atmosphere during temperature increase were achieved from the compositions of the coexisting phases. An updated phase diagram is suggested.