Topotactic Denitriding of Antiperovskite Compounds to Synthesize Novel Magnetic Intermetallics

Atomic disorder can be detrimental to the functional properties of intermetallic compounds. For example, in Heusler half metals, anti-site defects decrease the fraction of mobile polarized electron spins. Unfortunately, disorder can be difficult to avoid in materials with metallic bonding, where there is often only a small energetic difference between many atomic configurations. On the other hand, in ternary compounds with two metals and an anion, such as antiperovskite nitrides, it can be much more energetically costly to adopt metal site disorder. If order on the metallic sublattice is maintained during anion removal, antiperovskite nitrides could serve as precursors to new ordered intermetallic phases that can, (1) not be made using conventional approaches, and/or (2) be difficult to prepare without atomic disorder.<br/>Inspired by this idea, we have prepared several antiperovskite nitrides using conventional solid-state chemistry techniques, then topotactically removed the N to transform the antiperovskites into magnetic intermetallic phases. We focus on the transformation of Mn₃GeN to Mn₃Ge, which is achieved by reducing the precursor powder in flowing Ar gas (800 °C for 12 hours). Using synchrotron and neutron scattering, we track the effects of denitriding. Subtle structural distortions related to rotations of NMn₆ octahedra disappear when partially denitrided. The fully denitrited Mn₃Ge phase then undergoes an additional slight structural rearrangement but overall, the metal positions are almost entirely “locked” into their antiperovskite positions. While structural distortions are subtle a change in magnetic order is observed by emergent magnetic Bragg peaks in neutron diffraction and further confirmed by SQUID magnetometry which shows that while Mn₃GeN is magnetically soft, Mn₃Ge has a wide magnetic hysteresis loop and coercive field of ~1T at 300 K. In addition to the Mn₃GeN→Mn₃Ge transformation, we discuss the generality of topotactically denitriding antiperoskites using precursors of Mn₃GaN and Mn₃CuN powders, as well as thin films of Mn₃GeN and Co₃PdN. Overall, this work highlights an exciting approach for the synthesis of many novel ordered intermetallic phases.