Cation Site Ordering by Dissolution-Based Annealing during Cr2(NCN)3 Metathesis in a Molten Salt

Metal carbodiimides and cyanamides exhibit many opportunities for the discovery of materials with unique properties. For example, Cr2(NCN)3 is an insulating ferromagnet, which is a desirable and rare combination of properties. However, many of these materials must be made at moderate to low temperatures relative to traditional ceramic reactions, as primarily attributed to the decomposition of the (NCN)2- anion at elevated temperatures. Cr2(NCN)3 is typically made via salt metathesis between chromium and carbodiimide salts, facilitated by the presence of a molten alkali halide. We present how the reaction 2CrCl3 + 3Zn(NCN) takes place in eutectic LiCl-KCl from the analysis of in situ powder X-ray diffraction data. After complete dissolution of the precursors, the initial formation of Cr2(NCN)3 occurs with an imperfect registry of the metal layers. This is modeled by the inclusion of an additional Cr site and the reduced occupancy of the expected Cr position. The chromium site disorder orders over time and with a time scale consistent with a dynamic equilibrium of redissolution and regrowth rather than from solid-state diffusion. This provides insight into requirements for the synthesis and discovery of new carbodiimide materials.