Compressibility studies of [NiAs]-derived CoNCN and Hf(NCN)2

For reasons of packing efficiency NCN2−-containing solid state compounds typically adopt [NaCl]- or [NiAs]-derived crystal structures, similar to those presented by their chalcogenide counterparts. However, the extended linear NCN2− anion distinguishes itself from its spherical cousins with its ability to tilt. This has motivated us to carry out a comparative study of two [NiAs]-derived metal carbodiimides that we believe will display quite distinct compressibilities. In CoNCN (P63/mmc), the aristotype of a growing family of [NiAs]-derived metal carbodiimides, Co2+ cations reside in all of the octahedral holes which dictates that the NCN2− moieties are directed strictly along the 6-fold axis. By contrast, in vacancy ordered Hf(NCN)2 only 50 % of the octahedral voids are occupied such that the NCN2− dianion tilts away from the stacking axis by 28° in a porous wine-rack like structure. Indeed, recent DFT studies have predicted that CoNCN should exhibit zero linear compressibility along the stacking axis, whilst Hf(NCN)2 is expected to present negative linear compressibility along the same direction. We therefore propose to study the compressibility of these phases on the PEARL beamline via PND measurements up to 6 GPa in a Paris-Edinburgh press with a view to potential sensor applications. Furthermore, Rietveld analysis of the accrued data will give mechanistic insights into the behavior of the NCN2− scaffold as a function of pressure.