Synthesis and thermoelastic properties of Zr(CN₂)₂ and Hf(CN₂)₂

Two binary transition metal cyanamides, Zr(CN2)2 and Hf(CN2)2, were prepared by solid-state metathesis (SSM) reactions and separately controlled by differential thermoanalysis (DTA). The crystal structure of Hf(CN2)2 was solved and refined from a single-phase crystal powder by X-ray diffraction (XRD) in the space group Pbcn. Zr(CN2)2 was characterized by isotypic indexing. The crystal structure of M(CN2)2 compounds with M = Zr, Hf is closely related to that of LiY(CN2)2, but reveals large cavities due to the absence of lithium ions. Hf(CN2)2 exhibits thermoelastic properties characteristic of a flexible framework material. The calculated phonon energies, elastic tensor, and thermal expansion tensor are presented; the volumetric coefficient of thermal expansion is predicted to be near-zero under ambient conditions (αV = -3.5 × 10-6 K-1).