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 (alpha(V) = -3.5 x 10(-6) K-1).