Biaxial negative thermal expansion in Zn[N(CN)2]2

Negative thermal expansion (NTE) has both scientific and application value as a novel physical behavior, but the types of NTE materials are still limited. In the present work, a framework structure material, metal dicyanamide Zn[N(CN)2]2, has been found to show biaxial negative thermal expansion in a wide temperature range (αa = −11.82 × 10−6 K−1, αb = −11.75 × 10−6 K−1 from 173 K–473 K). Through a joint study of variable temperature X-ray diffraction, extended X-ray absorption fine structure, Raman spectroscopy, and the maximum entropy method, the NTE mechanisms in Zn[N(CN)2]2 have been revealed. The pronounced transverse vibration of C atoms in the multiatomic bridge linkage –NC–N– and the rotation of ZnN4 tetrahedra play a key role in biaxial negative thermal expansion. This work provides a new class of potential framework structure NTE materials.