Dynamic Bending of Carbodiimide Anions Derives Uniaxial Negative Thermal Expansion in ZnNCN

Abstract

Materials exhibiting negative thermal expansion (NTE), characterized by anomalous lattice contractions upon heating, have attracted considerable attention because of the unique attendant changes in crystal structure. A wide variety of compounds, including metal oxides, fluorides, and cyanides, show this phenomenon as a consequence of open-framework structures. However, compounds containing linear, triatomic dumbbell-like carbodiimide anion NCN2– have received less attention. Herein, we report that ZnNCN exhibits uniaxial NTE along the c-axis with a coefficient of −5.34(1) × 10–6 K–1 together with a coefficient of 4.12(3) × 10–6 K–1 along the a-axis at 300 K. Structural refinement using neutron diffraction data acquired at different temperatures indicates a contraction of the averaged N–N separation along the c-axis. Pair distribution functions (PDFs) generated from neutron total scattering data reveal that the ∠NCN angle decreases from 175° at 30 K to 160.5° at 673 K. Ab initio molecular dynamics calculations examining the local structure indicate an increased proportion of dynamical transverse bending of NCN2– anions, consistent with the PDF analysis. At higher temperatures, enhanced intramolecular bending of the anions is suggested to induce NTE along the c-axis in ZnNCN. Dynamical bending of molecular anions provides a design strategy for NTE materials, thereby expanding the scope of molecular-anion-based compounds.