Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN)2, studied by ab initio lattice dynamics

We report an ab initio calculation of crystal structure and lattice dynamics of cubic silicon dicarbodiimide, Si(NCN)₂, using density functional theory methods. The calculations reveal a low-energy spectrum of rigid unit modes that are shown to be associated with negative thermal expansion. Comparisons are drawn with the closely-related materials Zn(CN)₂ and the cubic-cristobalite phase of SiO₂. Instabilities in the spectrum of rigid unit modes point to the existence of disorder of the positions and orientations of the dicarbodiimide molecular anions.