Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN), studied bylattice dynamics.

We report ancalculation of crystal structure and lattice dynamics of cubic silicon dicarbodiimide, Si(NCN), 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)and the cubic-cristobalite phase of SiO. Instabilities in the spectrum of rigid unit modes point to the existence of disorder of the positions and orientations of the dicarbodiimide molecular anions.