Phonon dispersion and Gruneisen parameters of zinc dicyanide and cadmium dicyanide from first principles: Origin of negative thermal expansion.

Abstract

Zinc dicyanide, Zn(CN)2, and the isostructural cadmium dicyanide show neg. thermal expansion over a broad range of temps. By the use of d. functional theory calcns., we det. the phonon dispersion curves of these materials and show that the transverse acoustic modes give rise to a relatively sharp feature at about 2-4 meV in the phonon d. of states. In addn., we compute the Gruneisen parameters of these modes and show that they have large neg. values, indicating that they are the primary contributors to the neg. thermal expansion in this material. Our results are in broad agreement with recent neutron scattering expts. on Zn(CN)2 and the rigid unit mode anal. of these materials.