Negative thermal expansion in LnCo(CN)6 (Ln=La, Pr, Sm, Ho, Lu, Y): mechanisms and compositional trends

Abstract

Negative thermal expansion (NTE) is a comparatively rare phenomenon that is found in a growing number of materials.1 The discovery of new NTE materials and the elucidation of mechanisms underpinning their behavior is important both in extending the field and enabling tailored thermal expansion properties. NTE has been found throughout a broad family of cyanide coordination frameworks,2 arising from thermal population of low-energy transverse vibrations of the cyanide bridges, which reduce the average metal–metal distances, and thus the lattice parameters, with increasing temperature. More complex mechanisms have been established in metal–organic framework materials, in which both local and long-range modes contribute to NTE.3 The low-energy dynamics of metal-based materials are often modeled in terms of rigid unit modes (RUMs), wherein the metal-centered polyhedra are treated as rigid, with only the linkage being flexible. © 2013, Wiley-Vch Verlag GmbH & Co.