Negative Thermal Expansion in LnCo(CN)₆ (Ln=La, Pr, Sm, Ho, Lu, Y): Mechanisms and Compositional Trends

Abstract

RUM with a twist: The magnitude of negative thermal expansion (NTE) in the LnCo(CN)6 coordination frameworks increases with Ln ion radius rLn. The framework structure contains an unusual locally unstable trigonal prismatic LnN6 unit that participates in an NTE-contributing vibrational mode by twisting about its axis at low energies. This contrasts with the rigid unit modes (RUMs) prevalent in other systems.