Large negative thermal expansion in GdFe(CN)6 driven by unusual low-frequency modes

Abstract

Understanding the negative thermal expansion (NTE) mechanism is of great importance. In this work, we consider the new NTE compound GdFe(CN)6 (αv = −34.2×10-6 K-1) as a case study to investigate the NTE mechanism from the perspective of the lattice vibrational dynamics. The atomic mean-square displacements suggest that the NTE of GdFe(CN)6 comes from the strong tension effect induced by the transverse vibrations of the atomic –Fe–CN–Gd– linkages, with the largest contribution given by N atoms. Lattice dynamics calculations show that three low-frequency optical modes at about 50 cm-1 show the largest negative Grüneisen parameters thus providing the largest contribution to the NTE. The existence of these unusual low-frequency vibrational modes can be ascribed to the presence of GdN6 trigonal prisms in the framework structure of GdFe(CN)6.