Illuminating the negative thermal expansion mechanism of YFe(CN)6 via electronic structure and unusual phonon modes

Abstract

Understanding the negative thermal expansion (NTE) mechanism remains an important and challenging thing. In this work, we selected the case of YFe(CN)6 to investigate the structure-mechanism relation on the base of crystal structure, electronic structure and lattice dynamics. We expanded the NTE of YFe(CN)6 to 150 K, and the temperature dependence of volume and lattice constants was determined by temperature-variable synchrotron X-ray diffraction measurements. A large NTE was found in the system. Our theoretical calculations indicate that the Y–N bond exhibits a strong ionic feature through the calculated electron localization function (ELF), which has a strong influence on the anisotropic vibration of the N atom. The detailed lattice dynamics simulations suggest that the NTE of YFe(CN)6 may be related to the presence of the unusual low-frequency modes of the YN6 triangular prism (tri-prism) units. The optical branches with low frequencies are mainly related to the distortion and twisting modes of the YN6 tri-prism units, which contribute most to the NTE effect in the crystal.