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

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,electro nic 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 synchrotro n X-ray diffraction measure ments.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 YN_(6)triangular prism(tri-prism)units.The optical branches with low frequencies are mainly related to the distortion and twisting modes of the YN_(6)tri-prism units,which contribute most to the NTE effect in the crystal.