Zero thermal expansion in metal-organic framework with imidazole dicarboxylate ligands

Exploring new abnormal thermal expansion materials is important to understand the nature of thermal expansion.Metal-organic framework(MOF)with unique structure flexibility is an ideal material to study the thermal expansion.This work adopts the high-resolution variable-temperature powder x-ray diffraction to investigate the structure and intrinsic thermal expansion in Sr-MOF([Sr(DMPhH_(2)IDC)_(2)]_n).It has the unique honeycomb structure with one-dimensional(1 D)channels along the c-axis direction,the a-b plane displays layer structure.The thermal expansion behavior has strong relationship with the structure,ZTE appears in the a-b plane and large PTE along the c-axis direction.The possible mechanism is that the a/b layers have enough space for the transverse thermal vibration of polydentate ligands,while along the c-axis direction is not.This work not only reports one interesting zero thermal expansion material,but also provides new understanding for thermal expansion mechanism from the perspective of the structural model.

A new isotropic negative thermal expansion material of CaSnF_(6)with facile and low-cost synthesis

Double ReO_(3)-type fluorides have a great interest in the field of negative thermal expansion(NTE)and luminescent materials.However,their application is limited by the scarcity of quantity,expensive raw materials,and harsh preparation conditions.In this work we have found a new NTE material,CaSnF_(6),by applying the concept of the average atomic volume.More importantly,different from the previous solid-phase sintering and direct fluorination methods,the nano CaSnF6 has been synthesized for the first time by solvothermal method.The results of X-ray diffraction(XRD)and Raman spectroscopy show that a phase transition occurs from rhombohedral(R3)to cubic(Fm3m)structure at about 200 K,and a strong isotropic NTE(αv=−15.78×10^(−6)K^(−1))appears in the cubic phase.Lattice dynamics calculations from first-principles illustrate that the NTE is due to the transverse vibration of fluorine atoms excited by low-frequency phonons.This work not only broadens the NTE family of fluorides,but also provides a new facile and low-cost fabricati on method for the preparation of NTE fluorides.

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.

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

Understanding the negative thermal expansion(NTE)mechanism is of great importance.In this work,we consider the new NTE compound Gd Fe(CN)_(6)(α_(v)=-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 Gd Fe(CN)_(6)comes from the strong tension effect induced by the transverse vibrations of the atomic-Fe-C≡N-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 GdN_(6)trigonal prisms in the framework structure of Gd Fe(CN)_(6).