Synthesis And Mechanism Of Negative Thermal Expansion Non-oxides With Open-framework Structure

Negative thermal expansion（NTE）as an abnormal physical phenomenon has attracted a great attention in recent years,due to the potential practical application and complex physical mechanism.In this work,we focus on the NTE non-oxides which have open-framework structure.According to the studies on the local structure and lattice dynamics,we have achieved new insight to the NTE mechanism,find new NTE compounds,and explore the new direct way to control thermal expansion.The crystal structure and NTE in cubic GaFe（CN）₆ Prussian blue analogues（PBAs）were first reported.The Linear coefficient of thermal expansion（αl）is-3.95 × 10^-6K^-1 from 100 K to 475 K.The combined studies by synchrotron X-ray diffraction（SXRD）,X-ray total scattering,X-ray absorption fine structure（EXAFS）,neutron powder diffraction（NPD）,and density functional theory（DFT）calculations show that the NTE of GaFe（CN）₆ originates from the low-frequency phonons,which is directly related to the transverse vibrations of the atomic-Ga-N≡C-Fe-chains.Both the Ga-N and Fe-C chemical bonds are much softer to bend than to stretch.The transverse vibrational contribution to the NTE of GaFe（CN）₆ is dominated by N atoms not C atoms.The polyhedra of GaFe（CN）₆ are not rigid,which was,however,regarded as a necessary factor for the occurrence of NTE in those frameworks.The NTE mechanism can be vividly described by the "guitar-string" effect,which would be the common feature for the NTE property in many open-framework functional materials,such as PBAs,oxides,cyanides,metal-organic frameworks,and zeolites.This work proposes an effective method to control thermal expansion,in which guest ions（Na+）and molecules（H2O）can adjust the coefficient of thermal expansion from strong negative,to near zero,to positive in GaFe（CN）₆.Direct experimental evidence by high-resolution SXRD and EXAFS shows that the guest ions or molecules have intense dampening effect on the transverse vibrations of-Fe-C≡N-Ga-linkages responsible for NTE,especially for N atoms.The role of guests in controlling thermal expansion is attributed to the strong interaction of local environment-steric dampening.Electrochemical or redox intercalation of guest ions can be a general and effective method in controlling thermal expansionAfter the insertion of guest ions（K+）and molecules（H2O）,thermal expansion of YFe（CN）₆ can be largely switched from negative(αv=-33.67 × 10^-6 K^-1)to positive(αv = +42.72 × 10^-6K^-1).The mechanism was investigated both structure and lattice dynamics by joint studies using NPD,EXAFS,and DFT.The atom-volume-density descrease with the guest number increasing,corresponding the thermal expansion enhanced.It shows that the NTE is stronge related to the space of transverse vibrations of bridging atoms.There are strong ADPs of N and C atoms in YFe（CN）₆,while it is nearly identical in YFe（CN）₆.4H2O,due to the guest H2O molecules hindered.The mean square relative displacement（MSRDs）indicates that there is much larger transverse Fe-C vibrations in NTE YFe（CN）₆ than in PTE KYFe（CN）₆.Finally,most low-frequency modes with negative Gruneisen parameters in YFe（CN）₆ switch to positive when K+ions are inserted,thus resulting into PTE of KYFe（CN）₆.It is an effective method to control thermal expansion from positive to zero to negative by insertion of Li ions into the simple NTE framework material ScF3,doped with 10%Fe to enable reduction.The structure and composition of the lithiated Li0.06（Sc0.9Fe0.1）F3 was determined by joint studies of structure refinement based on NPD,STEM and X-ray absorption near edge structure（XANES）.The thermal expansion is related to the content of Li and Fe in the Lix（Sc0.9Fey）F3 which depend on annealing temperature.DFT calculation results show Li ions redistribute the fluorine vibrational motion locally,contributing to bond stretching thermal expansion.Li ions insertion weaken the anisotropic thermal vibration of fluorine ions,thus weaken the negative expansion coefficient.