Design And Mechanism Of Metal Cyanide And Fluoride Colossal Negative Thermal Expansion Materials

Most materials expand when heated,but some contract upon heating which are called negative thermal expansion(NTE).Because of their abnormal and intriguing properties,NTE materials have attracted considerable attention over the past 20 years.Such NTE materials provide the possibility in forming the composites with tailored thermal expansion even zero thermal expansion,which are useful for aerospace,precise mechanical or optical systems,such as fiber optics,coatings,electronics,mirror substrates to tooth fillings.In recent years,it has become one of the research hotspots in the fields of physics,chemistry and materials engineering.For the NTE materials,the most important characteristic parameters are the size of NTE coefficients and the range of NTE temperature zone.The wider the temperature range is,the wider the application range is.Here,first principles calculation combined with the quasiharmonic approximation theory is used to explore the NTE mechanism of typical wider framework structure of metal cyanides and fluorides,and design the materials with Colossal NTE coefficients.It mainly includes the following four parts:(1)The mechanism of MⅡ2MⅣ(CN)8(MⅡ=Ni,Co,Fe,Mn;MⅣ=Mo,W).Recently,growing interest has concentrated on the transition-metal cyanides and the Prussian blue materials(PBAs)with their unique linear diatomic bridge atoms(C≡N),which have relatively larger NTE coefficients because of their more flexible structures.It is found that the dimensionality of NTE is determined by the dimensionality of the geometric structure.For example,the cyanide MCN(M=Ag,Cu,Au)with 1D chain structure shows 1D NTE,Ni(CN)2 with 2D layered structure shows 2D NTE,and Zn(CN)2 is a 3D structure composed of tetrahedral units,showing 3D NTE.Ni2W(CN)8 is the first reported PBAs with(CN)8-based.It has a 2D and 3D topological structure and should have more abundant physical mechanisms.We demonstrate that the NTE properties is due to the synergistic effect of acoustic phonon and low-frequency optical phonon.More interestingly,when W atom is replaced by Mo atom,the NTE coefficient changes little,but if the Ni atom in MⅡ2W(CN)8 are replaced by Co,Fe or Mn atoms separately,the NTE coefficient increases with the increase of ion radius.Through the analysis of vibration modes which contribute a lot to the NTE,it is found that the frequency of these vibration modes decreases synchronously with the increase of ion radius.We call it the synergistic enhancement effect,which adjusts the NTE coefficient of MⅡ2MⅣ(CN)8(M=Ni,Co,Fe,Mn).(2)The abnormal doping effect in Lu1-xBixCo(CN)6.The LuCo(CN)6 is a typical framework compound possessing hexagonal symmetry(P63/mmc)owing to the combination of LuN6 trigonal prisms alternating with CoC6 octahedra.In the temperature range of 100～500 K,the NTE coefficient is-27.15×10-6 K-1.Based on phonon spectrum and Grüneisen parameters,it is found that the main vibration modes contributed to the NTE are rotating,rocking and antiprism rotating of LuN6 units,among which the antiprism rotation mode has the lowest frequency and the largest contribution to the NTE property.In general,doping could inhibit the NTE behavior and reduce the NTE coefficient.However,the doping of Bi atom in LuCo(CN)6 is abnormal.As the doping concentration increases,the NTE coefficient first increases and then decreases,showing a check mark form.First principles calculations show that Bi doping makes the LuN6 triangular prism structure twist,which is similar to the antiprism rotation mode of LuCo(CN)6.By calculating the torsional moment,it is found that the torsional residual stress controls the NTE of the doped system.(3)Design of ultra-large negative thermal expansion material MB(CN)4 based on the CN structure order in Zn(CN)2.Zn(CN)2 is a cyanide with simple structure and large NTE coefficient in a wide temperature range.Due to the small difference between the bond length of Z-C and Zn-N,there has been a debate on the ordered or disorder of CN structure and its origin of NTE.Here,the effect of the ordered or disorder of CN structure on the NTE is verified by first principles calculation.The results show that the ordered or disorder of CN has a little effect on the NTE of Zn(CN)2.The NTE mainly comes from the contribution of acoustic phonons and optical phonons.Based on the CN structure order,materials with MⅠMⅢ(CN)4 structure are designed by using equivalent substitution method,and materials with similar structure MB(CN)4(M=Ag,Cu,Li,Na,K)are found in experiment.Using first principles calculation,it is proved that the CN in these material is ordered,and the thermal expansion coefficient of NaB(CN)4 with simple cubic phase is-190×104 K-1,which is a colossal NTE material.By comparing the influence of CN order on the NTE coefficient in MB(CN)4(M=Ag,Cu,Li,Na)system,it is found that CN ordering can lead to greater NTE.(4)The NTE properties of 2D ScF3 ultrathin films.ScF3 is one of the few fluorides with cubic structure.It has a large NTE coefficient in a wide temperature range(the average linear expansion coefficient at 10～110 K is α1≈-1.1×10-5 K-1).Because of its simple structure and large NTE coefficient,it provides a good example for the study of NTE.The NTE coefficient can be adjusted by substituting doping,inserting ions into the gap,constructing bimetallic fluorides and preparing nanoparticles.However,there is still a lack of theoretical explanation for the low dimensional case.First principles calculations show that the stable structure of ScF3 ultrathin film is F atom as the terminated atom.For ScF3 ultrathin films with different thickness,the NTE coefficient changes little.Through the analysis of Griineisen parameters and spectrum,it is demonstrated that the NTE mainly comes from the membrane vibration,the rotation of ScF6 octahedral unit in the surface layer and in the inner layer.With the increase of the layer thickness,the membrane vibration decreases,and the rotation of the octahedron units in surface layer and inner increases.The common contribution of the three makes the NTE coefficient change little with the increase of the layer thickness.It provides theoretical guidance for the preparation of ScF3 ultrathin films.