Synthesis And Mechanism Of New Framework Structure Negative Thermal Expansion Compounds

Negative thermal expansion(NTE)is a counterintuitive thermophysical behavior,which is correlated with the coupled interaction among phonon,charge and crystal structure.The study of NTE can not only uncovers the origin of thermal expansion,but also promotes their application in the frontier fields such as thermal expansion control.In the present work,we focus on the new NTE materials with framework structure,involving Prussian blue analogues and oxides.The NTE mechanisms of them were invetigated from the aspect of crystal structure and lattice dynamic.The thermal expansion control of Cu2P2O7 and ?-Cu2V2O7 has been achieved by chemical substitution.A series of Prussian blue analogues MFe(CN)6(M=Fe,In,Sc,Ru)have been synthesized by coprecipitation method,and all of them display NTE in a wide temperature range.The sample of FeFe(CN)6 with perfect cubic morphology was selected to carry on the study of NTE mechanism.The NTE mechanism of FeFe(CN)6 has been revraled by combined analysis of synchrotron X-ray diffraction(SXRD),X-ray pair distribution function(PDF),and X-ray absorption fine structure(EXAFS).It has been found that the "ture" bond lengths of Fe-C and Fe-N obtained by EXAFS expand with increasing temperature while the "apparent"bond lengths shrink obtained by SXRD,indicating transeverse vibrations in Fe-C and Fe-N.By the calculation of atomic mean-square relative displacements(MSRDs),it has been confirmed that the transeverse vibrations of N atoms dominate the NTE behavior of FeFe(CN)6.The present results prove directly that the transverse thermal vibrations of C and N atoms are crucial for the occurrence of NTE of Prussian blue analogues.A simple and low-cost Cu2P2O7 has been found to exhibit the strongest NTE among the oxides(?v?-27.69×10-6 K-1,5K-375K).Cu2P2O7 is stable and nonhygroscopic in air.The complex NTE mechanism has been investigated by combined methods of high resolution SXRD,neutron powder diffraction(NPD),PDF,EXAFS and density functional theory calculations.Interesting,the direct experimental evidence reveals that the coupling twist and rotation of PO4 and CuO5 polyhedra are the inherent factors for the NTE nature of Cu2P2O7,which are triggered by the transverse vibrations of oxygen atoms O1 and 04,as the linkages of neighbouring PO4 polyhedra and PO4 and CuO5 polyhedra respectively.An ??? phase transition happens at around 375K,after which the transverse vibrations of O atoms are restricted strictly and NTE disappeared.The present new NTE material of Cu2P2O7 also has been verified the value for the practical application.The thermal expansion of Cu2P2O7 has been effectively controlled by chemical substitution.When substituted Cu by Zn,the NTE and the NTE operated temperature range are supperessed with increasing content of Zn.The sample shows near zero thermal expansion in Cu0.5Zn1.5P2O7(?v?-2.78 × 10-6 K-1,100-225K).When substituted P by V,the NTE operated temperature range is enlarged to higher temperature with incrasing content of V.Especailly in Cu2PVO7,the NTE temperature range is elevated by 273K compared with Cu2P2O7,and keep large NTE with ?v?-26 × 10-6 K-1 in the temperature range of 100-648K.In Cu2P0.5V1.5O7,the sample retains NTE in 100-750K.The transverse vibrations of O atoms bonded with V derive the coupled rotation of polyhedra leading to NTE,and the the more electrovalent bond in NTE temperature range facilates the distortion of polyhedra to cooperate the polyhedra rotaion.The same thermal expansion control method by chemical substitution has been applied in ?-Cu2-xZnxV2O7(x=0,0.1,0.2)solid solutions,in which the volumetric coefficients of thermal expansion are successful tuned from-10.19×10-6 K-1 to-1.58×10-6 K-1 in the temperature range of 100-475 K by increasing content of Zn2+.It has been revealed that the transverse vibrations of oxygen bonded with vanadium are dominant the contraction in the be plane leading to the overall volume NTE in?-Cu2V2O7.The introduction of Zn2+ densifies the crystal structure,which is presumed to suppress the space of transverse vibrations and results in the ZTE in?-Cu1.8Zn0.2V2O7.This work presents an effective method to realize ZTE in anisotropic framework systems.