Effect Of AMg（A=Li,Na,K,Rb） Double Cation Substitution On The Negative Thermal Expansion Performance Of In₂Mo₃O₁₂

The vast majority of materials expand in volume as the temperature rises,they show positive thermal expansion.However,there are very few materials that have the opposite property,contracted when heating and expanded when cooling,refer as to negative thermal expansion(NTE)materials.NTE materials have applications in aerospace,microelectronic devices,precision optical instruments and other fields.In recent years,A2M3O12 series of negative thermal expansion materials have attracted much attention since their discovery due to their excellent chemical flexibility at the A-position and their excellent negative thermal expansion properties.Among them,A2M3O12(A=Sc,Al,In,Fe,Cr;M=W,Mo)negative thermal expansion materials have the advantage of non-hygroscopicity,but there is a temperature-induced phase transition from monoclinic to orthorhombic structure in this type of materials.The phase transition occurs at 335℃,after which it presents excellent and stable negative thermal expansion properties,higher response temperature interval,high phase transition and low densities,and other drawbacks,which have certain limitations in its practical application.In this paper,In2Mo3O12 is studied and its properties are tuned using double cation equivalence substitution.AxMgxln2-xMo3O12(A=Li,Na,K,Rb)series ceramic materials were prepared by solid-phase reaction method using commercially available analytically pure A2CO3(A=Li,Na,K,Rb),MgO,In2O3 and MoO3 as raw materials.The effects of the introduction of(AMg)3+(A=Li,Na,K,Rb)ions on the phase composition,crystal structure,microscopic morphology and its thermal expansion properties of In2Mo3O12 were studied.The results of the research are as follows.(1)(LiMg)3+double cation co-doping regulates the material composition and thermal expansion behavior of In2Mo3O12.Mg2+ partially replaces In3+ to occupy the center of In2O3 octahedra,Li ions are located in the space by(ln2O3)/(MgO)octahedra and(MoO4)tetrahedra,and due to the small ionic radius of Li+,located in the channel,it has little effect on the(ln2O3)/(MgO)octahedra and(MoO4)tetrahedra.(LiMg)3+double cation co-doping at a doping ratio of 0.1≤x≤0.5,LixMgxln2-xMo3O12 forms a finite isomeric solid solution without changing the crystal structure of In2Mo3O12.The XRD pattern is consistent with In2Mo3O12,while the crystal morphology densities do not change significantly compared with In2Mo3O12.The thermal expansion curves of the samples first show positive thermal expansion,with a phase change point at about 340℃,and then show negative thermal expansion,also similar to In2Mo3O12(2)When the double cation was changed to(NaMg)3+,the crystal structure of In2Mo3O12 changed with the increasing doping ratio:when x=0.25,monoclinic In2Mo3O12 and hexagonal NaxMgxln2-xMo3O12 coexisted at this time,with the doping ratio x reached 0.5 and 0.75,NaxMgxln2-xMo3O12 are all hexagonal structure,and when the doping ratio x reached 1,the solid solution limit was reached and the MgMoO4 impurity appeared;and the Na,Mgxln2-xMo3O12 sample showed a significant grain growth and improved densities compared with In2Mo3O12.The(NaMg)3+ has an enhancing effect on the thermal expansion properties of In2Mo3O12,making the phase transition to disappear and widening the response temperature interval.In2Mo3O12 exhibits negative thermal expansion only from 380℃ to 700℃,with an average linear thermal expansion coefficient of-9.44×10-6℃-1.While the pure hexagonal Na0.5Mg0.5ln1.5Mo3O12 exhibits negative thermal expansion from room temperature to 700℃ with an average linear thermal expansion coefficient of-12.5×10-6℃-1.(3)When the double cation is replaced by(KMg)3+,the crystal structure of In2Mo3O12 changes again:when x=0.25,monoclinic In2Mo3O12 and monoclinic KxMgxln2-xMo3O12 coexist at this time,and with the increase of the doping ratio,when 0.5≤x≤1,the pure monoclinic KxMgxln2-xMo3O12 is presented.When the doping ratio of x further rises to 1.25 and 1.5,the sample reaches the solid solution limit and K2Mg2(MoO4)3 impurity appears.Meanwhile,compared with In2Mo3O12,the grain size of pure monoclinic KxMgxln2-xMo3O12 grows up,which promotes the expulsion of pores and enhances the densities to a certain extent.The pure monoclinic KxMgxln2-xMo3O12 exhibits excellent negative thermal expansion properties from room temperature to 700℃,and the phase transition disappears.Among them,K0.5Mg0.5ln1.5Mo3O12 has the most excellent performance with an average linear thermal expansion coefficient of-12.4×10-6℃-1.(4)When the double cation is replaced by(RbMg)3+,the change pattern of In2Mo3O12 crystal structure is similar to that of(KMg)3+.When x=0.25,the monoclinic In2Mo3O12 and monoclinic RbxMgxln2-xMo3O12 coexist at this time,and when 0.5≤x≤0.75,all are pure monoclinic RbxMgxln2-xMo3O12.as the ratio of x rises to 1,the solid solution limit is reached and MgMoO4 impurity appears.RbxMgxln2-xMo3O12(0.5≤x≤0.75)shows grain growth and increased densities.The pure monoclinic RbxMgxln2-xMo3O12 also exhibits excellent negative thermal expansion properties from room temperature to 700℃,with a broadening of the response temperature interval and disappearance of the phase transition.The average linear coefficient of thermal expansion of Rb0.75Mg0.75ln1.25Mo3O12 is-6.81×10-6℃-1.The different crystal structure variations of In2Mo3O12 described above are due to the combined effect of the introduction of(AMg)3+(A=Li,Na,K,Rb),while the different monovalent cation radic,which cause different degrees of crystal structure distortion,affect the rotational coupling of its polyhedra during temperature changes,which in turn enhances the negative thermal expansion behavior of the material.