Co-substitution And Modification Of Thermal Expansion In （AMg）_xSc_2-x（MoO₄）₃ （A=Li,Na,K,Rb）

It is a common understanding that solid materials expand on heating.However,several series of materials have been found to show an abnormal thermal expansion property,namely negative thermal expansion(NTE).In A2M3O12 family,Sc2(MoO4)3 shows the best NTE performance because it doesn’t absorb moisture in air and its phase transition temperature is as low as-93℃.However,the strength of Sc2(MoO4)3 ceramics is low due to its low density.Source material SC2O3 is an expansive and the magnitude of CTE of Sc2(MoO4)3 is smaller compared with other NTE materials in A2M3O12 family.Therefore,it is worthwhile to explore a new negative thermal expansion material with stronger negative thermal expansion coefficient,higher densities,and lower synthesis costs.In this study,the crystal structure,morphology and thermal expansion properties of Sc2Mo3O12 were investigated by using[(Li/Na/K/Rb)Mg]3+double cations as equivalent substitutes for Sc3+.(AMg)xSc2-xMo3O12(A=Li,Na,K,Rb)ceramics were synthesized via co-precipitation process after sintering at 1000℃ for 6 h using LiNO3(99.95%),Na2CO3(99.95%),K2CO3(99.95%),Rb2CO3(99.95%),Mg(NO3)2(99.95%),Sc(NO3)3·6H2O(99.99%)and(NH4)6Mo7O24·4H2O(99.99%)as source materials,respectively.The crystal structure of the samples was analyzed by X-ray diffractometer(XRD)combined with Raman;SEM and HRTEM were used to analyze the morphology and elemental distribution of the samples;Elemental valence states were analyzed using XPS;the macroscopic thermal expansion properties of the samples were analyzed by TMA.The results of the study are showed:(1)The LiMg3+ doping into Sc2Mo3O12 forms a finite isomorphic solid solution,and the crystal structure of the solid solution remains orthorhombic;When x≥0.75,the sample reaches its solid solution limit and the impurity MgMoO4 appears;(LiMg)xSc2-x(MoO4)3 consists of uniform-sized grains with higher densities than SC2Mo3O12 ceramics.With the increase of(LiMg)3+,the negative thermal expansion property of Sc2Mo3O12 gradually decreases,and among(LiMg)xSc2-x(MoO4)3 ceramics,(LiMg)0.4Sc1.6(MoO4)3 reaches its CTE of 0.25 × 10-6 ℃-1 in a wide temperature range of 30-700℃.(2)With the doping of(Na/K)Mg3+,the orthorhombic Sc2Mo3O12 change into the hexagonal[(Na/K)Mg]xSc2-x(MoO4)3;when x≤0.25,the orthorhombic SC2Mo3O12 is not completely become the hexagonal[(Na/K)Mg]xSc2-x(MoO4)3,the orthorhombic SC2Mo3O12 and the hexagonal[(Na/K)Mg]xSc2-x(MoO4)3 coexist in the sample;For the case where 0.25<x≤0.75,we have the following,the sample is the unique hexagonal[(Na/K)Mg]xSc2-x(MoO4)3.when x>0.75,(NaMg)xSc2-x(MoO4)3 reaches the limit of its solid solution and the impurity MgMoO4 is observed to be present.When x of KMg3+>1,(KMg)xSc2-x(MoO4)3 reaches its solid solution limit and impurity K2Mg2Mo3O12 appears.The doping of(Na/K)Mg3+makes the grain size gradually increase and promotes the pore discharge,and the densities of SC2Mo3O12 are significantly improved;With the increase of(Na/K)Mg3+,the negative thermal expansion performance of Sc2(MoO4)3 becomes more and more strong.Among the(NaMg)xSc2-x(MoO4)3 ceramics,(NaMg)0.5Sc1.5(MoO4)3 has the most excellent thermal expansion performance,and its CTE reaches-10.59×10-6℃-1 in the range of 30-700 ℃;Among the(KMg)xSc2-x(MoO4)3 ceramics,KMgSc(MoO4)3 has the most excellent thermal expansion performance,reaching a CTE of-9.10×10-6℃-1 over a wide temperature interval of 30-700℃.(3)With the doping of RbMg3+,the orthorhombic SC2Mo3O12 change into the monoclinic(RbMg)xSc2-x(MoO4)3;when x≤0.25,the orthorhombic SC2Mo3O12 is not completely become the monoclinic(RbMg)xSc2-x(MoO4)3,orthorhombic SC2Mo3O12 and the monoclinic(RbMg)xSc2-x(MoO4)3 coexist in the sample;When 0.25<x:5≤0.75,the sample is is the single monoclinic(RbMg)xSc2-x(MoO4)3;when x>0.75,(RbMg)xSc2-x(MoO4)3 reaches its solid solution limit and impurity Rb2Mg2Mo3O12 appears.The coefficient of thermal expansion of Sc2(MoO4)3 becomes more negative with the increase of RbMg3+.Among the(RbMg)xSc2-x(MoO4)3 ceramics,(RbMg)0.75Sc1.25(MoO4)3 has the most excellent thermal expansion performance,and its CTE reaches-6.53 × 10-6 ℃-1 in a wide temperature interval of 30-700℃.