Effect Of KB（B=Mg,Mn） Double Ionic Substitution On The Negative Thermal Expansion Properties Of Sc₂（MO₄）₃（M=Mn,W）

Generally speaking,when a solid material is heated,the average kinetic energy of its atomic motion increases,as well as the average distance between atoms,which makes the macroscopic volume thermally expand.However,some materials have a special crystal structure that exhibits "contraction on heating and expansion on cooling",which is known as negative thermal expansion（NTE）.Sc2M3O12（M=W,Mo）is a typical material in the A2M3O12 family,which is attracting attention due to its stable NTE properties and wide range of response temperatures.However,its intrinsic NTE coefficient is very low and the synthetic raw material Sc2O3 is expensive.In this paper,we propose a double ion substitution strategy for Sc3M3O12（M=W,Mo）materials to modulate their NTE properties and reduce the proportion of Sc ions.The effects of KB（B=Mg,Mn）double ion substitution on the crystal structure,phase composition,morphology,microstructure,density,and macroscopic as well as microscopic NTE properties of KxBxSc2-xM3O12（B=Mg,Mn,M=Mo,W）series ceramic materials and the corresponding NTE mechanisms were investigated.The KxBxSc2-x（MO4）3（B=Mg,Mn;M=Mo,W）series materials were prepared by solidstate reactions using commercially available analytically pure K2CO3,MnO,MgO,Sc2O3,WO3,and MoO3 as source materials,and their solid-phase preparation processes were explored.Based on XRD,Raman spectra,SEM,XPS,HRTEM and TMA analyses,the results are summarized as follows:（1）The KB（B=Mg,Mn）double cation substitution method was used to modulate the Sc2（MoO4）3 material components and their NTE properties.The results indicate that the KB（B=Mg,Mn）double cation successfully substitutes Sc3+equivalently and the crystal structure changes from orthorhombic to hexagonal phase as the amount of double ion substitution increases.When x=0.25,orthorhombic Sc2（MoO4）3 and the hexagonal KxBxSc2 x（MoO4）3（B=Mg,Mn）coexisted.when x=0.5,orthorhombic Sc2（MoO4）3 completely transformed into the single phase hexagonal KxBxSc2-x（MoO4）3（B=Mg,Mn）.When x>1,the material reaches its solid solution limit and the impurity phase K2B2（MoO4）3（B=Mg,Mn）formed.Meanwhile,the amount of double ion substitution increases,the size of the KxBxSc2x（MoO4）3 grains gradually increases,the inter-grain pores gradually decrease and the densities increase.When 0≤x≤1.25,The KxBxSc2-x（MoO4）3（B=Mg,Mn）exhibited excellent NTE,and the double ion substitution played a role in enhancing the NTE properties of Sc2（MoO4）3.The average linear thermal expansion coefficient of the single orthorhombic phase Sc2（MoO4）3 is about-4.74 ×10-6℃-1（50-700℃）;while the average linear thermal expansion coefficients of the single hexagonal structures KxMgxSc2-x（MoO4）3（0.5≤x≤1）and KxMgxSc2x（MoO4）3（0.6≤x≤0.9）are higher than that of Sc2（MoO4）3.The average linear coefficients of thermal expansion of K0.5Mg0.5Sc1.5（MoO4）3 and K0.6Mn0.6Sc1.1（MoO4）3 were as high as8.39×10-6℃-1（50-700℃）and-13.98×10-6℃-1（50-700℃）respectively.The change in the crystal structure of the material shows that the Mg2+/Mn2+replaces the Sc3+ site during the double ion substitution process and that the difference in the atomic radius after the substitution leads to a slight distortion of the crystal structure.This distortion enhances the rotational coupling of the polyhedra in the crystal structure during temperature changes,which in turn enhances the negative thermal expansion behaviour of the material.（2）Modulation of Sc2（WO4）3 composition and NTE properties was carried out using KB（B=Mg,Mn）double ion substitution method.The results show that the KB（B=Mg,Mn）double ion successfully substituted Sc3+,and the crystal structure changed similarly to the Sc2（MoO4）3 series,from orthorhombic phase to hexagonal phase with the increase of the double ion substitution.When x=0.25,the material contains both the orthorhombic Sc2（WO4）3 and the hexagonal KxBxSc2-x（WO4）3（B=Mg,Mn）.when x=0.5,the material component is a single hexagonal KxBxSc2-x（WO4）3（B=Mg,Mn）.When x>1,the material reaches the solid solution limit and the impurity phase BWO4（B=Mg,Mn）appears.The increase in the amount of double ion substitution leads to a gradual increase in the size of the material grains,a gradual disappearance of inter-grain pores and an increase in the densities.At the same time,KxBxSc2x（WO4）3（B=Mg,Mn）exhibits excellent NTE properties within the double ion substitution ratios from 0 to 1.The NTE coefficient of KxBxSc2-x（WO4）3（B=Mg,Mn）tends to increase as the amount of double ion substitution increases.The average linear coefficient of thermal expansion of the single orthorhombic phase Sc2（WO4）3 is about-2.27×10-6℃-1（50-700℃）;the average linear coefficients of thermal expansion of the single hexagonal phase structures K0.5Mg0.5Sc1.5（WO4）3,K0.75Mg0.75Sc1.25（WO4）3 and K1Mg1Sc1（WO4）3 are-7.48×10-6℃-1（50700℃）,-9.95×10-6℃-1（50-700℃）and-14.06×10-6℃-1（50-700℃）,respectively.The average linear coefficients of thermal expansion of the single hexagonal phase structures of K0.5Mn0.5Sc1.5（WO4）3 and K0.75Mg0.75Sc1.25（WO4）3 were-8.83×10-6℃-1（50-700℃）and8.94×10-6℃-1（50-700℃）,respectively.The change in the coefficient of thermal expansion is related to the change in its crystal structure.The Mg2+/Mn2+ replaces the Sc3+site when double ion substitution occurs and the difference in its atomic radius causes a distortion in the crystal structure.The high distortion of the crystal structure due to the high substitution ratio is therefore the main reason for the enhanced negative thermal expansion behaviour of the material.