Synthesis And Thermochromic Properties Of Rare Earth Molybdate Ln₆MoO₁₂（Ln=La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu）

Thermochromic materials have the ability to change optical properties in response to temperature stimulation,which have been used in many fields,such as intelligent temperature control coatings,temperature sensors,laser warning,color clothing and temperature indicators.Thermochromic materials are currently classified into inorganic,organic and liquid crystal categories.Compared with the other two types of thermochromic materials,inorganic thermochromic materials have the advantages of high temperature resistance,acid and alkali resistance,light resistance and easy preparation.Currently,there are fewer types of inorganic thermochromic materials and their development is slow,which plays a great limitation to the application in complex scenarios.It is crucial to develop new environmentally friendly inorganic thermochromic materials.Rare earth molybdate Ln₆MoO₁₂ has attracted extensive attention from researchers due to its non-toxic,stable physical and chemical properties and good optical properties.The rich f-f transition of rare earth cations in the visible region of Ln₆MoO₁₂ gives it a variety of colors,and its properties can be regulated by the rare earth ion radius,sintering temperature and stoichiometric ratio.In this paper,a series of rare earth molybdate Ln₆MoO₁₂ thermochromic materials were synthesized by regulating reaction temperature and element proportion.The thermochromic properties of the materials were regulated by controlled doping of the lattice sites of rare earth and Mo.The specific research content and results are as follows:1.Synthesis,structural regulation and thermochromic properties of rare earth molybdate Ln₆MoO₁₂（Ln=La,Nd,Sm,Eu,Gd,Dy,Ho,Er,Tm,Yb,Lu）.All samples were synthesized by sol-gel method at temperatures of 800-1400°C.All Ln₆MoO₁₂samples can be cubic phases of fluorite structure（space group:Fm-3m）,for Ln=（Tm,Yb,Lu）,the synthesis result is cubic phase below 1000°C,and when the temperature is higher than 1000°C,the final product is rhombohedral phase（space group:R-3）.All samples exhibited reversible thermochromic properties in the range of RT to 500°C,and rhombohedral phase showed higher color difference before and after color change than cubic phase samples with the same chemical composition.The in-situ detection of samples by temperature-dependent XRD spectra,temperature-dependent Raman spectra and temperature-dependent UV-Vis-NIR spectra showed that the thermochromic mechanism of this series of materials was charge transfer absorption of O^2-→Mo⁶⁺caused by lattice expansion,charge transfer of defective energy levels caused by oxygen vacancies,and f-f transition of heat-activated rare earth ions.2.Dy_6-xNd_xMoO₁₂（x=0.5,1.0,1.5,2.0,2.5,3.0）were successfully synthesized using cubic phase Dy₆MoO₁₂ as the base material and Nd³⁺as the dopant.The f-f transition of Nd³⁺in the visible light region was used to change the initial color of Dy₆MoO₁₂,and the color-changing effect of the brightness L value decreased after the color change of Dy₆MoO₁₂ itself was superimposed to obtain the thermochromic properties from Light straw green（L=75,a=-9,b=30）to dark khaki（L=63,a=1,b=39）,and the material achieved a color gamut leap from the perspective of providing f-f transition.3.Rhombohedral phase Yb₆MoO₁₂ was selected as the control object,and Yb₆W_xMo_1-xO₁₂（x=0.2,0.4,0.6,0.8,1.0）were successfully synthesized by doping W⁶⁺to control structure and thermochromic properties at Mo site.After W⁶⁺enters the lattice,the color of the sample changes significantly,from bright cream to white gradually.With the increase of W⁶⁺doping amount,the material band gap gradually increases.The mechanism of material discoloration and the relationship between ion regulation and material band gap can be further understood by UV-Vis-NIR spectroscopy.