Study On The Synthesis And Optical Properties Of Rare Earth Molybdates

Rare earth molybdate has the advantages of strong light absorption,corrosion resistance,low toxicity and high stability,which is widely used in many fields such as photoelectric information,indicator,color-changing sensor,photocatalytic degradation,catalytic hydrogen storage,scintillator,etc.It is an indispensable inorganic material in industrial chemistry.However,the tedious preparation process,complicated mechanism and uncontrollable performance make it not well studied in some fields of application.Thus,it is important to explore the application of rare earth molybdate in more fields.Rare earth molybdate materials with soft and pure colors,good color saturation and brightness,which enable them to have a broad development prospect as near-infrared reflective materials and thermochromic materials.In this paper,two rare earth molybdates were designed and synthesized.Their thermochromic properties and near-infrared reflectance properties were also investigated.The main contents are as follows.（1）Three structural Nd₂MoO₆samples were synthesized at different temperatures using high-temperature solid-state reactions.During the increase of synthesis temperature,the samples underwent phase transformation accompanied by significant color changes.The sample Nd₂MoO₆was a monoclinic phase with space group C12/c1 at 900℃.When the synthesis temperature was increased to 1000℃,the sample formed a cubic phase with a similar structure to Nd₆MoO₁₂with space group Fm-3m（225）,and at the synthesis temperature of 1350℃,the sample formed a high-temperature tetragonal phase with space group I41/acd.As the temperature increases,the symmetry of the crystal structure increases and the color changes from light blue to green to light purple.XRD,Raman spectroscopy and UV-Vis diffuse reflectance spectroscopy all confirm that the color change is attributed to a phase transition.（2）To further investigate the color properties of Nd₂MoO₆,the second part focuses on the monoclinic structure Nd₂MoO₆compound prepared at 900℃.It exhibits reversible thermochromic behavior in the temperature range of 30-300°C.The XRD characteristic peaks of the samples gradually move to lower diffraction angles as the temperature increases.The intensity of the absorption peaks at 480 nm and 592 nm of the diffuse reflectance spectra gradually increased and the color of the samples changed from light blue to green.The temperature decreases,the characteristic peak returns to its original position,and the color of the sample returns to its initial color.The shift of the XRD peaks suggests that the thermochromic phenomenon is directly related to the lattice expansion at high temperatures.We attribute the reversible color change to the expansion/contraction of the lattice.（3）In the third part,a new environmentally friendly inorganic yellow pigment Bi_2-xEr_xMoO₆was prepared by solid-state reaction method.The effects of Er³⁺on the structure,morphology and optical properties of Bi₂MoO₆were comprehensively investigated.As the concentration of Er³⁺increases,the yellow color of the pigment deepens,which can meet the demand for decorative effects.Meanwhile,a series of Bi_2-xEr_xMoO₆（x=0,x=0.2,x=0.4,x=0.6,x=0.8,x=1.0）samples have high NIR reflectance,all of them are higher than 88.1%.At x=0.6,the NIR reflectance and NIR solar reflectance are 106.1%and 99.6%,respectively.Under the irradiation of the infrared lamp,the bottom temperature of the aluminum plate coated with Bi_2-0.6Er_0.6MoO₆sample decreased by 3℃compared with the bottom temperature of the aluminum plate coated with titanium dioxide pigment.Therefore,the synthetic pigments exhibited better NIR reflection and thermal insulation properties.In the acid and alkali resistance test,the color differencesΔE*≤5 before and after the immersion of the pigment confirmed the chemical stability of the pigment.