Synthesis and optical properties of NIR pigments with high solar reflectance of monoclinic and tetragonal AMoO4 phase materials with different “A” elements

This study reports an analysis of the AMoO 4 series, which includes various "A" phases (Mg, Ca, Sr, and Ba) that often reveal fascinating structural and physical properties, making them significant for numerous applications. Different methods, including ball-milling, mechanical stirring, and co-precipitation, were employed to synthesize and stabilize these AMoO 4 phases. However, regardless of the synthesis method used, the monoclinic MgMoO 4 phase was not successfully obtained in the as-prepared state. The samples derived from the A = Ca, Sr, and Ba series, when initially prepared, predominantly crystallize in a tetragonal structure. This tetragonal structure can be either entirely pure, or it may contain one or two minor impurity phases. The subsequent annealing (650°C/3 h) of the samples that were ball-milled and mechanically stirred resulted in an improved formation of the pure monoclinic phase of MgMoO 4 , in contrast to the annealed co-precipitated product, which did not achieve the same outcome. The FTIR spectra provide evidence for the AMoO 4 structure, as indicated by the distinct bands corresponding to Mo-O and A-O bonds. The UV-Vis absorbance and NIR reflectance spectra of AMoO 4 series materials are affected by the synthesis techniques used, exhibiting a pronounced absorption edge λ max at 246 ∼278 nm, which corresponds to the charge transfer in the Mo-O species within the AMoO 4 structure. The calculated values of the energy gap (E g ) for the AMoO 4 series range from 3.69 to 4.22 eV. The monoclinic MgMoO 4 and tetragonal AMoO 4 (A = Ca, Sr, Ba) materials, synthesized via diverse methods, exhibit high solar near-infrared (NIR) reflectance. The solar NIR reflectance of scheelites (AMoO 4 ) varies depending on the specific "A" element present in the compound. The high NIR reflectance of AMoO 4 series materials makes them suitable for thermal insulation applications as cool pigments.