Low-temperature synthesis of bismuth and vanadium substituted LaPO₄ near-infrared reflective pigment and its aging resistance nature on PMMA

A new series of pigments having a chemical composition of La_1-xBi_x(P_1-yV_yO₄) [x = y = 0.02, 0.05, 0.1, 0.2, 0.5] were prepared via low-temperature hydrothermal synthesis. Phase identification (XRD), functional group analysis (FT-IR), optical properties studies (UV-Vis DRS), and morphological studies were carried out. CIE-L*a*b* parameter and near-infrared reflectance were calculated. The mismatch in the radius of doped metal ions switches the phase of the system from hexagonal to monoclinic. The band gap value lowered with an increase in dopant concentrations. For a co-doped LaPO₄ compound at x = y = 0.5 concentration, the bandgap significantly reduced to 2.11 eV from 5.37 eV (LaPO₄). This is due to the formation of a new hybrid orbital by the additional contributions of the Bi_6s orbital in the valence band and the V_5d orbital in the conduction band. Among the synthesized compounds, the most effective composition La_0.8Bi_0.2(P_0.8V_0.2O₄) having a b* value of ⁓38 with ⁓75% reflectance value was selected for stability and anti-aging studies. The shape of this composition is found to be nanorod using HR-TEM analysis. Chemical stability was tested using acid and alkaline medium, and thermal stability was evaluated by TG–DTA analysis. Aging resistance application had been evaluated for a selected composition with PMMA. The difference in the color parameter after heat treatment reflects that the pigment enhances the aging resistance properties of PMMA. The La_0.8Bi_0.2(P_0.8V_0.2O₄) yellow pigment has a greater potential to be used as a cool pigment for walls and roofs, and its anti-aging characteristic promises its application to extend the lifetime of plastic materials under hot circumstances.