Origin of Luminescence in La₂MoO₆ and La₂Mo₂O₉ and Their Bi-Doped Variants

In this article, lanthanum molybdenum oxides (La₂MoO₆ and La₂Mo₂O₉) and their Bi-doped derivatives were investigated as potential rare-earth-free phosphors. An X-ray diffraction analysis coupled with an EDX study confirmed the purity of the samples and the insertion of bismuth in a 1 molar % amount. Kubelka-Munk-transformed reflectance spectra clearly indicated that the insertion of Bi induces a shortening of the optical gap in La₂MoO₆ but has no impact on that of La₂Mo₂O₉. Moreover, excitation and emission spectra evidenced a strong temperature quenching effect in all materials. Also, the CIEx,y parameters at 77 K are almost identical with or without Bi doping for the two host lattices. Clearly, it was shown, by combining experimental data, ab initio calculations, and the empirical positioning of absorption bands that the luminescence of the Bi-doped La₂MoO₆ sample is mainly related to the host lattice itself and distortions induced by La/Bi substitution. The role of the Bi³⁺ dopant is indirect, and the luminescence is mainly due to a Mo-O charge transfer rather than an on-site Bi³⁺ ³P₁,₀ → ¹S₀ transition. Concerning La₂Mo₂O₉, there is no effect following the insertion of Bi, implying that the role of Bi is insignificant.