Fig. 1. Photoluminescence properties and thermal stability analysis.

a Diffuse reflectance (DR) spectra of La_4-xGeO₈:xBi³⁺ (LGO:xBi³⁺, x = 0, 0.007) samples; the inset presents the calculated optical bandgap values. b PLE and PL spectra of an LGO:0.007Bi³⁺ sample and commercially available yellow YAG:Ce³⁺ phosphor, which are monitored at the optimal wavelength. c The chromaticity coordinates of the representative LGO:0.007Bi³⁺ sample (λ_ex = 397 nm) and YAG:Ce³⁺ sample (λ_ex = 460 nm) in the CIE 1931 color space. d Photographs of LGO:0.007Bi³⁺ under natural light (left) and 365 nm n-UV light (right) radiation. e XPS spectra of LGO:xBi³⁺ (x = 0.01, 0.03) samples and a standard α-Bi₂O₃ sample, which is used for comparison. f The variation in the integrated emission intensity (λ_ex = 397 nm) of LGO:0.007Bi³⁺ vs the heating temperature (298–573 K); the inset presents the corresponding temperature-dependent PL spectra