. XRD patterns of the precursor powders synthesized at 200 ℃ for 12 h (K⁺/Na⁺=3:1) with different [OH^-] concentrations (a), XRD patterns of KNN:xEu (x=0.02, 0.04, 0.06, 0.08, 0.10) powders synthesized at 200 ℃ for 12 h (K⁺/Na⁺=3:1, [OH^-]=11 mol/L) (b) and zoomed XRD patterns from (b) within 30°-33° (c)

. SEM image of KNN:0.06Eu ceramics synthesized at 1140 ℃ for 4 h

. Excitaion (λ_em=618 nm) spectra of the KNN:0.06Eu ceramics and emission (λ_ex=465 nm) spectra of KNN:xEu ceramics at room temperature (a) and the dynamic decay curves on Eu³⁺ concentrations for KNN:Eu samples under 465 excitation (b)

. Reflectance spectra for the KNN:0.06Eu by UV light irradiation (0 s, 30 s, 60 s, 2 and 3 min) (a), difference absorption (Δabs) spectra for the KNN:0.06Eu by UV irradiation 3 min with inset showing photographs of color changes of ceramic before and after UV irradiation (b), the Δabs vs Eu concentration (c) and reflectance intensity changes (d) of KNN:0.06Eu by alternating UV irradiation and heat treatment

. Changes of emission spectra (λ_ex=465 nm) of KNN:0.06 Eu ceramics before and after UV irradiation for 3 min (a), luminescence switching ratio (ΔR_t) at 615 nm as a function of Eu concentration (b) and ΔR_t of KNN:0.06Eu ceramics by alternating UV light irradiation and thermal stimulus for 7 cycles (c)

. Luminescence modulation ratio (ΔR_t) as a function of irradiation time under different irradiation wavelengths (a) and schematic diagram of luminescence modulation upon photochromic reactions for KNN:xEu ceramics (V_O is oxygen vacancy, and V_A is K and Na vacancy) (b)