Incommensurately Modulated Structures and Luminescence Properties of the AgxSm(2–x)/3WO4 (x = 0.286, 0.2) Scheelites as Thermographic Phosphors

Ag+ for Sm3+ substitution in the scheelite-type AgxSm(2–x)/3□(1–2x)/3WO4 tungstates has been investigated for its influence on the cation-vacancy ordering and luminescence properties. A solid state method was used to synthesize the x = 0.286 and x = 0.2 compounds, which exhibited (3 + 1)D incommensurately modulated structures in the transmission electron microscopy study. Their structures were refined using high resolution synchrotron powder X-ray diffraction data. Under near-ultraviolet light, both compounds show the characteristic emission lines for 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, and 11/2) transitions of the Sm3+ ions in the range 550–720 nm, with the J = 9/2 transition at the ∼648 nm region being dominant for all photoluminescence spectra. The intensities of the 4G5/2 → 6H9/2 and 4G5/2 → 6H7/2 bands have different temperature dependencies. The emission intensity ratios (R) for these bands vary reproducibly with temperature, allowing the use of these materials as thermographic phosphors.