Excitation-dependent luminescence and emission-center identification in mixed-phase SrAl2O4/Sr4Al14O25:Eu,Dy probed by a hard X-ray nanoprobe

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Abstract
The emission of a luminescent material is governed not only by the centers it contains but by how they are excited, a distinction that becomes decisive when several emitting phases coexist. Eu- and Dy-doped strontium aluminates, the most widely used persistent-luminescence phosphors, are frequently multiphase, which complicates the assignment of their emission. Here, a mixed-phase Eu,Dy-doped strontium aluminate ceramic, with estimated phase fractions of 71.4% SrAl2O4 and 28.6% Sr4Al14O25 from powder X-ray diffraction, is characterized at the Taiwan Photon Source 23A hard X-ray nanoprobe by X-ray-excited optical luminescence (XEOL) spectroscopy and mapping, X-ray fluorescence (XRF) mapping, and X-ray absorption spectroscopy (XAS). The dominant spectral contributions depend on the excitation channel: under 9670 eV excitation the XEOL is dominated by a 493 nm Eu2+ band characteristic of the minority Sr4Al14O25 phase, whereas ultraviolet (UV)-excited photoluminescence (PL) is dominated by the 519 nm band characteristic of the majority SrAl2O4 host. Because broadband hard X-ray excitation is less site-selective than direct UV excitation, XEOL additionally reveals Eu3+ and Dy3+ lines scarcely visible in PL and sharp Cr3+-related emission near 692–694 nm, and XAS supports these oxidation-state assignments. Energy-dispersive XRF detected trace Mn, whose K-edge absorption profile was most consistent with Mn2+, although no Mn2+ emission was resolved. A weak violet band near 408 nm is tentatively assigned to an F-type color center, although a second-site Eu2+ contribution cannot be excluded. This approach resolves multiple emission features in a multiphase phosphor and demonstrates excitation-dependent spectral weighting consistent with different phase contributions.

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