Photoreduction of Sm³⁺ in Nanocrystalline BaFCl

Abstract

We demonstrate that exposure of nanocrystalline BaFCl:Sm(3+) X-ray storage phosphor to blue laser pulses with peak power densities on the order of 10 GW/cm(2) results in conversion of Sm(3+) to Sm(2+). This photoreduction is found to be strongly power-dependent with an initial fast rate, followed by a slower rate. The photoreduction appears to be orders of magnitude more efficient than that for previously reported systems, and it is estimated that up to 50% of the samarium ions can be photoreduced to the divalent state. The main mechanism is most likely based on multiphoton electron-hole creation, followed by subsequent trapping of the electrons in the conduction band at the Sm(3+) centers. Nanocrystalline BaFCl:Sm(3+) is an efficient photoluminescent X-ray storage phosphor with possible applications as dosimetry probes, and the present study shows for the first time that the power levels of the blue light have to be kept relatively low to avoid the generation of Sm(2+) in the readout process. A system comprising the BaFCl:Sm(3+) nanocrystallites embedded into a glass is also envisioned for 3D memory applications.

New Insights into the Influence of the 4f55d1 State in the 4f6 Electronic Configuration of Sm2+ in Crystal Hosts

“…Materials doped with Sm 2+ have gained a growing interest over the past decades in various applications, starting back to the conception of lasers in the 1960s. − They take advantage of specificities of the Sm 2+ electronic structure over other compounds. Its 5 D 0 – 7 F 0 transition is thereby nondegenerate, and in cases where it is observable (especially in C n and C nv compounds where it is symmetry-allowed), the number of lines experimentally detected directly indicates the number of crystallographic sites actually occupied by the ions. , The Sm 2+ luminescence being also pressure- and temperature-dependent, some doped crystals are used as sensors in a more extended and precise way than the historically used ruby. − Sm 2+ -activated matlockites have also been the subject of numerous studies about their photon-gated spectral hole-burning properties for optical data storage. − More recently, BaFCl:Sm 2+ has proved to be a stable and reliable X-ray and UV storage phosphor. − Its mechanism is based on the photoreduction of Sm 3+ upon X-ray or UV exposure and the subsequent strong 5 D J – 7 F J luminescence arising from the divalent ion . With the view of extending the range of applications for Sm 2+ -doped materials in the future, it appears necessary to gain more insight into its electronic structure.…”

In a Different Light: Deciphering Optical and X-ray Sensitization Mechanisms in an Expanded Palette of LaOCl Phosphors

“…The enhanced luminescence of Eu 3+ in the presence of Tb 3+ is explicable as per the Dieke diagram based on energy transfer from the latter to the former as illustrated in Figure . − , However, in stark contrast, fundamental understanding of activation channels, recombination pathways, and sensitization mechanisms upon high-energy excitation has not thus far been elucidated in this system. VUV and soft X-ray excitation near the elemental edges of the host lattice gives rise to high-energy electron–hole pairs that can thermalize to activate luminescent centers, which then decay through a variety of radiative and nonradiative recombination pathways. ,− Interestingly, while some authors have demonstrated Sm 3+ to Sm 2+ reduction in similar matlockite structures upon deep UV exposure, prolonged exposure of the LaOCl nanocrystals presented here to soft X-rays results in beam damage and nonspecific diminution of XEOL intensity without any discernible photoinduced reduction of Eu 3+ sites (which would be observed as an increase in intensity of Eu 2+ bands). , As noted above, exposure at a particular spot is limited to 10 ms in order to avoid beam damage from high-intensity synchrotron radiation.…”

“…23,46−48 Interestingly, while some authors have demonstrated Sm 3+ to Sm 2+ reduction in similar matlockite structures upon deep UV exposure, prolonged exposure of the LaOCl nanocrystals presented here to soft X-rays results in beam damage and nonspecific diminution of XEOL intensity without any discernible photoinduced reduction of Eu 3+ sites (which would be observed as an increase in intensity of Eu 2+ bands). 49,50 As noted above, exposure at a particular spot is limited to 10 ms in order to avoid beam damage from highintensity synchrotron radiation. Figure 6 shows La N 4,5 -edge (4d → 4f) XANES spectra acquired in FLY mode for solid-solution La 0.889 Eu 0.111 OCl, La 1−x−y Tb x Eu y OCl, and La 0.911 Tb 0.089 OCl phosphors in the range between 95−150 eV.…”

Photoluminescence of X-ray-Induced Divalent Tm Ions in BaLiF3:Tm3+ Nanocrystals

“…Rare-earth ion-doped alkaline-earth fluorohalides with the PbFCl (matlockite) structure (e.g., BaFCl and BaFBr) exhibit high storage efficiencies when subjected to X-irradiation and are widely used in computed radiography, especially for intraoral dental imaging . They also have considerable potential for applications in dosimetry and optical data storage. − Recently, BaLiF 3 has emerged as a promising alternative host material for storage phosphors, in addition to its applications as an ion conductor, a scintillator, , and a tunable ultraviolet laser. , The storage mechanism in alkaline-earth fluorohalides doped with rare-earth ions is reasonably well-understood. ,,− In contrast, fluoroperovskites (ABF 3 ) are less well-understood, and they remain the subject of an active area of research with some attention focused on BaLiF 3 . As such, for a better understanding and to realize potential practical implementations, the investigation of BaLiF 3 activated with a broad range of rare-earth elements is required.…”