Controlled Generation of Tm2+ Ions in Nanocrystalline BaFCl:Tm3+ by X-ray Irradiation

Luminescence of nanocrystalline BaFCl codoped with Eu2+/3+ and Tb3

BaFCl:Eu,Tb nanocrystals were synthesized via hot-injection thermolysis of metal chlorodifluoroacetates and their luminescence response characterized between 80 and 430 K. Though unintended, partial reduction of Eu3+ to Eu2+ occurred during the synthesis stage, leading to luminescent nanocrystals coactivated by Eu2+, Eu3+, and Tb3+. Their emission was dominated by Eu2+, specifically by radiative transitions 4f7 (6P7/2) → 4f7 (8S7/2) and 4f65d1 (lowest level) → 4f7 (8S7/2). Owing to the thermal coupling between 4f7 (6P7/2) and 4f65d1 (lowest) levels, line-like emission from this manifold at 80 K evolved into band-like emission at 430 K. The change in the shape of the emission profile of Eu2+ was used to demonstrate that BaFCl:Eu,Tb may serve to realize bandshape luminescence thermometry as a more straightforward alternative to the usual ratiometric approach. The energy gap between 4f7 (6P7/2) and 4f65d1 (lowest) levels and their radiative transition probabilities were estimated from time-resolved variable-temperature decays.

Local atomic environment of Yb3+ in alkaline-earth fluorohalide nanocrystals

The local atomic environment of Yb3+ ions doped into alkaline-earth fluorohalide nanocrystals was probed using Yb L2 edge EXAFS spectroscopy. Two structural models are proposed for the first coordination shell of Yb3+.

Modified Density Functional Dispersion Correction for Inorganic Layered MFX Compounds (M = Ca, Sr, Ba, Pb and X = Cl, Br, I)

MFX (M = Ca, Ba, Sr, Pb and X = Cl, Br, I) compounds have received considerable attention due to their technological application as X-ray detectors, pressure sensors, and optical data storage materials, when doped with rare-earth ions. MFX compounds belong to the class of layered materials with a tetragonal Matlockite crystal structure, characterized by weakly stacked double-halide layers along the crystallographic c-axis. These layers predominantly determine phase transitions, elastic, and mechanical properties. However, the correct description of the lattice parameter c is a challenge for most standard DFT functionals, which tend to overestimate the lattice parameter c. Because of the weak interactions between the halide layers, dispersion-corrected functionals seem to be a better choice. We investigated 11 different inorganic layered MFX compounds for which experimental data are available, with standard and dispersion-corrected functionals to assess their performance in reproducing the lattice parameter c, structural, and vibrational properties of the MFX compounds. Our results revealed that these functionals do not describe the weak interactions between the halide layers in a balanced way. Therefore, we modified Grimme's popular DFT-D2 dispersion correction scheme in two different ways by (i) replacing the dispersion coefficients and van der Waals radii with those of noble gas atoms or (ii) increasing the van der Waals radii of the MFX atoms up to 40%. Comparison with the available experimental data revealed that the latter approach applied to the PBE (Perdew-Burke-Ernzerhof)-D2 functional with 30% increased van der Waals radii, which we coined PBE-D2* (S_rvdw 1.30) is best suited to fine-tune the description of the weak interlayer interactions in MFX compounds, thus significantly improving the description of their structural, vibrational, and mechanical properties. Work is in progress applying this new, computationally inexpensive scheme to other inorganic layered compounds and periodic systems with weakly stacked layers.

Mechanochemically prepared SrFCl nanophosphor co-doped with Yb3+ and Er3+ for detecting ionizing radiation by upconversion luminescence

We report a novel method for detecting ionizing radiation by employing the phenomenon of upconversion luminescence. Nanocrystalline SrFCl:Yb³⁺/Er³⁺ was prepared by ball-milling and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photoluminescence properties of nanocrystalline SrFCl:Yb³⁺, SrFCl:Er³⁺ and SrFCl:Yb³⁺/Er³⁺ before and after X-irradiation were investigated. The results demonstrate that both Yb³⁺ and Er³⁺ ions in the SrFCl host are reduced to their divalent state upon X-ray exposure. Under 980 nm infrared excitation, SrFCl:Yb³⁺/Er³⁺ nanocrystals displayed efficient upconversion luminescence. The upconversion luminescence intensity gradually decreased with increasing X-irradiation in a double exponential fashion with rate constants of k₁ = 0.08 Gy^-1 and k₂ = 0.01 Gy^-1. In comparison with other X-ray storage phosphors, the present system shows a much higher stability of stored information since it is not subject to photobleaching in the read-out process. This is the first report on detecting ionizing radiation by upconversion luminescence, with the potential for improved read-out performance over traditional storage phosphors. Possible applications of the present phosphor include bioimaging and in vivo cell-level X-ray dose monitoring.