Photoluminescence of X-Ray-Generated Sm²⁺ in Co-Precipitated SrF₂:Sm³⁺ Nanocrystals

Abstract

We report on X-ray-induced Sm³⁺ → Sm²⁺ reduction in SrF₂:Sm³⁺ nanocrystals of ~40 nm size synthesized via a co-precipitation method. Non-irradiated samples show characteristic Sm³⁺ f-f ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, and ⁶H_11/2 emissions, while X-irradiation induces intense low-temperature Sm^{2+ 5}D₀ → ⁷F₁ emission and other Sm²⁺ lines. The evolution of Sm³⁺ and Sm²⁺ photoluminescence intensities with X-ray dose (0–300 Gy) follows first-order kinetics, consistent with a trapping–detrapping mechanism. Compared to CaF₂:Sm³⁺, SrF₂:Sm³⁺ exhibits faster Sm³⁺ reduction due to the higher X-ray absorption cross section of strontium compared to calcium for Cu-Kα (8 keV) radiation, highlighting its potential as a nanoscale X-ray storage phosphor.

1. Introduction

Samarium-doped materials are widely studied for photonic and radiation-related applications, including optical data storage [1,2,3], solid-state lasers [4,5], and ionizing-radiation storage phosphors [6,7]. A key mechanism is the radiation-induced Sm³⁺ → Sm²⁺ valence state conversion, which results in characteristic Sm²⁺ luminescence arising from 4f-4f or 4f⁵5d¹ → 4f⁶ transitions. This behavior has been reported in various hosts, including fluoride glasses [8], alkaline-earth fluorohalides [9], borate [10], phosphate glasses [11,12], and oxyfluoride glass-ceramics [13]. Although Sm³⁺ (−1.55 V) is thermodynamically more difficult to reduce than Eu³⁺ (−0.36 V) and Yb³⁺ (−1.10 V) [14], the Sm³⁺/Sm²⁺ redox pair remains highly advantageous for irradiation-induced reduction studies. The formation of Sm²⁺ is strongly influenced by defect states and local lattice environments, and its host-dependent 4f-5d emission provides sensitive spectroscopic signatures for probing radiation-induced charge-transfer processes.

Fluorite-type MF₂ (M = Ca, Sr, Ba) compounds offer high IR-VUV transparency and low phonon energies (BaF₂: 319 cm⁻¹, SrF₂: 366 cm⁻¹, CaF₂: 466 cm⁻¹) with the latter lowering the nonradiative relaxation probability and hence enhancing the quantum yield [15,16,17]. Previous studies on Sm-activated alkaline-earth fluorohalides (MFX) nanocrystals (M = Ca, Ba, Sr; X = Br, I, Cl) [18,19], fluoroperovskites such as BaLiF₃ [20], and CaF₂ nanocrystals [21] have shown that Sm³⁺ → Sm²⁺ conversion can be induced by optical excitation (blue-violet and UV-C light) or ionizing radiation (β-, γ-, and X-rays). Enhanced Sm³⁺ → Sm²⁺ conversion in nanocrystals such as BaFCl, compared with bulk materials, appears to be linked to higher defect densities and larger surface-to-volume ratios. In contrast, oxide-ion impurities in close proximity to the Sm³⁺ ions in microcrystals tend to stabilize the trivalent oxidation state, thereby limiting reduction.

However, Sm²⁺ generation by X-irradiation in SrF₂:Sm³⁺ nanocrystals has not yet been examined, despite the favorable properties of SrF_2, i.e., low phonon energy, and higher X-ray absorption relative to CaF₂. Understanding Sm³⁺ reduction in this host is therefore of both fundamental and applied relevance.

In this work, we investigate X-ray-induced Sm³⁺ → Sm²⁺ conversion in co-precipitated SrF₂:Sm³⁺ nanocrystals, analyze their pre- and post-irradiation photoluminescence, and quantify the reduction kinetics as a function of X-ray dose, with comparison to CaF₂:Sm³⁺ nanocrystals [21]. This study provides insight into the roles of host composition and X-ray absorption in facilitating radiation-driven valence state conversion.

2. Results and Discussion

The powder XRD pattern of the as-prepared SrF₂:Sm³⁺ (0.5 mol% Sm) nanocrystals and its Rietveld refinement are presented in Figure 1. The standard SrF₂ PDF obtained from the Crystallography Open Database (http://www.crystallography.net/cod/index.php, accessed on 1 December 2019) is also shown for comparison [22]. The Rietveld refinement was performed using the MAUD 2.93 software package [23], applying an isotropic approximation within the default MAUD “Delf” size-strain model, and the refined parameters are summarized in

Table 1. Rietveld refinement parameters obtained from MAUD. R_wp and R_exp are the weighted-profile and expected R-factor, respectively. G is the goodness of fit (R_wp/R_exp).

Average Crystallite Size (nm)	Lattice Parameter, a (Å)	Rietveld Refinement
		Rwp (%)	Rexp (%)	G
42 ± 1	5.8092 ± 0.0002	14.3	9.9	1.4

. As seen in the figure, all diffraction peaks are consistent with the standard SrF₂ data, confirming that nanocrystals adopt a cubic fluorite-type structure with the standard fluorite space group Fm-$\overline{3}$m [24]. The average crystallite size obtained from the refinement is 42 ± 1 nm, with a lattice parameter of 5.8092 ± 0.0002 Å, which is only slightly (~0.2%) larger than that of macroscopic crystals (5.7996 Å) [25]. The goodness of fit value of G = 1.4 implied a good refinement.

The 27-K photoluminescence spectrum of SrF₂:Sm³⁺ nanocrystals before and after 150 Gy X-irradiation, demonstrating the generation of Sm²⁺ by X-irradiation, is shown in Figure 2. In the inset, a schematic Sm²⁺ energy-level diagram with the relevant transitions is depicted [26]. The 4f⁵5d excited state lies 450 cm⁻¹ above the ⁵D₀ 4f⁶ level, with a separation of 115 cm⁻¹ between A_1u and T_1u levels [27,28]. As follows from the figure, the non-irradiated sample exhibits the Sm³⁺ f-f transitions ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, and ⁶H_11/2 at 566, 599, 645, and 704 nm, respectively, confirming the incorporation of trivalent samarium into the SrF₂ lattice [29]. Notably, X-irradiation causes a decrease in Sm³⁺ luminescence. Unlike the CaF₂:Sm²⁺ system, the 4f⁵5d levels in SrF₂:Sm²⁺ lie above the 4f^{6 5}D₀ excited state [30,31]. Consequently, the relatively sharp Sm²⁺ emission at 696.7 nm observed after 150 Gy X-irradiation corresponds to the magnetic dipole-allowed transition from the 4f^{6 5}D₀ (A_1g) excited state to the 4f^{6 7}F₁ (T_1g) ground state level (labeled as “d”) [31,32]. Several relatively strong vibronic sidebands are also present [28,32,33,34]. Additional Sm²⁺ lines include the 675.6 nm (a) electric dipole-allowed 4f⁵5d (T_1u) → ⁷F₀ (A_1g) transition, the 683.8 nm (c) very weak magnetic dipole-forbidden ⁵D₀ ↔ ⁷F₀ transition, and the 663.5 nm (b) strongly forbidden 4f⁵5d (A_1u) → ⁷F₀ (A_1g) transition, previously observed in an external magnetic field [26,28,31].

Figure 3 shows the temperature dependence of the Sm²⁺ photoluminescence spectrum of SrF₂:Sm³⁺ nanocrystals after 200 Gy X-irradiation in the temperature range from 27 to 293 K. With increasing temperature, the prominent 696.7 nm peak (d) broadens and the vibronic sidebands intensify. The 675.6 nm transition (a) is very weak at 27 K, as the T_1u level is scarcely populated at low temperature, consistent with a Boltzmann distribution [35]. Note that the Sm²⁺ emission is quenched above ~200 K in SrF₂ and BaF₂, while partially preserved in CaF₂ [36]. As the temperature rises, the broad d-f luminescence (a) starts to dominate over the sharp f-f emission. This quenching of the f-f emission, i.e., the nonradiative crossover between Sm²⁺ 5d ↔ ⁵D₀ states, has been quantitatively modeled in some early work by Fonger and Struck [37]. Importantly, the emission is eventually quenched at higher temperature via a two-step quenching pathway, ⁵D₀ → 5d → ⁷F [35,38], resulting from thermally activated crossover from the ⁵D₀ level to the 5d state, followed by rapid multiphonon relaxation [39].

The dependence of the Sm³⁺ → Sm²⁺ reduction in SrF₂:Sm³⁺ nanocrystals under Cu-Kα (8 keV) X-irradiation was investigated by monitoring the photoluminescence intensities of the Sm^{2+ 5}D₀ (A_1g) → ⁷F₁ (T_1g) transition at 696.7 nm (d) and the Sm^{3+ 4}G_5/2 → ⁶H_7/2 transitions at 599 nm (see Figure 4). As seen in Figure 4b, the Sm³⁺ luminescence decreased by ~78% upon 300 Gy X-irradiation. The dose dependence of both Sm²⁺ and Sm³⁺ intensities, $I_{{\mathrm{Sm}}^{\mathrm{x}}}$ (Figure 4c,d), were well fitted by single-exponential functions:

$$I_{{\mathrm{Sm}}^{\mathrm{x}}} = a_0+a_1\exp \left(-k\times \mathrm{dose}\right)$$

(1)

with fitting amplitudes $a_0$ = 1.02 and $a_1$ = −1.05 for Sm²⁺, and $a_0$ = 0.22 and $a_1$ = 0.76 for Sm³⁺, yielding a rate constant $k$ = 0.01 Gy⁻¹ for both Sm²⁺ and Sm³⁺. These results indicate that the formation of the Sm²⁺ and the depletion of Sm³⁺ under X-irradiation follow simple first-order kinetics up to doses of 300 Gy using 8 keV Cu-Kα radiation. A simple possible mechanism for the Sm³⁺ → Sm²⁺ reduction in SrF₂:Sm³⁺ under X-irradiation is proposed in Equation (2):

$${\mathrm{Sm}}^{3+}+{\left(\mathrm{trap}\right)}^{-}+(\mathrm{X}-\mathrm{irradiation})\to {\mathrm{Sm}}^{2+}+\left(\mathrm{trap}\right)$$

(2)

The Sm³⁺ ions occupy Sr²⁺ lattice sites and are charge-compensated by defects such as interstitial F⁻ ions or O²⁻ impurities [40,41]. X-irradiation generates electron-hole pairs, and some electrons are then captured and trapped directly, or indirectly via F-centers, by Sm³⁺ to form Sm²⁺, while the holes are trapped by interstitial F⁻ ions or O²⁻ impurities.

The presence of defect-related trapping states within the bandgap stabilizes the reduced Sm²⁺ species by suppressing immediate recombination of charge carriers. These traps prolong electron lifetimes and enhance the reduction in Sm³⁺. Such processes, involving F-centers and V_k centers, are well known in alkaline-earth fluorides and play an important role in charge transfer and recombination dynamics [37,42].

Figure 5a compares the Sm²⁺ photoluminescence generated upon 150 Gy X-irradiation of SrF₂:Sm³⁺ and CaF₂:Sm³⁺ nanocrystals (0.5 mol% Sm relative to Sr or Ca). In CaF₂:Sm³⁺ nanocrystals, the prominent Sm²⁺ luminescence is the electric dipole-allowed 4f⁵5d (A_1u) → ⁷F₁ (T_1g) transition centered at 708.2 nm (d), whereas the intense Sm²⁺ luminescence at 676.9 nm (d) in SrF₂:Sm³⁺ corresponds to the magnetic dipole-allowed 4f^{6 5}D₀ (A₁) → 4f^{6 7}F₁ (T_1g) transition. Figure 5b shows the corresponding decrease in Sm³⁺ luminescence of both SrF₂:Sm³⁺ and CaF₂:Sm³⁺ nanocrystals with increasing X-ray dose (0–300 Gy), which is well described by a single-exponential behavior, as expressed by Equation (1). The fit of CaF₂:Sm³⁺ yielded a rate constant of $k$ = 0.007 Gy⁻¹ with amplitudes of $a_0$ = 0.50 and $a_1$ = 0.48.

As shown in Figure 5b, the Sm³⁺ → Sm²⁺ conversion in SrF₂: Sm³⁺ proceeds markedly faster, with approximately 78% of Sm³⁺ reduced after a 300 Gy X-ray dose, compared with only ~46% in CaF₂:Sm³⁺ under the same conditions. This higher reduction efficiency in SrF₂ is due to its higher X-ray mass attenuation coefficient compared to CaF₂ [43], which results in more electron-hole pair creation [44] and, consequently, a greater degree of Sm³⁺ → Sm²⁺ reduction. The X-ray absorption coefficients computed using the SRS Absorption Calculator (https://11bm.xray.aps.anl.gov/absorb/, accessed on 29 November 2025) yield values of ~246 cm⁻¹ for CaF₂ and ~340 cm⁻¹ for SrF₂ at the Cu-Kα photon energy of 8 keV, demonstrating that SrF₂ absorbs X-rays approximately 1.4 times more strongly than CaF₂, which is in good agreement with the ratio (1.43) of the initial conversion rates for the two systems. As shown in the inset of Figure 5b, the Sm³⁺ reduction up to 850 Gy demonstrated a dispersive first-order kinetics for the latter host.

3. Materials and Methods

Nanocrystalline SrF₂:Sm³⁺ was synthesized by co-precipitation of SrCl₂ꞏ2H₂O, SmCl₃ꞏ6H₂O, and NH₄F (ACS grade, Sigma-Aldrich, Bayswater, Australia). Briefly, 37.4 mg of SmCl₃ꞏ6H₂O (0.5 mol% of Sm relative to Sr) was added to 100 mL of an aqueous SrCl₂ꞏ2H₂O solution (0.3 M). Subsequently, 100 mL of an aqueous NH₄F solution (0.6 M) was added, and the mixture was stirred for 3 h. The resulting precipitate was collected by centrifugation (12 min, 4000 rpm), thoroughly washed with deionized water, and dried at 60 °C overnight.

The phase purity of the sample was characterized using a Rigaku Miniflex 600 benchtop XRD diffractometer (Rigaku Corporation, Tokyo, Japan) (Cu-Kα radiation, λ = 0.154 nm; 40 kV, 15 mA). The diffraction patterns were recorded over 2θ range of 20° to 90° with a step size of 0.01° and a scan rate of 0.5° min⁻¹.

The photoluminescence spectra were carried out using a 462 nm laser diode as the excitation source and accumulated on a Spex 500 M monochromator (Spex Industries Inc., Edison, NJ, USA) equipped with an Andor iDus Model DV401A-BV CCD camera(Andor Technology, Belfast, Northern Ireland, UK). Powder samples were placed in an aluminum holder with a 0.5 mm (depth) × 5 mm (diameter) counterbore and mounted on the cold finger of a closed-cycle cryostat (CTI-Cryogenics Cryodyne Model 22, CTI-Cryodyne, Haverhill, MA, USA).

The Sm³⁺ to Sm²⁺ reduction by X-irradiation was performed using the Rigaku diffractometer at a fixed 2θ angle of 30°. The delivered dose was cross-calibrated with a Sirona HELIODENT Plus Dental X-ray Source (Sirona Dental Systems GmbH, Erlangen, Germany).

4. Conclusions

We have demonstrated the reduction in Sm³⁺ → Sm²⁺ upon X-irradiation in nanocrystalline SrF₂:Sm³⁺ synthesized via co-precipitation of SrCl₂, SmCl₃, and NH₄F. The as-prepared sample, with an average crystallite size of ~40 nm, exhibits clear Sm²⁺ photoluminescence upon exposure to ionizing radiation. The evolution of Sm²⁺ and Sm³⁺ emission intensities as a function of X-ray dose (0–300 Gy) follows first-order kinetics, indicating a basic trapping–detrapping mechanism associated with radiation-generated charge carriers. Compared with CaF₂:Sm³⁺ nanocrystals, the reduction in the SrF₂ host is more efficient, i.e., ~78% of Sm³⁺ ions in SrF₂:Sm³⁺ are converted to Sm²⁺ after a 300 Gy dose, versus ~46% in CaF₂:Sm³⁺ under identical conditions. This enhanced reducibility is attributed to the higher X-ray absorption cross section of SrF₂ relative to CaF₂ at 8 keV (~1.4×), which results in more effective generation of electron-hole pairs that participate in the redox conversion. The pronounced radiation sensitivity and robust Sm²⁺ emission observed in SrF₂:Sm³⁺ nanocrystals highlight its potential as a nanoscale X-ray storage phosphor for high dose dosimetry.