Sm²⁺ ion exhibits a broad absorption band in the ultraviolet-visible region due to its 5d state absorption, which can be tuned by crystal field effects and co-doping with other ions without causing self-absorption during 4f→4f emission transitions. Sm²⁺ ion also holds significant research interest in fields such as fluorescence lighting, photocatalysis, scintillation, temperature sensing, pressure sensing, and radiation dose monitoring. Fluorosilicate glasses doped with Sm²⁺ ions have promising applications in optical storage and information encryption. However, the efficient reduction of Sm³⁺ ion to Sm²⁺ ion remains a challenge due to the relatively low reduction potential of Sm³⁺/Sm²⁺. This paper proposes a simple and cost-effective method for preparing Sm²⁺-doped fluorosilicate glass in ambient air atmosphere.

The nominal base molar composition was 52SiO₂-16Al₂O₃-6AlF₃-7.5BaF₂-18NaF-0.5Sm₂O₃. Al particles were added to a raw material at different mass fractions (i.e., 0%, 0.04%, 0.08%, 0.20%, 0.60%, and 0.80%, respectively). 25 g of analytical-grade raw material was mixed in required proportions and ground in an agate mortar for 15 min. The ground mixed material was placed in an alumina crucible covered with alumina and graphite plates and heated in a high-temperature elevator furnace in air at 1 550 ℃ for 30 min. Afterwards, the molten glass was poured onto a graphite plate and rapidly pressed flat using a stainless steel mold to form glass samples. These glass samples were labeled as G0, G0.04, G0.08, G0.20, G0.60, and G0.80, corresponding to the amount of elemental Al added in the raw materials. Finally, the glass samples were cut and polished to a thickness of 1 mm for the spectral measurements.

To investigate the effect of annealing temperature on the stability of Sm²⁺ ion, the samples G0.04 and G0.60 were selected for heat-treatment at 440, 460, 480, 500 and 520 ℃, respectively. We also investigated the effect Sm²⁺ doping concentration on the luminescence properties of glass samples. To ensure the effective reduction of Sm ions, the concentration of Sm ions in the glass samples was varied at a metallic Al content of 0.80%. The nominal molar composition of the glass was 52SiO₂-16Al₂O₃-6AlF₃-7.5BaF₂-18NaF-xSm₂O₃ (i.e., x=0, 0.062, 0.125, 0.250, 0.500, and 1.000, respectively).

At Sm doping amount of 1% (in mole fraction), the characteristic emission of Sm²⁺ increases, while that of Sm³⁺ decreases with increasing Al content in the raw materials. At Al doping content of 0.60% (in mass fraction), the characteristic emission peak of Sm³⁺ ion completely disappears. These results demonstrate that the incorporation of metal Al during glass melting enables efficient, stable and controllable conversion of Sm³⁺ ion to Sm² ion.

At Al doping content of 0.04%, no crystallization peaks appear in either the original sample or the heat-treated samples at various temperatures. However, at Al doping content of 0.60%, the crystallization peaks appear even before heat-treatment, corresponding to the diffraction plane (220) of Si. The addition of metallic Al in the raw materials can affect the crystallization behavior of the glass. The sample G0.04 exhibits a dramatic decrease in the fluorescence intensity as the heat treatment temperature increases. During this process, a reversal phenomenon occurs in the relative intensities of the fluorescence peaks at 600 nm and 683 nm. This indicates that some of the Sm²⁺ ions in the G0.04 sample were oxidized during the heat treatment process. In contrast, for the sample G0.60, the relative intensities of the fluorescence peaks at 683 nm and 600 nm remain unchanged after heat- treatment at various temperatures. An increase in Al doping content contributes to maintaining the stability of the Sm²⁺ valence state at higher temperatures. This phenomenon may be attributed to the formation of a number of low-valence silicon species surrounding Sm²⁺ ions in the glass at a greater Al doping content. This arrangement potentially prevents the oxidation of Sm²⁺ ion by Si⁴⁺ ion during heat-treatment.

The results show that the transmittance of the glass samples in the 300-600 nm range rapidly decreases, and the absorption bands gradually red-shift towards longer wavelengths as Sm doping content increases. Under 350 nm excitation, the fluorescence intensity at 683 nm gradually increases with increasing Sm doping content. However, it suddenly decreases when the Sm doping concentration reaches 1% (in molar fraction, same hereafter), which can be attributed to concentration quenching. The effect of concentration quenching on the luminescence is further corroborated by the fluorescence lifetime decay curves. As the Sm²⁺ concentration in the samples increases, the fluorescence lifetime at 683 nm gradually decreases.

Sm²⁺-doped fluorosilicate luminescent glasses were prepared via adding an appropriate amount of metallic aluminum as a reducing agent in raw materials. The results showed that Sm³⁺ could be fully reduced when Al doping content reached 0.60% (in mass fraction, same hereafter). Furthermore, the influence of heat treatment temperature on Sm²⁺ stability was investigated. The oxidation of Sm²⁺ during heat-treatment could be prevented at Al doping content of 0.60%. In addition, the impact of Sm²⁺ doping content on the luminescence performance was also explored. The fluorescence intensity of the sample reached its maximum value at Sm doping content of 0.5% (in molar fraction).