With the development of technology and the increasing emphasis on patent protection, anti-counterfeiting technologies are emerging one after another. Among numerous anti-counterfeiting technologies, photoluminescence anti-counterfeiting technology is widely used in the field of anti-counterfeiting due to its excellent optical properties. However, traditional fluorescent powder anti-counterfeiting only changes its color when the UV excitation light source is changed, lacking dynamic changes in the time dimension. Therefore, it is necessary to develop an efficient anti-counterfeiting new luminescent material. Currently, the use of cation substitution and multiple occupied sites has become a popular strategy for tunable emission. Due to the interaction between activators and anions, the energy difference between the energy levels of the luminescent center ion can be altered. Therefore, anion substitution can also be an effective strategy to adjust the photoluminescence performance. This article describes the preparation of Mg_2-xNa_xAl₄Si₅O_18-xQ_x:1%Eu²⁺ (Q=Cl, F) by glass relaxation crystallization method, which introduces Na⁺ sites and replaces 1O^2– with 2F^–/2Cl^– to achieve the opening of the [Al/Si]₆O₁₈ hexagonal ring structure, providing a rich field environment for the central ion Eu²⁺ to enter the α-Mg₂Al₄Si₅O₁₈ structure: Na⁺ sites, Mg²⁺ sites, and hexagonal ring channel sites. By regulating the Eu²⁺ occupation, ${\text{Eu}}_{\text{vac}}^{2+}$, ${\text{Eu}}_{\text{Mg}}^{2+}$ and ${\text{Eu}}_{\text{Na}}^{2+}$ structures are formed, achieving blue to yellow light emission.MethodsBased on the composition of 2MgO–2Al₂O₃–5SiO₂, a series of Mg_2-xNa_xAlSiO_18–xQ_x:1%Eu²⁺ (Q=Cl, F) fluorescent powders (abbreviated as 1Eu²⁺, 1Eu²⁺-xNaCl and 1Eu²⁺-xNaF) were prepared by introducing NaCl (A.R.), NaF (A.R.), and Eu₂O₃ (A.R.), and glass crystallization after quenching the reactants in ice water. Doping concentration: NaCl are 0, 1%, 2%, 3%, and 5% molar fraction and NaF are 1%, 2.0%, 2.5%, and 3.0%. Weigh according to the set reactant stoichiometry, and grind the mixture thoroughly in an agate mortar for 30 minutes to ensure even mixing. Transfer it to a graphite crucible, place it in a well furnace, heat it up to 1550 ℃ at a heating rate of 5°/min, and keep it in an argon atmosphere for 30 minutes. Then, pour the molten reactants into ice water for rapid cooling, dry and grind to obtain the precursor glass sample PG. Finally, the precursor glass sample PG will be kept in a reducing atmosphere at 1100 ℃ for 1 hour, and then ground into a crystalline sample after cooling in the furnace.Results and discussionWhen single doped with Eu²⁺, Eu²⁺ occupies the Mg²⁺ and channel sites in the α- cordierite structure, forming ${\text{Eu}}_{\text{Mg}}^{2+}$ and ${\text{Eu}}_{\text{vac}}^{2+}$ luminescent structural units. When NaF is introduced, Eu²⁺ occupies the Na⁺ site to form a new luminescent structural unit ${\text{Eu}}_{\text{Na}}^{2+}$. Introducing NaCl/NaF can enhance the occupation of channel sites by Eu²⁺. When F^–/Cl^– replaces O^2– and partially opens the hexagonal ring of the channel, the channel sites can accommodate more Eu²⁺, which makes it easier to explain the increased ${\text{Eu}}_{\text{vac}}^{2+}$ structure formed when NaCl/NaF is introduced. When NaCl is introduced, Eu²⁺ hardly occupies the Na⁺ sites, while when NaF is introduced, as the amount of NaF introduced increases, Eu²⁺ occupies more Na⁺ sites. That is because the stronger electronegativity of O^2– than Cl^–, replacing O^2– with Cl^– can introduce more covalences in the lattice,that means Eu²⁺- Cl^– has stronger contradiction and polarization than Eu²⁺–O^2–. Moreover, the ionic radius of Cl^– differs significantly from that of O^2–, making the formation of Eu²⁺–Cl^– more difficult. Therefore, when Cl^– is introduced, Eu²⁺ does not occupy the Na⁺ sites. Similarly, when F^– is introduced, Eu²⁺ is more likely to occupy Na⁺ sites. In addition, the amount of Eu²⁺ introduced is constant, and when Na⁺–Cl^– is introduced, Eu²⁺does not occupy the Na⁺ sites. As the ${\text{Eu}}_{\text{vac}}^{2+}$ structure increases, the corresponding generated ${\text{Eu}}_{\text{Mg}}^{2+}$ structure will decrease. As a ligand, compared to Mg²⁺–O^2–, the introduction of Na⁺–F^– has a smaller steric hindrance and greater bonding ability due to the smaller radius and greater electronegativity of F^– (compared to O^2–) ions. Eu²⁺ will preferentially occupy Na⁺ sites with similar radii and then occupy Mg²⁺ sites. As the amount of NaF introduced increases, more ${\text{Eu}}_{\text{Na}}^{2+}$ structures are formed while the corresponding ${\text{Eu}}_{\text{Mg}}^{2+}$ structures decrease.ConclusionsThe main conclusions of this paper are summarized as following. This article mainly studies the effect of introducing Na⁺ sites and Cl^–/F^– substituted O^2– on the occupancy of Eu²⁺ in the α- cordierite structure. Exploring the mechanism of distinguishing subgrain structure evolution for luminescent structural probes. By introducing Na⁺ sites and anionic Cl^–/F^– substituted O^2– to regulate the selective occupancy of Eu²⁺, temperature sensitive emission from blue white to yellow light is achieved. Cl^–/F^– substituted O^2– opens the hexagonal ring structure of the channel, which facilitates the occupation of channel sites by Eu²⁺. The proportion of the formed structure increases from 33.32% to 64.89%. When introducing Na⁺ sites, the presence of F^– increases the proportion of Eu²⁺ occupying Na⁺ sites to form structures to 18.63%. This series of fluorescent powders can significantly improve their thermal quenching performance when NaF is introduced, and have superior thermal stability at 150 ℃. This series of fluorescent powders can be widely used in the fields of adjustable luminescence, detection, and anti-counterfeiting.