A series of SrAl₁₂O₁₉:x mol% Cr³⁺ (x = 0.1, 0.5, 1, 2, 4) phosphors were synthesized via a one-step high-temperature solid-state method. The high-purity raw materials include SrCO₃ (99%, Sinopharm Chemical Reagent), Al₂O₃ (99.99%, Sinopharm Chemical Reagent), Cr₂O₃ (99.95%, Aladdin), Nd₂O₃ (99.99%, Aladdin), Yb₂O₃ (99.99%, Macklin), and Er₂O₃ (99.99%, Macklin).

These reagents were weighed according to the stoichiometric ratio and transferred to an agate mortar. An appropriate amount of ethanol was added as a mixing medium. The mixture was thoroughly ground and homogenized until the ethanol largely evaporated. After that, the precursors were subsequently transferred to the forced-air drying oven and set to 80 °C to ensure complete ethanol removal. The dried precursor powder was then loaded into an alumina crucible and sintered in a high-temperature box furnace at 1,250-1,650 °C for 4 h under an air atmosphere. All samples were heated from room temperature to 1,000 °C at a rate of 10 °C/min, and then heated to 1,250-1,650 °C at a rate of 5 °C/min. After holding at 1,250, 1,350, 1,450, 1,550, 1,650 °C for 4 h, the samples were subsequently cooled down naturally within the furnace chamber. Finally, the as-synthesized block samples were ground into fine powder using an agate mortar and passed through a 150-mesh stainless steel sieve for subsequent characterizations and application designs.

The phase purity of the synthesized SrAl₁₂O₁₉:x%Cr³⁺ compounds was analyzed by X-ray diffraction (XRD) using an Ultima IV high-resolution diffractometer (Rigaku Corporation, Japan), with data collected in the range of 10-120°. The Rietveld refinement was performed using the GSAS-II software. The platelike morphology of the samples was captured by a field emission scanning electron microscope (FESEM) on a Gemini SEM 560 instrument (ZEISS, Germany). Elemental analysis was conducted using energy-dispersive X-ray spectroscopy (EDS) with an Oxford cryogenically cooled spectrometer ULTIM MAX (Oxford, UK). Microstructural characterization was performed by aberration-corrected transmission electron microscopy (TEM) on a Titan Cubed Themis G2 300 microscope (Thermo Fisher Scientific, The Netherlands). PL spectra and fluorescence decay curves were measured with an Edinburgh FLS1000 spectrometer (Edinburgh Instruments, UK). Quantum efficiency (QE) was measured with Quantaurus-QY Plus C13534-12 (Hamamatsu Photonics, Japan). The ML experiments were conducted with a custom-built setup including a digital push-pull gauge, a metal slider driven by a linear motor, and an optical fiber connected to the spectrometer. The ML spectra were acquired using an QE65 PRO high-sensitivity spectrometer (Ocean Optics, USA) for the visible range and an NIR QUEST 512 spectrometer (Ocean Optics, USA) for the NIR region. For ML testing, the films were prepared using a PTFE mold (5 × 5 × 0.03 cm) featuring five parallel 10 × 2 mm rectangular sections. The material block was first crushed and sieved through a 150-mesh stainless steel screen to obtain a uniform powder. This powder was then evenly distributed into the mold’s rectangular sections. After removing any excess, the shaped powder strips were thermally sealed with PET laminating film to create the final test samples. The ML signal was subsequently obtained by scraping the film surface with the metal slider at a distance of 20 mm and a speed of 16 mm/s. The PDMS used in the application section was prepared by mixing powder and PDMS in a 1:1 mass ratio. After being placed in an oven for 6 h, it was cured for subsequent testing. All the NIR images and videos were captured using an ONV3+ night vision device (ORPHA, Germany) and a smartphone. The infrared thermal video was captured by a TI25 instrument (Fluke, USA).

The magnetite-type SrAl₁₂O₁₉ matrix belongs to the hexagonal crystal system with a space group of P6₃/mmc. Rietveld refinement of the XRD pattern demonstrates an excellent agreement between the calculated (red line) and observed (black circles) data for SrAl₁₂O₁₉:2%Cr³⁺ sample [Figure 1A]. The refinement parameters were Rp = 5.70%, Rwp = 7.85%, and GOF = 2.25. After introducing Cr³⁺ into the host lattice, the unit cell parameters of the SrAl₁₂O₁₉:2%Cr³⁺ sample increased (a = b = 5.5685 Å, c = 22.0114 Å) compared with the undoped SrAl₁₂O₁₉ matrix (a = b = 5.5666 Å, c = 22.0018 Å). This lattice expansion can be attributed to the replacement of Al³⁺ ions (0.535 Å, CN = 6) by larger Cr³⁺ ions (0.615 Å, CN = 6)^[27]. These results provide clear evidence that Cr³⁺ ions successfully replaced Al³⁺ sites in the host structure. Supplementary Figure 4 shows the XRD patterns of SrAl₁₂O₁₉:x%Cr³⁺ synthesized with varying Cr³⁺ doping concentrations (x = 0, 0.1, 0.5, 1, 2, 4) at 1,650 °C for 4 h. All diffraction patterns remain nearly identical and closely match the standard card of SrAl₁₂O₁₉ (PDF#80-1195), confirming the doping of Cr³⁺ does not induce any secondary phases or alter the host crystal structure. Similarly, Supplementary Figure 5 shows the XRD patterns of samples SrAl₁₂O₁₉:2%Cr³⁺ synthesized at different sintering temperatures (1,250, 1,350, 1,450, 1,550 and 1,650 °C) for 4 h. At 1,250 °C, several peaks from Al₂O₃ impurity (PDF#88-0826) are evident. However, these peaks are largely eliminated when the temperature reaches 1,350 °C, and a pure SrAl₁₂O₁₉ phase is gradually obtained (PDF#80-1195), indicating that a synthesis temperature of 1,350 °C or higher is required to achieve phase-pure SrAl₁₂O₁₉ material.

As illustrated in Figure 1B, the crystal structure of SrAl₁₂O₁₉:Cr³⁺ comprises one [AlO₄] tetrahedron, one [AlO₅] trigonal bipyramid, and three [AlO₆] octahedra, while the Sr²⁺ occupies the 12-oxygen-coordinated site^[28]. The rich crystal structure diversity provides an extremely high compatibility host lattice for doping with various transition metals and rare-earth ions. Furthermore, SrAl₁₂O₁₉:Cr³⁺ is composed of alternately stacked mirror-image dense spinel structural units and relatively loose mirror layers. The oxygen ions are densely packed in the spinel structural units but sparsely distributed in the mirror layers^[29]. This structural anisotropy promotes preferential crystal growth along the a-b plane, while the large c/a axial ratio (3.95) suppresses growth along the c-axis, ultimately resulting in a stable hexagonal layered structure for SrAl₁₂O₁₉:Cr^3+[30].

Due to the hydrolysis of the high-performance ML material SrAl₂O₄:Eu²⁺, Dy³⁺, we specifically conducted comparative experiments and evaluated the aqueous stability of SrAl₁₂O₁₉:2%Cr³⁺. The sample synthesized at 1,650 °C was immersed in deionized water for a month. Afterward, the powder was dried and characterized as shown in Supplementary Figure 6, the XRD pattern of the water-immersed SrAl₁₂O₁₉:2%Cr³⁺ sample remains nearly identical to the non-water-immersed sample and the standard card SrAl₁₂O₁₉ (PDF#80-1195), with no detectable phase transitions or structural changes. The result indicates the outstanding chemical stability of the SrAl₁₂O₁₉:2%Cr³⁺ in aqueous environments, supporting its potential for applications in underwater NIR ML devices. The excellent waterproof performance is intrinsically related to its unique crystal structure.

To identify the anisotropic growth, Figure 2A presents the SEM image of the SrAl₁₂O₁₉:2%Cr³⁺ sample. The result reveals a stacked platelike structure, each comprising multiple single-crystal layers. These platelike morphologies are attributed to the preferential crystal growth along the (001) plane. Notably, the platelike structure of SrAl₁₂O₁₉ is achieved through one-step high-temperature solid-state synthesis, eliminating the need for molten salt or secondary processing, thereby offering considerable advantages for NIR ML coating applications.

Elemental mappings via EDS analysis [Figures 2B-G] confirm the homogeneous distribution of Sr, Al, and O, indicating the successful co-doping of both Cr³⁺ and Nd³⁺ into the SrAl₁₂O₁₉ matrix. Figure 2H presents a high-resolution TEM image. Specifically, since Sr atoms present a significantly higher atomic mass than Al atoms, the heavier Sr atoms in the SrAl₁₂O₁₉ matrix appear as brighter white spots due to pronounced scattering effects than Al atoms under dark-field conditions. The elemental mappings of the enlarged area in Supplementary Figure 7 further show that Sr, Al, O atoms exhibit a uniform layered distribution, and the Cr atoms are successfully doped into the Al layer. Figure 2I reveals the Fourier transform image, with the corresponding (110) lattice spacing of 0.278 nm. Figure 2J displays the selected area electron diffraction (SAED) pattern of the SrAl₁₂O₁₉:2%Cr³⁺ sample. The periodic diffraction spots further indicate a high crystallinity.

The PL properties of SrAl₁₂O₁₉:Cr³⁺ were systematically investigated. Figure 3A presents the photoluminescence excitation (PLE) and PL spectra of SrAl₁₂O₁₉:2%Cr³⁺. The PLE spectrum, monitored at 688 nm emission, exhibits two intense excitation bands centered at 422 and 584 nm, corresponding to the spin-allowed [⁴A₂ → ⁴T₁] and [⁴A₂ → ⁴T₂] transitions of Cr³⁺. Under excitation at 422 nm, the emission spectrum is characterized by both the R-line and broadband emission. Specifically, the sharp peaks at 688 and 694 nm arise from the spin-forbidden [²E → ⁴A₂] transition, and the broadband emission from 750 to 950 nm is ascribed to the spin-allowed [⁴T₂ → ⁴A₂] transition^[31]. The co-existence of sharp R-line and the broadband emissions indicates that the Cr³⁺ ions occupy the distorted octahedral sites in the SrAl₁₂O₁₉ matrix, with an intermediate crystal field strength^[32].

The PLE and PL spectra of SrAl₁₂O₁₉:x%Cr³⁺ are presented in Supplementary Figure 8. The PL spectra are dominated by sharp R-line emissions at 688 and 694 nm with Cr³⁺ concentration below 1%. As the Cr³⁺ concentration increases, the PL intensity of broadband emission gradually increases and finally surpasses that of sharp emissions. Specifically, the PL intensity reaches a maximum at 1% Cr³⁺ concentration, followed by a concentration quenching with increasing Cr³⁺ concentration. Supplementary Figure 9 presents the PLE and PL spectra of SrAl₁₂O₁₉:2%Cr³⁺ synthesized at different temperatures. The PL intensities show monotonic increase as the synthesis temperature is raised. The PL decay curve of SrAl₁₂O₁₉: x%Cr³⁺samples are presented in Figure 3B and Supplementary Figure 10 as a function of Cr³⁺ concentration. The corresponding lifetimes were calculated using Equation 1^[33].

(1) $$ \tau_{\text {ave }}=\left(A_{1} \tau_{1}^{2}+A_{2} \tau_{2}^{2}\right) /\left(A_{1} \tau_{1}+A_{2} \tau_{2}\right) $$

where A₁ and A₂ are the amplitudes, τ₁ and τ₂ are the decay lifetimes of the individual components, respectively. As a result, the lifetime decreases from 3.22 ms to 0.57 ms as the Cr³⁺ concentration increases. The reduction is attributed to an enhanced probability of energy migration among adjacent Cr³⁺ ions and finally to quenching sites, which collectively promote non-radiative decay pathways. Table 1 summarizes the quantum efficiency measured within 650-950 nm for SrAl₁₂O₁₉:x%Cr³⁺. As Cr³⁺ concentration increases, the internal quantum efficiency (IQE) presents a continuous decrease, whereas the external quantum efficiency (EQE) initially increases and then decreases. Notably, the SrAl₁₂O₁₉:2%Cr³⁺ sample demonstrates the highest EQE value of 29%.

Consistent with the PL spectra, the ML spectra of SrAl₁₂O₁₉:x%Cr³⁺ are composed of a sharp R-line emission at 690 nm and a broadband emission spanning 750-950 nm [Supplementary Figure 11]. The sharp R-line corresponds to the spin-forbidden [²E → ⁴A₂] transition of Cr³⁺, whereas the broadband emission arises from the spin-allowed [⁴T₂ → ⁴A₂] transition of Cr³⁺. As shown in Figure 3C and Supplementary Figure 11, the ML intensity of SrAl₁₂O₁₉:x%Cr³⁺ increases with the Cr³⁺ doping concentration increases from 0.1% to 1.0%, reaching maximum at 1.0%. The enhancement is attributed to the increasing quantity of available Cr³⁺ luminescent centers. At higher Cr³⁺ doping concentrations from 1.0% to 4.0%, the ML intensity decreases due to concentration quenching. The reduced interionic distance between Cr³⁺ ions facilitate enhanced energy migration, which promotes non-radiative energy transfer and consequently diminishes the luminescence efficiency. Furthermore, the spectrum exhibits a concentration-dependent shift: the ML is dominated by the R-line emission at Cr³⁺ concentrations below 1.0%, while the broadband emission becomes predominant at Cr³⁺ concentrations exceeding 1.0%. This unique spectral shift properties arouse our research interest. The SrAl₁₂O₁₉:2%Cr³⁺ sample, in which the sharp peak and broadband intensities are comparable, was selected as the primary subject for further investigation. The dual-mode NIR luminescence of platelike material SrAl₁₂O₁₉:x%Cr³⁺ were further explored, as schematically shown in Figure 3D. To explore the relationship between applied force and ML intensity, Figure 3E displays the ML spectra of the SrAl₁₂O₁₉:2%Cr³⁺ sample under linearly increasing applied loads (F = 10-50 N). The corresponding fitting curve demonstrates a nearly linear correlation between the ML intensity and the applied force (R² = 0.95), favorable for stress feedback applications [Figure 3F]. Besides, the self-recoverable property of SrAl₁₂O₁₉:2%Cr³⁺ was also verified by subjecting the prepared PET film to 100 sliding cycles under 5 N load, as shown in Figure 3G. The recorded ML signals remained stable throughout the test, demonstrating robust ML cyclic stability, the ML intensity remains at 86% of its average value even after 100 cycles. To eliminate the influence of thermal effects on ML, we recorded the SrAl₁₂O₁₉:2%Cr³⁺ PET scraping process using an infrared thermal imaging camera. The results showed no significant heating phenomenon [Supplementary Video 1]. These results indicate that SrAl₁₂O₁₉:Cr³⁺ belongs to the category of elastico ML materials. Moreover, the material exhibits exceptional chemical stability, as evidenced by the negligible change in the ML before and after water-immersed [Supplementary Figure 12].

Temperature-dependent ML measurements were also performed, and the result shows that the ML intensity progressively diminishes as temperature increases [Supplementary Figure 13]. This phenomenon is attributed to the enhanced lattice vibrations, which generally promote non-radiative relaxation of excited electrons. The ML integrated intensity as shown in Figure 3H for different Cr³⁺ doping concentrations indicate that excessively high doping concentrations weaken the ML intensity, which is attributed to the concentration quenching effect. Additionally, the effect of different sintering temperatures (1,250-1,650 °C) on the ML intensity for the SrAl₁₂O₁₉:2%Cr³⁺ sample is also shown in Figure 3I and Supplementary Figure 14. The result indicates that no ML signal can be detected below 1,250 °C, whereas a continuous enhancement is observed with further increasing temperature. The peak position of the ML emission remains unchanged across temperatures. Furthermore, the holding time during reaction was found to have a negligible impact on the ML intensity and spectral curve [Supplementary Figure 15]. Besides, the SrAl₁₂O₁₉:2%Cr³⁺ sample was also sintered in a reducing N₂/H₂ atmosphere and exhibits a slight enhancement in ML intensity [Supplementary Figure 16]. This observation is primarily ascribed to limited oxygen incorporation during synthesis, thereby stabilizing oxygen vacancies, which is favorable for promoting local lattice distortion and constructing piezoelectric potential, leading to enhanced ML performance.

Figure 4A illustrates a proposed mechanism for the ML process. Although the host lattice of SrAl₁₂O₁₉ is centrosymmetric, the doping Cr³⁺ ions and oxygen vacancies result in local structural distortion. Under external mechanical force, an internal piezoelectric field is generated, leading to the separation of electron-hole pairs. These electrons subsequently relax to the valence band maximum and recombine with holes. The released energy in the form of photons excites the Cr³⁺ ions to produce the NIR ML. Furthermore, the incorporation of lanthanide ions (Nd³⁺, Yb³⁺, Er³⁺) enables distinct NIR-Ⅱ ML via efficient energy transfer from the excited Cr³⁺.

Additionally, for SrAl₁₂O₁₉, the Sr²⁺ site is favorable for doping lanthanide ions to construct energy transfer pathway and realize NIR-Ⅱ ML. Therefore, the co-doped and tri-doped SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ (or 1%Yb³⁺ or 1%Er³⁺ or 1%Yb³⁺, 5%Er³⁺) samples were synthesized to modulate the emission spectra^[34,35]. Compared with Cr³⁺ singly doped sample, the co-doped samples present distinct emission at 900/1,050, 977, and 1,524 nm under 422 nm excitation, ascribed to the intra-configurational ⁴F_3/2→⁴I_9/2/⁴I_11/2, ²F_5/2→²F_7/2, and ⁴I_13/2→⁴I_15/2 transitions of Nd^3+[36,37], Yb^3+[38], and Er³⁺, respectively [Supplementary Figure 17]. Besides, the introduction of lanthanide ions leads to reduced luminescence lifetimes [Supplementary Figures 18-21], which provide evidence for energy transfer from Cr³⁺ to lanthanides. Specifically, the lifetime of Cr³⁺, Nd³⁺ co-doped sample decreases substantially, indicating a significantly higher energy transfer efficiency. The corresponding ML spectra are presented in Figure 4B-D. In contrast to SrAl₁₂O₁₉:2%Cr³⁺ sample, the co-doped samples exhibit additional ML emissions within the NIR-II region, which is consistent with the PL spectra. For example, the SrAl₁₂O₁₉:2%Cr³⁺,1%Nd³⁺ sample presents pronounced ML emissions at 864, 903 and 1,064 nm, while weak emission ranging 700-900 nm, which can be ascribed to the efficient energy transfer from Cr³⁺ to Nd³⁺ ions. Specifically, the emission at 903 nm, originating from the ⁴F_3/2 → ⁴I_9/2 transition, exhibits a doublet structure attributed to the Stark effect. The SrAl₁₂O₁₉:2%Cr³⁺, 1%Yb³⁺ sample presents a significantly weak ML at 980 nm of Yb³⁺ ions, indicating a much weaker energy transfer efficiency from Cr³⁺ to Yb³⁺. The SrAl₁₂O₁₉:2%Cr³⁺, 1%Er³⁺ sample presents a substantially red-shifted ML at 1,543 nm of Er³⁺ ions. These results collectively demonstrate that the ML spectrum can be effectively tuned via doping lanthanide ions and controlled energy transfer pathways.

Due to the excellent mechanical-to-photon conversion, ML materials have already been developed for information encryption^[39,40], self-powered illumination^[41,42], and so on. Leveraging the NIR ML properties of SrAl₁₂O₁₉:2%Cr³⁺,1%Nd³⁺, a dual-mode flexible NIR ML paper was fabricated. The fine powder was homogeneously blended with dehydrated paper pulp block in water at a powder/block mass ratio of 5:1. The mixture was subsequently transferred using a paper-making screen, followed by drying to obtain the final product. Figure 5A and B respectively compare the difference of this paper sheet under ambient light and UV illumination. Under ambient light, the result shows that the resultant paper exhibits off-white bodycolor under ambient light, while dark red PL under UV excitation. Besides, the uniform PL across the paper sheet indicates the homogeneous distribution of the ML powder. A piece of paper sheet was cut and encapsulated in a PET for the ML test. As shown in Figure 5C, the ML spectrum confirms that the paper sheet composite retains the characteristic emission from the pristine SrAl₁₂O₁₉:2%Cr³⁺,1%Nd³⁺ powder.

The microstructure of the paper composite was analyzed by FESEM. As shown in Figure 5D and Supplementary Figure 22, the SEM images show the unique platelike morphology with uniform distribution of the SrAl₁₂O₁₉:2%Cr³⁺,1%Nd³⁺ sample within the paper fibers of the sheet. The EDS confirmed the presence of all constituent elements [Supplementary Figure 23]. Figure 5E shows a long-exposure image captured with an ONV3+ night vision device, showing the numbers 0-9 and the 26 letters of the alphabet written directly onto the NIR-ML paper sheet using a glass rod. Supplementary Video 2 demonstrates the bending and writing operations performed on the paper sheet in which the NIR ML is clearly captured under night vision device. The paper sheet presents excellent ML repeatability even after 100 sliding experiments [Supplementary Figure 24]. Therefore, this paper exhibits excellent anti-counterfeiting characteristics, as the written information remains invisible to the naked eye but detectable in the NIR spectrum using specialized equipment.

Besides, the SrAl₁₂O₁₉:2%Cr³⁺ phosphor, which exhibits composition and luminescence stability under aqueous environment [Supplementary Figures 6 and 12], shows great potential for applications in underwater NIR ML devices. Accordingly, Figure 5F-H demonstrates the application of this ML coating composed of SrAl₁₂O₁₉:2%Cr³⁺ phosphor on various substrates, including black boxes (tested both in air and underwater) and banknotes [Supplementary Videos 3-5]. The ML induced by sand and stone abrasion on the seabed-simulated black box, as well as the ML response from manually scratching the banknotes were simulated. Figure 5I also demonstrates the creation of English letter patterns employing SrAl₁₂O₁₉:2%Cr³⁺ 1%Nd³⁺ coating after passing through a perforated plate mask. The patterns were imaged under three conditions: ambient light, UV excitation, and through night vision device with UV excitation. When viewed through night vision device, the intense emission confirms the material’s efficient NIR ML capability. Therefore, this dual-mode luminescence - comprising both NIR PL and ML which are only detectable using night vision device under light and mechanical stimulation [Supplementary Video 6] - enables effective application in advanced anti-counterfeiting and specialized marking, significantly enhancing the complexity of information encryption. Additionally, we conducted exploratory ultrasonic testing on PDMS films mixed with SrAl₁₂O₁₉:2%Cr³⁺,1%Nd³⁺. Experimental results demonstrate the material’s excellent response to ultrasonic forces, offering new insights for NIR stress luminescence applications within biological organisms [Supplementary Video 7].