Introduction

Stimulus-responsive chromogenic (SRC) compounds are a class of intelligent materials that undergo reversible color changes in response to external stimuli such as light, heat, electric potential, or mechanical forces1,2,3. Over the past decades, considerable progress has been made in the development of single-SRC materials, with a predominant focus on light- and temperature-induced chromic responses. For instance, various transition metals, ferroelectric oxides, silver halides, and organic molecules have demonstrated notable photo- and/or thermo-induced coloration phenomena4,5,6,7,8,9,10,11,12,13,14. Mechanical strain, a ubiquitous physical phenomenon, manifesting through actions such as rubbing, pressing, twisting, bending, squeezing, and curling, has recently attracted considerable attention due to its potential to induce optical behavior15,16. The development of force-induced luminescence and chromic materials has facilitated advanced applications in fields such as artificial intelligence, stress sensing, and biotechnology17,18,19,20. Organic mechanochromophores (e.g. bis-naphthopyran21, multinetwork polymers22, and Rhodamine-spiropyran conjugates23) are predominant in current research. However, they face significant challenges, including complex synthesis and limited stability24,25,26, which restrict their practical applications compared to inorganic alternatives. To date, inorganic mechanochromic materials remain relatively underexplored, to the best of our knowledge.

Multi-SRC materials possess an edge in the diversity of optical properties over single-SRC systems, which enable the intuitive reflection of multiple environmental changes through visual color signals, thereby facilitating their application in complex scenarios such as smart windows, human–machine interaction, and robotics27,28,29. At present, multi-SRC materials are typically fabricated by incorporating various single-SRC molecules into a unified system30,31,32,33; however, integrating the multi-SRC phenomenon within an inorganic host material remains a significant obstacle. In contrast, SRC inorganic materials are generally limited to single-color responses34,35, while multicolor chromism could enhance their photophysical properties, which is highly desirable for high-end applications such as switchable photovoltaics and encryption systems36,37. Currently reported inorganic multicolor chromic materials include perovskite halides38, chloroapatites34,39, and sodalites40, in which only light or/and heat stimulation is employed to achieve multicolor chromism. Therefore, leveraging a combination of force, light, and heat stimuli to induce multicolor chromism in inorganic materials represents a pioneering and promising approach.

Reversible luminescence modulation based on photo- and thermal-induced coloration has garnered considerable attention, exhibiting prospective in optical storage4, security24, fingerprint acquisition25, etc. The microstructure such as cationic valence and defect distribution can be influenced by mechanical strain41,42,43, while the impact of inorganic mechanochromism on luminescent properties remains insufficiently understood. In this work, we fabricate a europium tungstate phosphor with mechanochromic properties, exhibiting a reversible orange-to-black color transition under pressure and a black/orange-to-white negative photo-/thermochromism, thereby achieving force-light-heat stimulation-induced multicolor chromism in a single inorganic unit. The mechanisms underlying mechanochromism, photochromism, and thermochromism are systematically investigated. By utilizing the multi-SRC effect, the luminescence of Eu3+ is reversibly regulated, with complete luminescence quenching observed upon saturation of the mechanochromic response. Due to the rich photophysical properties of europium tungstate phosphors, their multi-functional applications in optical storage, security, and pressure sensing are explored.

Results

The orange EuWO4 phosphors were synthesized using a high-temperature solid-phase method sintered at 1150°C in a reducing atmosphere (95%N2 + 5%H2). The phase determination of EuWO4 phosphors is analyzed by powder X-ray diffraction (XRD), exhibiting the diffraction peaks consistent with the standard card of EuWO4-JCPDS No. 22-1101 (Supplementary Fig. 1a). Rietveld analysis of as-prepared EuWO4 through XRD refinement yields a deeper understanding of its crystallographic structure, as shown in Fig. 1a and Supplementary Table 1. The calculated deviations in statistical parameters Rwp, Rp, and χ2 respectively are 4.53%, 4%, and 0.942, indicative of a close alignment between the fitted and observed XRD patterns. Figure 1b describes the crystal structure of EuWO4, which belongs to the body-centered tetragonal scheelite phase with the space group of I41/a44. The W cations bond with four oxygen atoms in nearly regular tetrahedra, while Eu cations are surrounded by eight oxygen atoms, forming distinctive bisdisphenoidal polyhedra44. Field emission scanning electron microscopy (FESEM) imaging reveals a micron-scale particle size, with energy-dispersive X-ray spectroscopy (EDS) results indicating a uniform distribution of W, Eu, and O elements within the crystal (Fig. 1c, Supplementary Fig. 1b).

Fig. 1: Mechanochromic-thermobleaching properties of EuWO4.
figure 1

For the EuWO4-A (EWO-A) sample, a Rietveld refinement of the X-ray Diffraction (XRD) profile; b crystal structure of the EuWO4 lattice. c Field Emission Scanning Electron Microscope (FESEM) and corresponding Energy Dispersive X-ray Spectroscopy (EDS) element mapping images of W, Eu, and O elements; d the diagram of the mechanochromic and thermo-bleaching processes; e the diffuse reflectance spectra as a function of pressure with holding time of 10 min; f the plot of coloration contrast at 710 nm versus pressure, g the pressure-dependent Eu3+ photoluminescence (PL) spectra with holding time of 10 min (λex = 395 nm); h the variation of Eu3+ PL luminescent regulation rate (615 nm) with pressure. i The diffuse reflection spectra of the EuWO4-B (EWO-B) sample during the thermo-bleaching process at various temperatures and durations. j The variation of Eu3+ PL intensity and reflectance for EuWO4 by alternating pressure stimuli and heat treatment in five cycles.

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As illustrated in Fig. 1d, EuWO4 undergoes a reversible color transition between orange and black by alternating force and heat stimuli, respectively designated as EWO-A and EWO-B. To elucidate the light-absorbing capabilities across different states, pressure-dependent diffuse reflection spectra are presented in Fig. 1e. It is found that EWO-A predominantly absorbs light with energies above 2.48 eV, while light in the 500–1200 nm range is partially reflected. The extent of color change in force-induced chromic EuWO4 can be quantified by the coloration contrast (\({\Delta {{\rm{R}}}}_{{{\rm{c}}}}\)), defined as \(\Delta {{{\rm{R}}}}_{{{\rm{C}}}}=\frac{{{{\rm{T}}}}_{{{\rm{o}}}}-{{{\rm{T}}}}_{{{\rm{c}}}}}{{{{\rm{T}}}}_{0}}\times 100\%\) 24. In this formula, To and Tc represent the reflectance before and after chromism. In the mechanochromic process of EWO-A, the maximum coloration contrast reaches approximately 77.55% at a wavelength of 710 nm (Supplementary Fig. 2). As pressure (P) increases progressively from 0 to 30 MPa with a hold time of 10 min, the coloration contrast at 710 nm linearly increases (Fig. 1f), which can be well-described by the equation of \({\Delta {{\rm{R}}}}_{{{\rm{c}}}}=1.8*P+20.44\), achieving a fit accuracy of 0.98. At elevated pressures ranging from 15 to 90 MPa, the mechanochromic response is significantly accelerated (Supplementary Fig. 3), leading to a noticeable coloration within 3 s. These results suggest that EuWO4 shows potential as a material for application in pressure-sensing technologies. Apart from pressure, the coloration contrast is also influenced by the duration of force stimulation, as illustrated in Supplementary Fig. 4. Under a 30 MPa stimulus, the color of EWO-A deepens and reflectance decreases with pressing time lengthening, reaching saturation at 10 min. Notably, when the pressure is below the 4 MPa threshold, no observable color change occurs, even with pressing times exceeding 1 h (Supplementary Fig. 5).

The photoluminescence (PL) spectra of EWO-A phosphor display emission peaks at 593 nm, 615 nm, 653 nm, and 702 nm, corresponding to the 5D0 → 7F1-4 transitions of Eu3+ ions. The emission intensities gradually decrease with pressure increasing, eventually leading to a complete PL quenching in the EWO-B state (Fig. 1g). The external photoluminescence quantum yield (PLQY) of the EWO-A exhibits a relatively low value of 5.86%, while the red emission can still be observed in a dark environment. Reviewing past publications (Supplementary Fig. 6), the highest photo/thermochromism-induced luminescent regulation rate is approximately 99.2%, while achieving a complete light off-on remains challenging. In contrast, the mechanochromism-induced 100% luminescence modulation is demonstrated in our cases (Supplementary Fig. 7), surpassing the performance of most light- or heat-responsive chromic inorganic materials. Figure 1h presents the plot of the luminescent regulation rate (\({\Delta {{\rm{R}}}}_{{{\rm{m}}}}\)) as a function of pressure, which can be fitted with the linear equation \({\Delta {{\rm{R}}}}_{{{\rm{m}}}}=3.23*{{\rm{P}}}+3.14\). Notably, the value of the slope for the \({\Delta {{\rm{R}}}}_{{{\rm{m}}}}\)-P relationship is greater than that of the \({\Delta {{\rm{R}}}}_{{{\rm{c}}}}\)-P linear fit, which is attributed to the combined effects of reabsorption and nonradiative transitions that collectively contribute to luminescence quenching (Supplementary Fig. 8). This demonstrates that dual-mode pressure sensing, utilizing both coloration and PL, can be achieved in EuWO4. Placing the black EWO-B sample in a reducing atmosphere furnace for sintering at various temperatures gradually restores its reflectance (Fig. 1i). Through thermal treatment at 500 °C for 1 h, the EWO-B state can be reversibly converted back to the orange EWO-A state. Leveraging the reversible color change between orange and black, a PL off-on switch is achieved in EuWO4, exhibiting good anti-fatigue and reproducibility in coloration-bleaching and PL modulation processes (Fig. 1j, Supplementary Fig. 9).

Understanding the coloration and bleaching mechanism of EuWO4 can pave the way for developing inorganic mechnochromic materials. In 2005, Errandonea reported the pressure-induced reversible phase transition of EuWO4 between the tetragonal I41/a structure and the monoclinic I2/a structure, typically occurring at high pressures, around 8.5 GPa44. Accordingly, the XRD patterns and corresponding Rietveld refinement of EWO-A and EWO-B are compared in Supplementary Fig. 10, revealing no phase transition before and after mechnochromism. It is found that the diffraction peaks broaden after force stimulation, which is due to the lattice shrinkage and the polyhedral distortion under pressure stimulation (Supplementary Fig. 10). Additionally, the emission intensity ratio of peaks at 615 nm (5D0 → 7F2 transition) and 593 nm (5D0 → 7F1 transition) in the Eu3+ spectrum can be used as an indicator to explore crystal field symmetry45. The ratio of I615/I593 gradually increases with pressure, further indicating the occurrence of polyhedra distortion in the EuWO4 lattice during mechanochromism. The X-ray photoelectron spectroscopy (XPS) technique was employed to investigate the cationic valence transformation and the oxygen vacancy in EuWO4. Figure 2a shows the Eu 3 d high-resolution spectra of EuWO4 with 0 MPa, 17 MPa and 30 MPa pressure-stimulated for 10 min, featuring four Gaussian peaks at 1125.0 eV (Eu2+ 3d5/2), 1135.0 eV (Eu3+ 3d5/2), 1154.8 eV (Eu2+ 3d3/2), and 1164.5 eV (Eu3+ 3d3/2), respectively. In the EWO-A state, europium exists in two oxidation states, the 4f7 configuration of Eu2+ and the 4f6 configuration of Eu3+, while the Eu2+/Eu3+ ratio decreases from 0.124 to 0.087 in the EWO-B state. Due to the small energy difference between the valence states of Eu2+ and Eu3+, external pressure stimulation may enable orbital intermixing41, resulting in a force-induced transformation in europium cationic valence. The transition between Eu2+ and Eu3+ causes a charge imbalance in the EWO-A system, potentially leading to valence changes in tungsten ions or the loss of oxygen vacancies. During the mechanochromic process, the proportion of W5+ ions increases slightly from 78% to 87%, returning to its original level following thermal bleaching (Fig. 2b). Previous studies have reported that the intervalence charge transfer transitions between W5+ and adjacent W6+ ions can produce an intense broad absorption band in the visible and near-infrared spectral region, which may be responsible for the coloration of the tungstate46,47.

Fig. 2: The mechanism of reversible mechanochromism in EuWO4 phosphors.
figure 2

Eu 3 d a, W 4 f b, and O 1 s c high-resolution X-ray photoelectron spectra (XPS) spectra of the original, mechnochromic (17 MPa and 30 MPa for 10 min), and fully bleached EuWO4. d Eu L-edge X-ray Absorption Near-Edge Structure (XANES) spectra of the EWO-A, EWO-B, purchased EuO, and commercial Eu2O3. Enlarge figures correspond to the pre-edge and white lines of the EWO-A and EWO-B. The right panel is the average valence calculation for the Eu element based on white-line energy. e W L-edge XANES spectra of the EWO-A, EWO-B, tungsten foil, and standard WO3. The right panel is the average valence calculation for the W element using the integral area of the white line. f The schematic diagram of the pressure-induced coloration in EuWO4.

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The asymmetrical O 1 s XPS spectra are generally deconvoluted into two or three peaks in past literature, corresponding to the lattice oxygen, oxygen vacancy, and absorbed oxygen. Figure 2c shows that the O 1 s peak shifts toward the higher binding energy as the pressure increases, which may be attributed to the combination of charge compensation and the formation of color centers48. However, the XPS technique, which measures the kinetic energy of electrons emitted from core or valence levels, cannot directly detect oxygen vacancies due to the lack of distinct characteristic peaks49. Therefore, complementary techniques are required to assist in elucidating the evolution of oxygen vacancies, which is crucial for a comprehensive understanding of the mechanochromic mechanism. In general, the Ramman peaks at near 805 cm−1 and 715 cm−1 in the WO3 lattice are derived from the tensile vibration of O-W-O and W-O50,51 Thus, the Ramman peak at 774 cm−1, which shifts toward a higher wavenumber in the EWO-B state (Supplementary Fig. 13a), may suggest a weakening W-O bond strength or a decrease in oxygen vacancy. The electron paramagnetic resonance (EPR) signal is located at around g = 2 and can be assigned to reduced europium (S = 7/2) or single-electron-trapped oxygen vacancies (\({V}_{O}^{\cdot }\))52. Compared with the EPR signal of Eu2+, the \({V}_{O}^{\cdot }\) signal is quite narrow with a line width of about 553, so the enhancement in the EPR signal provides further evidence of the generation of color centers during the mechanochromic process (Supplementary Fig. 13b). Positron annihilation lifetime (PAL) spectroscopy measures the lifetime of the positrons, which is extended in regions of low electron density, making it a sensitive method for detecting defects in materials54. Supplementary Fig. 14 describes the PAL spectra of the EWO-A and EWO-B, which can be well fitted by two-component fitting (Supplementary Table 3). The shorter and longer lifetime components are both extended, while the average lifetimes decrease, further demonstrating that the \({V}_{O}^{\cdot }\) works more effectively during the force stimulation process (see more detail in Supplementary Note 1).

X-ray absorption near-edge spectroscopy (XANES) is an element-selective technique used to quantitatively investigate the local valence states in oxides55. The Eu L3-edge XANES spectra of EuWO4 before and after 30 MPa pressure stimulation for 10 min are described in Fig. 2d, with commercial EuO and Eu2O3 serving as references for Eu2+ and Eu3+ species. A distinct white line can be observed, which allows for the examination of the Eu average valence states of the EuWO4 host based on the photon energy of the white line. The calculation results indicate an increase in Eu average valence from 2.72 to 2.77 after applying 30 MPa pressure for 10 min, further confirming the cationic transformation from Eu2+ to Eu3+ during force-induced coloration. Figure 2e depicts the W L3-edge XANES spectra of EWO-A, EWO-B, tungsten foil, and standard WO3. By subtracting the differential XANES spectrum of tungsten foil (W0) from that of EWO-A and EWO-B (Supplementary Fig. 15), the average oxidation states of W element are derived using the integration of the white-line peak, revealing a decrease in W valence from 5.35 to 5.15 under pressure stimulation. These results align with conclusions drawn from the XPS technique, and the mechanochromic and PL regulation mechanisms in EuWO4 are summarized in Fig. 2f and Supplementary Fig. 16. Upon force stimulation, the valence transformation from Eu2+ to Eu3+ occurs, accompanied by the W6+ → W5+ transition and a decrease in oxygen vacancies for charge compensation. Subsequently, the intervalence charge transfer absorption band between different tungsten oxidation states and the formation of color centers collectively contribute to the coloration of EuWO4. Exploiting the strong visible light absorption of the black phase and the additional nonradiative transition channels, the Eu3+ PL is entirely quenched in EWO-B, thereby enabling a reversible PL on-off switching.

Reversible multicolor chromism of europium tungstate is achieved through various external stimuli (Fig. 3a), including force, light, and temperature fields. Negative photochromism of EWO-B is observed under 532 nm laser irradiation (process 3), resulting in a color change from black to white. For simplicity, the white europium tungstate state is marked as EWO-C. The reflectance of the EWO-B state in the whole visible spectral region gradually increases with the 532 nm laser (60.37 W/cm2) irradiation duration lengthening, saturated at 3 s (Fig. 3b). In addition to the 532 nm laser, negative photochromism of the black EWO-C is also observed under 473 nm laser irradiation, with the coloration rate being dependent on both the wavelength and power density of the laser (Supplementary Fig. 17). Additionally, the 808/980 nm lasers and a low-power 532 nm (7.59 W/cm2) laser were employed as light sources to further study the properties in process 3 (Supplementary Fig. 18), suggesting that the color change from black to white is primarily attributed to negative photochromism rather than laser-induced thermal effects. To revert the white EWO-C sample to its original orange EWO-A state, a heat treatment in a reducing atmosphere (95% N2 + 5% H2) is required. Figure 3c illustrates the diffuse reflection spectra of the EWO-C heated at 700 °C and 800 °C for various holding times, showing a redshift for the absorption band edge after thermal treatment at reducing atmosphere. After 800 °C sintering for 10 min, the white EWO-C state can completely return to the initial orange EWO-A state (process 4).

Fig. 3: Force-light-heat stimulation induced multicolor chromic phenomenon in europium tungstate.
figure 3

a Diagram of multi-color chromism. b Diffuse reflection spectra and corresponding digital photos of EWO-B as a function of 532 nm laser (60.37 W/cm2) irradiation durations. c Diffuse reflection spectra of EWO-C sintered at various temperatures and durations in a reducing atmosphere. d XRD patterns of EWO-A, EWO-B, and EuWO4-C (EWO-C). e W 4 f (bottom) and Eu 3 d (top) high-resolution XPS spectra of the EWO-C state. f W (bottom) and (top) L-edge XANES spectra of the EWO-C state. Tungsten foil and standard WO3 are used as references for W testing, and commercial EuO, and Eu2O3 are employed as standards for Eu determination. g The PLE spectra of the EWO-A and EWO-C monitoring at 614 nm. The insert shows the [WO6] octahedra and [WO4] tetrahedra in Eu2W2O9 and EuWO4 lattices, respectively.

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To investigate the mechanism underlying the multicolor chromism, specifically the transition from orange or black to white in europium tungstate, XRD patterns of EWO-A, EWO-B, and EWO-C were compared, as shown in Fig. 3d. The phase transition from EuWO4 to Eu2W2O9 is induced by 532 nm irradiation of EWO-B, which reverts to the EuWO4 phase upon sintering in a reducing atmosphere. The band gap of the EWO-A and EWO-C states was estimated from their absorption spectra, as shown in Supplementary Fig. 19, using the Tauc method56. The calculated bandgap is 2.06 eV (EWO-A) and 3.30 eV (EWO-C), aligning with the reported values for EuWO4 (1.97 eV) and Eu2W2O9 (3.2 eV) in the literature57,58. To determine the valence states of europium and tungsten in the EWO-C, high-resolution XPS spectra of Eu 3 d and W 4 f for the EWO-C state were measured (Fig. 3e). The Eu 3 d XPS spectra were deconvoluted into four Gaussian peaks, confirming the coexistence of Eu2+ and Eu3+ ions. The W 4 f XPS spectra exhibit peaks at 35.2 eV and 37.38 eV, with a spin-orbit splitting of 2.18 eV, corresponding to the 4 f5/2 and 4f7/2 spin-orbit doublets of W6+ ions in Eu2W2O9. The W 4f XPS peaks of the EWO-C state shift to higher binding energies compared to those of the EWO-B state (Supplementary Fig. 20a), suggesting a decrease in the electron density around the W in Eu2W2O9. This shift is likely attributed to the higher coordination of W with oxygen and the weaker polarization effect of the W-O bond in Eu2W2O9, relative to that in EuWO459. The W and Eu L3-edge XANES spectra were collected to further quantitatively confirm the valence of W and Eu in the EWO-C state (Fig. 3f, Supplementary Fig. 21). The average valence of W and Eu are calculated to be 5.83 and 2.87 in the EWO-C state, respectively. The above results demonstrate that, in the EWO-C state, the proportion of Eu2+ and W5+ ions remains low compared to the EWO-A and EWO-B states, which leads to a relatively lower concentration of oxygen vacancies (Supplementary Fig. 20b).

A Eu2W2O9 unit cell contains two Eu sites and a W site, in which the Eu1 site is coordinated by eight oxygen, the Eu2 site is surrounded by nine oxygen, and two W ions are both coordinated by six O2− ions forming [WO6] octahedra57. The different lattice structures of EuWO4 and Eu2W2O9 cause the variation in their PL spectra (Supplementary Fig. 22a). Figure 3g describes the photoluminescence excitation (PLE) spectra of the EWO-A and the EWO-C states monitoring at 614 nm, consisting of a broad band from 200 nm to 350 nm and several sharp peaks. It is found that the broad band of the EWO-A can be deconvoluted into two Gaussian peaks at 275 nm and 310 nm. In general, the excitation band at 275 nm in AWO4 (A=Ca, Sr, Eu) is associated with the charge transfer band of W6+--O2- within [WO4]2- unit60,61, while the excitation peak at 310 nm is usually assigned to the Eu3+--O2- charge-transfer states62. Only an excitation band centered at 310 nm can be detected in EWO-C, which is typically assigned to the absorption of W6+--O2- state within [WO6]6- group57,63. The difference in the PL and PLE spectra in the ultraviolet-to-visible spectral region provides additional evidence of a phase transition between Eu2W2O9 and EuWO4 during the process 4. Accordingly, the color change from black or orange to white should be attributed to the phase transition in europium tungstate.

Reversible color change from orange to white of europium tungstate can be achieved upon laser or heat treatment (process 5). Figure 4a describes the sintering time-dependent diffuse reflection spectra and corresponding digital photos of the EWO-A at 350 °C, 450 °C, and 550 °C in an air atmosphere. It is worth noting that the color of the sample in process 5 can be affected by the sintering temperature and durations. As the sintering time increases from 0 to 5 h, the color change of the EWO-A progresses from orange to brownish green, yellow ochre, ivory, and ultimately to white. The XRD patterns of the EWO-A sintering at 350 °C for various durations are presented in Fig. 4b to study the temperature-dependent multicolor mechanism in europium tungstate. The results show that the structure of the EWO-A gradually transitions from EuWO4 to Eu2W2O9 in process 5, with two-phase coexistence stages. The discrepancy in the proportion of the two phases leads to the generation of different color states. In addition to heat treatment in an air atmosphere, the negative photochromism of the orange EWO-A can also be achieved through laser irradiation, utilizing lasers with wavelengths of 473 nm (30.08 W/cm2), 532 nm (36.38 W/cm2), 808 nm (8.29 W/cm2), and 980 nm (9.67 W/cm2). Figure 4c illustrates the diffuse reflection spectra of the EWO-A sample upon exposure to these four laser stimuli, demonstrating a saturated negative photochromism after irradiation for 1 s. The saturation time is observed to be dependent on the laser power density (Supplementary Fig. 23), with lower power densities requiring longer irradiation durations to reach saturation. High-power density laser irradiation typically induces a significant localized thermal effect. Consequently, the surface temperature of the samples was monitored using thermal imaging (Fig. 4d, Supplementary Fig. 24). Upon laser irradiation for 1 s, the surface temperature of the EWO-A sample rapidly escalates, reaching approximately 350 °C. This observation suggests that the negative photochromism observed in EWO-A is predominantly driven by the laser-induced thermal effect.

Fig. 4: Reversible color change and luminescence modulation between the EWO-A and the EWO-C states.
figure 4

a Thermal treatment time-dependent diffuse reflection spectra of EWO-A at 350°C, 450°C, and 550°C in an air atmosphere. b XRD patterns of EWO-A sintering at 350°C at various time. Diffuse reflection spectra c and corresponding thermal mapping d of EWO-A by 473 nm (30.08 W/cm2), 532 nm (36.38 W/cm2), 808 nm (8.29 W/cm2), and 980 nm (9.67 W/cm2) laser irradiation for 1 s. e Under 395 nm excitation, PL spectra of the EWO-A by 808 nm laser irradiation at various power for 1 s (left) and the EWO-C is heated at various temperatures for various durations (right). f Emission decay curves (λex = 395 nm, λem = 614 nm) of the EWO-A and EWO-C states. g The variation of PL intensity at 614 nm in five cycles.

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Leveraging the distinct light-absorbing capacities of the EWO-A and EWO-C states, the PL intensity of the europium tungstate can be reversibly modulated (Fig. 4e). Under 395 nm excitation, the PL intensity at 614 nm increases by approximately 51.4-fold due to the reduced light absorbance in the visible region of the EWO-C state. Figure 4f depicts the PL emission decay curves (λex = 395 nm, λem = 614 nm) of the EWO-A and EWO-C states, which can be respectively well-fitted by a bi-exponential and mono-exponential equation (Supplementary Table 4). In the EWO-A state, the fast decay component should be attributed to the energy transfer from the Eu3+ to the traps associated with the high oxygen vacancy concentration64. Alternating between process 4 and process 5, the PL intensity of the europium tungstate is reversibly regulated within 5 cycles, with excellent stability and reproducibility (Fig. 4g).

Europium tungstate exhibits versatile multicolor chromic behaviors induced by force-light-heat stimuli, enabling its application in complex scenarios. For instance, the development of scientific submersibles is crucial for exploring deep-sea environments, which harbor unique ecosystems, untapped mineral resources, and critical geological structures. Modern scientific submersibles are capable of operating at depths of several kilometers, where the pressure can reach several tens of megapascals. Accurate and real-time pressure detection is vital for these submersibles to ensure structural integrity, supporting the safe execution of deep-sea scientific operations under extreme hydrostatic stress. The pressure-induced color change in EWO-A can be utilized for pressure monitoring during submersible operation, as illustrated in the proof-of-concept diagram in Fig. 5a. The operational depth of a submarine directly determines the hydrostatic pressure it must withstand, as pressure increases proportionally with depth in the marine environment. Under identical force stimuli, a smaller four-pointed star undergoes a more pronounced color change than a larger seven-pointed star, indicating greater pressure. The above results demonstrate that PL-off occurs when EWO-A fully transitions to EWO-B. Given the superior sensitivity of light signals over color signals, PL monitoring can be utilized to implement a pressure alarm system.

Fig. 5: Multi-functional applications of europium tungstate.
figure 5

a Pressure sensing based on mechanochromism and a light on-off switch in the EWO-A state. b Schematic diagram of the laser direct writing technique and mask method for optical storage and security. Digital photos of the sample for anti-counterfeiting c and optical storage d applications, achieved through processes 3 to 5.

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Additionally, the optical storage and security applications of europium tungstate can be developed by leveraging its negative photochromic process. In contrast to traditional photochromic materials, negative photochromism induces PL enhancement, leading to a higher signal-to-noise ratio for readout. Figure 5b shows the schematic of applying europium tungstate in optical storage and anti-counterfeiting. Using laser direct writing technology (808 nm and 532 nm laser), high-precision QR codes and dot patterns can be inscribed on the surface of the sample for information writing. The high luminescent efficiency of the EWO-C state allows for clear and accurate information readout. Various shapes of patterns can also be drawn on the surface of the sample through mask-based methods. Figure 5c presents the digital photos of the input flower patterns and Chinese characters using 808 nm and 532 nm laser irradiation, showing clear white patterns on the orange and black pallets. Red luminescence images with high resolution can be observed under 365 nm ultraviolet light excitation. The complex QR codes and dot patterns can also be accurately written on the surface of europium tungstate pallets, as shown in Fig. 5d. For example, The QR codes of “KUST” and “YZW” are input, which can be recognized through smartphone scanning mode. Aging test indicates the good chemical stability of EWO-A in various solutions (e.g. pure water, ethanol, and sea water) by storing over half year (Supplementary Fig. 25). These results demonstrate the prosing multi-functional applications of europium tungstate on pressure sensing, security, and optical memory based on its force-light-heat stimulation-induced multicolor chromic behaviors.

Discussion

In this study, europium tungstate phosphors were synthesized via a conventional high-temperature solid-state method, exhibiting a multicolor transition from orange to black to white upon force, light, and heat stimulation. The mechanisms underlying mechanochromism, negative photochromism, and thermal recovery are systematically explored. The color transition from orange to black is attributed to the pressure-induced W6+ to W5+ reduction and the formation of color centers. The negative photochromic shift from orange/black to white is primarily driven by the phase transition from EuWO4 to Eu2W2O9. Exploiting the exceptional visible light absorption properties of the black sample, the luminescence of Eu3+ is fully quenched, enabling the switching of light-on and light-off states. This demonstrates the potential of europium tungstate phosphors as pressure sensors, particularly for applications in maritime navigation. Furthermore, the reversible photo-induced color change behaviors allow for the creation of complex patterns and QR codes on the surface of europium tungstate pallets, highlighting their promising multifunctional applications in security and optical data storage.

Methods

Synthesis of europium tungstate

EuWO4 phosphors were prepared by a conventional high-temperature solid-state reaction method, in which Eu2O3(99.99%, Alladin) and WO3(99.99%, Alladin) were used as raw materials without further purification. In a typical synthesis procedure, 10 mmol Eu2O3 and WO3 were stoichiometrically weighted and then thoroughly mixed in an agate mortar by adding several drops of ethanol. Subsequently, the mixtures were transferred into a corundum crucible, which was respectively sintered at 950 °C, 1050 °C, and 1150 °C for 4 h with a heating rate of 5 °C/min at a reducing (95%N2 + 5%H2) atmosphere. It is worth noting that the airflow speed required to be held at 100 mL/min in the sintering process. The final products were obtained by cooling to room temperature and re-grinding process.

Characterization

A series of semiconductor continuous lasers were utilized to investigate the photo-induced coloration properties, including 473 nm (30.08 W/cm2), 532 nm (36.38 W/cm2), 808 nm (8.29 W/cm2), and 980 nm (9.67 W/cm2). A manual tablet press (L-30T, KEJING, China) was employed to carry out the mechanochromic experiment. The phase structure of the samples was determined by the D8 ADVANCE X-ray powder diffractometer (Bruker, Germany). Morphology and elemental distribution were examined using a scanning electron microscope with the type of TESCAN MIRA LMS (TESCAN, Czech Republic), installed with an energy dispersive spectrometer. The photoluminescence spectra were recorded by an F-7000 spectrophotometer (Hitachi, Japan) using a 150 W Xenon lamp as the light source. Diffuse reflection spectra were evaluated by a U-4100 UV-Vis-NIR spectrophotometer (Hitachi, Japan) equipped with an integrating sphere accessory. The emission decay curves were collected by an FLS1000 spectrometer (Edinburgh, UK) using a microsecond flash Xenon lamp as the light source. The electron paramagnetic resonance spectra were performed by a paramagnetic resonance spectrometer with type of Bruker X-band A300-6/1 (Bruker, Germany), in which a 9.2 GHz frequency was employed. The X-ray photoelectron spectra were measured by a K-Alpha X-ray photoelectron spectrometer system (ESCALAB 250Xi, Thermo Fisher, USA) with an Al Kα X-ray source (1486.6 eV) under vacuum conditions. The operating voltage was 14.6 kV, and the filament current was 13.5 mA. The W and Eu L3 edges of the sample were determined by the extended X-ray absorption fine structure technique using the BL14B2 instrument (SPring-8, Japan), and the corresponding results were sorted out by Athena and Artemis in the Demeter package. The PAL spectra were performed using a PAL spectrometer (DPLS3000, Anhui Nuclear and Electronics Technology Co., Ltd., China) with a time resolution of approximately 180 ps and a source intensity of 20 µCi, in which the tested sample were prepared disc with dimensions of 12 mm in diameter and 2 mm in thickness. The PAL results were calculated using the spectrum lifetime software Lifetime9. The PLQY of EWO-A was measured using a steady-state and transient fluorescence and phosphorescence spectrometer (Horiba Fluorolog-QM, Canada) at room temperature.