Introduction

Lead halide perovskites (LHPs), APbX3 (A = Cs, methylammonium MA, formamidinium FA; X = Cl, Br, I), continue to receive immense attention as burgeoning narrow-band emitters (NBEs) for optoelectronic devices, such as light-emitting diodes (LEDs), super-resolution imaging, and quantum light sources1,2,3,4. The emission bandwidth (EMW) of NBEs is of prime importance for improving efficiency of light sources and color purity of high-definition display5,6,7. Thanks to the narrow EMW with the full width at half-maximum (FWHM) of <40 nm, LHPs can be the access to opportunity for entering display market8,9,10. Especially, research progress in EMW of full-inorganic CsPbX3 and hybrid FAPbBr3 nano-crystals is a prompter for LPHs as single-photon sources facilitated by charged exciton fast nonradiative Auger decay11,12. However, the luminescence efficiency of LHPs single-photon sources plummets with EMW dwindling. Additionally, there remain issues with lead toxicity and unsatisfactory stability of LHPs.

Lead-free rare-earth Eu(II)-based perovskitoids (EuPs) have been developed recently as creating an alternative class of NBEs profited from strongly localized Eu2+-bound excitons and low phonon frequencies ( ~ 200 cm–1)13,14,15. They feature fortunate thermal stability and ultrahigh external quantum efficiency (ФEQE) of photoluminescence of over 50%. Yet, further suppression of EMW in EuPs remains a formidable challenge because the mechanism behind the small EMW is not well understood16,17,18,19. The relationship between the structure and the EMW at the microscopic level is critical to achieve ultra-narrow EuP-based emitters for state-of-the-art optoelectronic devices. Theoretically, the broadening of emission band is originated from inhomogeneous distribution of emission centers (inhomogeneous broadening) and the coupling with phonon vibrations in an excited electronic state (e.g., exciton) that shifts adiabatic potentials from the ground electronic state (Fig. 1a)20. The latter is an intrinsic property for a single emission center, and refers to as so-called the Franck–Condon shift (FCs), described by homogeneous broadening (Fig. 1a)18,21,22. Since the FCs is associated with lattice relaxation in the excited electronic state, and it can be strongly suppressed by tightly-connected coordination polyhedrons. It is therefore possible to reduce the EMW in self-activated EuPs by designing Eu(II) ions in tightly connected polyhedrons with uniform structural characteristics (Fig. 1b).

Fig. 1: The emission broadening mechanism.
figure 1

a Schematics of broadening mechanisms for the EMW with homogeneous broadening and inhomogeneous broadening. b Schemes of structural relaxations for softly/tightly-connected coordination polyhedrons.

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Here we design and employ five organic cations with distinct electrostatic potential (ESP) to dictate the structures of Eu(II)-halide polyhedrons. All of as-prepared EuPs consist of Eu octahedrons in face-shared connected pattern. The TEtEuBr3, characterized with more symmetrical Eu-Br octahedrons, displays highly efficient and thermally stable ultra-narrow-band cyan emission (FWHM ~ 36 nm, ФEQE ~ 59%, 93%@120 °C). By first-principles density-functional-theory calculations, the origin of the very small EMW in TEtEuBr3 is elucidated in terms of suppressed electron–phonon coupling. Accordingly, the phosphor-converted light-emitting diodes (pc-LEDs) have been fabricated and demonstrate low blue light hazard index (one-sixth of traditional white LEDs) and superior performances, including high color-rendering index of 96% in the lighting technology and larger color gamut of ~ 115% Rec. 2020 for display applications.

Results

Design of Eu(II)-based perovskitoids

We sought to design tightly-connected Eu(II)-halide polyhedrons by engaging five bulky organic ammonium molecules (Tetraethylammoniumwith TEt, triethylmethylammonium MeEt, Benzyltriethylammonium ByEt, 1-propenyl-3-methylimidazolium Emim, 1- ethyl-3-methylimidazolium Pmim) with different alkyl chains, phenyls and imidazoles (Fig. 2a). The electrostatic potential (ESP) of organic molecules were calculated (lower panel of Fig. 2a), indicating the electron-withdrawing ability of the vacant p orbital of nitrogen atom and iminazole/benzene units containing π–π interactions23,24. Moreover, the alkylamine groups (TEt and MeEt) exhibit stronger and more homogeneously symmetrical electron affinity, resulting in an increased barrier for distorted Eu-centered local structures.

Fig. 2: Structural design of narrow-band Eu(II)-based perovskitoids.
figure 2

a Molecular structures (upper panel) and electrostatic potential (lower panel) of organic cations (Tetraethylammoniumwith TEt (color bar, −0.091-0.091 eV), triethylmethylammonium MeEt (color bar, −0.086–0.086 eV), Benzyltriethylammonium ByEt (color bar, −0.062–0.062 eV), 1-propenyl-3-methylimidazolium Emim (color bar, −0.033–0.033 eV), 1- ethyl-3-methylimidazolium Pmim (color bar, −0.028–0.028 eV)). b Crystal structures of TEtEuBr3 and Pmim2EuBr4 perovskitoids plotted using VESTA. c Local connection patterns of face-shared [EuBr6]4– octahedrons in as-prepared Eu(II)-based perovskitoids. Carbon, hydrogen, nitrogen, bromine and europium are shown in dark gray, light gray, nattier blue, purple and pink, respectively.

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Therefore, we obtained single crystals of several Eu(II)-based perovskitoids (see the Experimental Section for details) and the crystal data were obtained by performing single-crystal X-ray diffraction (SCXRD measurements (details and crystal cell parameters are provided in Supplementary Tables 1, 2), namely TEtEuBr3, MeEtEuBr3, ByEtEuBr3, EmimEuBr3, Pmim2EuBr4 (Fig. 2b and Supplementary Fig. 1). Importantly, the compounds contain face-shared [EuBr6]4– octahedrons isolated by organic molecules (Fig. 2c), forming one-dimensional (1D) perovskitoid structures except for 0D Pmim2EuBr4 (Fig. 2b). This sort of face-shared structures showcase strong coupling between [EuBr6]4– octahedrons and good structural rigidity25. In particular, the 1D perovskitoid structures possess higher rigidity over the 0D one, as evidenced by the shorter Eu–Eu metallic bonding in 1D chains (3.945–4.005 Å) than in 0D clusters (4.033 Å) (Fig. 2c). Besides, we characterized powder X-ray diffraction (PXRD) patterns of all compounds to ensure their high purity (Supplementary Fig. 2).

Photoluminescence characterizations of Eu(II)-based perovskitoids

To evaluate effects of organic molecules on emission behaviors, we studied photoluminescence (PL) and photoluminescence excitation (PLE) spectra (Supplementary Fig. 3) of as-prepared Eu(II)-based perovskitoids. All compounds exhibit narrow-band cyan emissions peaking at about 490 nm (Fig. 3a). The Eu(II)-based perovskitoids with alkylamine groups display narrower EMWs than those including heterogeneous iminazole/benzene units, for example, TEtEuBr3 (FWHM ~ 36 nm) versus Pmim2EuBr4 (FWHM ~ 50 nm). This suggests that 1D perovskitoid structures are more beneficial than 0D perovskitoid structures for decreasing excited-state structural relaxation and hence reducing the EMW. The narrow-band cyan emissions are ascribed to localized 5d–4 f transitions of six-coordinated Eu(II)26, as also authenticated by PL decay times of 610–750 ns (Supplementary Fig. 4). Furthermore, PL internal/external quantum efficiency (ФIQE/ФEQE) measurements (Fig. 3b and Supplementary Figs. 5 and 6) show that TEtEuBr3 exhibits higher PL efficiencies (ФIQE ~ 96%, ФEQE ~ 59%) than those of other EuPs (Supplementary Table 3). Moreover, the emission intensity of TEtEuBr3 remains 93% of that at room temperature when increasing temperature up to 120 °C, showing excellent thermal stability (Fig. 3c). Interestingly, the TEtEuBr3 establishes relatively good air stability (PL intensity above 80% after 6 h) under humidity of 50% (Supplementary Fig. 7). Thus, TEtEuBr3 displays superior PL performance in optoelectronic device applications, which could be connected with its characteristic 1D perovskitoid structures.

Fig. 3: Photoluminescence of narrow-band Eu(II)-based perovskitoids.
figure 3

a Normalized PL spectra at room temperature. b PL internal/external quantum efficiency (ФIQE/ФEQE) and absorption efficiency (Abs), (the error bar, ±5%, from three measurements). c Temperature-dependent emission intensity with temperature range of 20–120 °C. d The time-resolved photoluminescence of TEtEuBr3, MeEtEuBr3, ByEtEuBr3, EmimEuBr3, Pmim2EuBr4 under 340 nm pulse laser diode excitation, where the x-axis is the PL decay time of each emission wavelength, y-axis is emission wavelength with spacing of 10 nm, the depth of color represents PL intensity (color bar, 10−3–103 arb.u.). e The FWHM of as-prepared Eu(II)-based perovskitoids by summarizing average values of 15 samples. f The full width at half maximum (FWHM) of as-prepared Eu(II)-based perovskitoids under variable excitation wavelengths (300–450 nm), frequencies (0.2–20 MHz) and power (5–100 mW) with 375 nm laser, (the error bar, ±3 nm, from five samples).

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The time-resolved photoluminescence (TRPL) of as-prepared Eu(II)-based perovskitoids revealed a similarly narrow-band cyan emission and PL decay times without extra emission peaks (Fig. 3d and Supplementary Tables 4, 5). Moreover, the FWHMs of as-prepared Eu(II)-based perovskitoids were measured by summarizing average values of FWHMs (20 samples) (Fig. 3e), and under variable excitation wavelengths, frequencies, and power (Fig. 3f). The FWHMs are nearly unchanged, consistent with the PL results (Fig. 3a), indicating that narrow-band emission of as-prepared EuPs demonstrates strong anti-interference performance.

Mechanisms of narrow-band emission in Eu(II)-based perovskitoids

As mentioned, distribution of emission sits and lattice relaxation in the excited electronic state caused by electron–phonon coupling contribute to inhomogeneous and homogeneous broadening of the emission band, respectively. To evaluate the influence of inhomogeneous site distribution, the FWHMs for narrow-band cyan EuPs including the five compounds under study and those reported previously were collected14,27, and the FWHM variation versus the number of crystallographically distinct Eu(II) sites is depicted in Fig. 4a. There is no clear correlation between the FWHM and the Eu(II) site number, implying that the site difference is not large enough to affect the FWHM. This point is also supported by the stable singly emission peak observed in TRPL under variable excitation wavelengths, frequencies, or power.

Fig. 4: Mechanisms of narrow-band emission.
figure 4

a The number of crystallographically distinct Eu(II) sites dependent full width at half maximum (FWHM) for narrow-band cyan Eu(II)-based perovskitoids. b The distortion index (Δd) and the bond angle variance (σ2) of polyhedral distortion in TEtEuBr3. c Huang–Rhys factor (S), FWHM and electrostatic potential (ESP). d The one-dimensional configurational coordinate model of TEtEuBr3 with ground state (A, D dots) and excited state (B, C dots). e The coordinate position change of each atom in TEtEuBr3 at the excited state relative to the ground state. f The bond length change of Eu-Br in TEtEuBr3 at the excited state (ES-C dot) relative to the ground state (GS-A dot). g Calculated vibrational density of states (DOS) (upper panel) and Raman spectrum (lower panel) in the experiment for TEtEuBr3. h The distribution map of calculated Huang–Rhys factor. i Calculated emission spectrum compared with the experimental result. The calculated curve was shifted to align its maximum with that of the experimental emission spectrum for better comparison.

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As for the contribution of homogeneous broadening to the EMW, we first collected the structural characteristics of each distinct [EuBr6]4– octahedrons in the five EuPs, in terms of the distortion index (Δd) to characterize the deviation in bond lengths, and the bond angle variance (σ2) to quantify the distortion in bond angles28. The results are displayed in Fig. 4b, showing that the overall polyhedral distortion in TEtEuBr3 is the smallest, in line with the narrowest EMW observed for this compound. Note that 0D Pmim2EuBr4 with soft structure exhibits the largest variation in Δd. Thus, more symmetrical Eu(II) sites bring about smaller excited-state lattice relaxation resulting in narrower emission bands.

Next, the temperature dependence of the FWHM of the emission band was investigated by using the classical equation from low-temperature PL measurements (Supplementary Fig. 8)22,29

$${{{\rm{{FWHM}}}}}\left(T\right)=2.36\sqrt{S}{{\hslash }}\omega \sqrt{\coth \frac{{\hslash }\omega }{2{kT}}}$$
(1)

where S is Huang–Rhys (H-R) factor describing electron−phonon coupling strength, ħω is the phonon vibrational frequency, k is the Boltzmann constant, and T is temperature. By fitting the above expression to the measured temperature-dependent FWHMs (Supplementary Fig. 9a), we found that the H-R factor is the smallest for TEtEuBr3 (S = 1.66), in agreement with the above structural analysis. Interestingly, the derived H-R factors exhibit a positive correlation with ESPs of the organic molecules in the five compounds (Fig. 4c). Thus, organic molecules with stronger and more homogeneously symmetrical ESPs produce face-shared [EuBr6]4– octahedrons with higher local symmetries, leading to weaker electron–phonon coupling and hence smaller EBWs. Moreover, TEtEuBr3 displays the smallest variation of the emission peak wavelength with temperature (Supplementary Fig. 9b), indicating again its high structural rigidity. The thermal quenching activation energy for TEtEuBr3 was estimated to be 187.6 meV which is the highest among the five compounds (Supplementary Fig. 9c), in consistence with its high PL efficiency.

To delve into the intricacies of the microscopic mechanism behind the small EMW, we conducted explicit calculations on the electron–phonon coupling for TEtEuBr3. Hybrid DFT calculations in the 1×1×2 supercell model (containing 528 atoms with 16 Eu atoms) were performed on the structural and electronic properties, with the lowest excited electronic state being simulated by constraining the occupancies of the DFT eigenlevels. The density of states (DOS) for the ground and excited electronic state shows that the valence band top and the conduction band bottom are composed of Eu(II)-4f and 5 d orbitals (Supplementary Fig. 10a), respectively, and the promotion of an electron from occupied 4 f to empty 5 d eigenlevels results in the appearance of an occupied 5 d state inside the band gap (Supplementary Fig. 10b). Charge densities projected onto the 4f-hole and the 5d-electron states reveals that both states are mostly localized on the same Eu(II) center (Supplementary Fig. 11), reflecting the strong electron-hole attraction. The attraction energy was calculated to be Eattr = E(GS) + Eg – E(ES) = 0.63 eV, where Eg = 3.48 eV is the fundamental bandgap energy, and E(GS) and E(ES) are the ground and unrelaxed excited electronic state energies, respectively. The narrow-band emission in the EuP, therefore, is originated from 5d–4 f transitions localized on individual Eu(II) centers.

The calculated FCs for the ground (EFC,g) and excited (EFC,e) electronic states of TEtEuBr3 are 0.059 eV and 0.138 eV, respectively (Fig. 4d). The excited-state potential energy landscape is steeper than that of the ground state, showing larger crystal-field interaction of the 5 d electron than of the 4 f electrons, which is beneficial for narrowing the emission band. Figure 4e and Supplementary Fig. 12 depict atomic-position rearrangements in the excited electronic state (point C in Fig. 4d) relative to the ground electronic state (point D in Fig. 4d). The structural changes are mainly concentrated on the 6 coordinating Br atoms and the two adjacent Eu atoms, with the Eu–Br bond lengths and Eu–Eu distances shortened by 0.055–0.065 Å and 0.068 Å, respectively (Fig. 4f). Therefore, the equilibrium structure in the excited electronic state in TEtEuBr3 differs little from that in the ground electronic state, which is conductive to generating the narrow band emission.

We calculated the lattice vibrations of the TEtEuBr3 supercell, and the atom-projected phonon density of states are shown in the upper panel of Fig. 4g. The phonon frequency distribution agrees well with experimental Raman spectrum (lower panel of Fig. 4g). It shows that the Eu(II)- and Br-associated vibrations lie in the low frequency range of 0–150 cm–1. Similar behaviors were observed for the other four compounds (Supplementary Fig. 13). By projecting the atomic-position rearrangements from the ground to the excited electronic state onto the phonon eigenvectors, the contribution of each phonon mode to the structural change were evaluated, as described by H-R factors (lower panel of Fig. 3h). The largest H-R factors (1.12, 0.53 and 0.36) are associated the phonon modes involving mainly the motions of Eu(II) and Br atoms with frequencies of 104.4 cm–1 (upper panel of Fig. 4h), 104.2 cm–1, and 104.1 cm–1, respectively (Supplementary Fig. 14). With the calculated H-R factors, the emission band shape was calculated by the generating function approach, and a good agreement with the experimental data is shown in Fig. 4i. These results support that TEtEuBr3 is characterized with low electron–phonon coupling, giving rise to ultra-narrow-band emission.

LEDs for lighting and display by Eu(II)-based perovskitoids

Despite extensive efforts to improve efficiency and color purity in recent decades, existing LEDs cannot be reconfigured to meet harmless light source due to service of blue excitation source30. For instance, the most potent bule region (446–477 nm) suppresses melatonin in healthy human subjects, inducing circadian-phase shifting or autonomic stimulation31. Herein, we construct a white pc-LED device (LED3) using TEtEuBr3 with the commercial phosphors (YAG:Ce3+ and K2SiF6:Mn4+) embedding into NUV InGaN LED chips (λ  =  405 nm) to evaluate the practical performance as harmless light source (Fig. 5a). By comparison with two commercial consulting solutions of white pc-LEDs, i.e., blue (LED1) /NUV (LED2) LEDs combining to phosphors (YAG:Ce3+ and K2SiF6:Mn4+), LED3 has an excellent color render index (Ra = ~96), a color temperature (CCT) of 6243 K, CIE color coordinate (0.311, 0.341), as well as lower blue light hazard index (Lλ of ~one-sixth of traditional white LEDs) (Fig. 5b and Supplementary Fig. 15a). It can act as harmless light source for full-spectrum lighting. Additionally, LED3 also exhibits good fatigue life and stability, with unchanged emission intensity after 500 h of continuous operation (Fig. 5c).

Fig. 5: Applications in LED devices.
figure 5

a The emission spectra of white LED1, LED2 and LED3 devices at a current of 20 mA. b The color render index (Ra) and light hazard index (Lλ) of white LED1, LED2 and LED3 devices, (the error bar, ±4, from three LED devices). c The fatigue life and stability of LED3 device, the inset is photograph of LED3 (scale bar, 5 mm). d The emission spectra of four-color LED device. e Chromaticity (x, y) of as-fabricated four-color LED device with RGB band pass filters in comparison with Rec. 2020. f Comparison of color gamut for as-prepared four-color LED device and commercial LED display screens. g Schematic diagram of color mixing technique for four-color display, the inset is fluorescence microscopy picture of as-prepared micro-LED (scale bar, 10 μm) plotted using Microsoft PowerPoint. h The imaging of four-color display by as-prepared micro-LED display screen with light up four-color emission “SCUT” (scale bar, 1 cm).

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Moreover, TEtEuBr3, as an innovated narrow-band emitter, represents an attractive candidate for wide color gamut display. We proposed and fabricated four-color LED devices with the green-emitting Eu(II) hybrids, commercial red phosphor K2SiF6:Mn4+ and a 450 nm blue InGaN chip. Figure 5d displays the emission spectrum of the LED device under a current of 50 mA, obtaining high luminous efficacy of 102 lm W−1, CCT ~ 7280 K and CIE color coordinate (0.303, 0.298) (Supplementary Fig. 15b). Benefitting from expanded four-color display technology, the four-color LED device achieves an ultrahigh color gamut of 115% Rec. 2020 standard (Fig. 5e), which is superior to the state-of-the-art LED display technologies, including OLED, CC Mini-LED, and Micro-LED5,32, as shown in Fig. 5f. Additionally, we fabricated pc-LED devices using TEtEuBr3 and commercial narrow-band phosphors (β-SiAlON:Eu2+ and BaSi2O2N:Eu2+) for comparison, as shown in Supplementary Fig. 16. The pc-LED using TEtEuBr3 reaches a high photoelectric efficiency of ~25% at 20 mA, and excellent long-term operational stability (>96% after 600 h), thermal stability (>90% @ 120 °C), and humidity stability (Supplementary Figs. 17–19). Taking advantage of characteristics of solution-processed TEtEuBr3, we fabricated micro cyan conversion layers in quartz panels with pixel size ~5 μm (inset of Fig. 5g). By color mixing technique (Fig. 5g), four-color display image “SCUT” is illustrated, as shown in Fig. 5h.

In summary, we develop lead-free Eu(II)-based perovskitoids as a burgeoning family of narrow-band emitters. The emission bandwidths have been decreased via employing rigid face-shared connected [EuBr6]4– octahedrons, and the suppressed electron–phonon coupling contributes to reduce the homogeneous broadening of Eu(II) therein. Typically, TEtEuBr3 with symmetrical octahedrons accomplishes ultra-narrow-band cyan emission (FWHM ~ 36 nm). The as-fabricated pc-LEDs show a lower blue light hazard index (one-sixth of traditional white LEDs), with superior performance (high color-rendering index of 96% as full-spectrum white light sources and larger color gamut of ~ 115% Rec. 2020) for four-color display devices. Our results open an untapped avenue to design and tailored narrow-band emitters for state-of-the-art optoelectronic applications.

Methods

Synthesis of TEtEuBr3, MeEtEuBr3, TEtEuBr3, and EmimEuBr3 perovskitoids

A total of 1 mmol of EuBr2 (99.5%, Grirem Advanced Materials Co. Ltd.) and 1 mmol of the corresponding organic molecule (tetraethylammonium bromide, triethylmethylammonium bromide, benzyltriethylammonium bromide, or 1-propenyl-3-methylimidazolium bromide; 99%, Alfa Aesar) were combined with 3–6 mL of anhydrous methanol (99.7%, Macklin) in a 10 mL Pyrex bottle. The mixture was heated to 60 °C under continuous stirring for 1 h to obtain a homogeneous solution. Subsequently, the bottle was placed in a glovebox for crystal growth via slow evaporation at room temperature. Viridescent single crystals formed after one week. All manipulations were conducted in a nitrogen-filled glovebox with H2O and O2 levels maintained below 0.1 ppm.

Synthesis of Pmim2EuBr4 perovskitoids

For the synthesis of Pmim2EuBr4 perovskitoids, 1 mmol of EuBr2 (99.5%, China Grinm Group Co. Ltd.) and 2 mmol of 1-ethyl-3-methylimidazolium bromide (99%, Alfa Aesar) were mixed and sealed in a vacuum quartz tube at approximately 5 × 10−4 Pa (Partulab device, MRVS-1002). The sealed quartz tubes were heated to 230 °C for 12 h and then cooled to 25 °C at a rate of 3 °C h−1 to obtain single crystals. All procedures were performed in a nitrogen-filled glovebox with H2O and O2 levels maintained below 0.1 ppm.

Fabrication of pc-LEDs

The fabrication of pc-LEDs involved several configurations: LED1 was created by adding commercial phosphors (YAG:Ce3+ and K2SiF6:Mn4+) onto a blue LED chip (λmax = 450 nm). LED2 was assembled using the same commercial phosphors (YAG:Ce3+ and K2SiF6:Mn4+) on near-ultraviolet (NUV) InGaN LED chips (λ = 405 nm). LED3 was fabricated by incorporated TEtEuBr3 along with commercial phosphors (YAG:Ce3+ and K2SiF6:Mn4+) on NUV InGaN LED chips (λmax = 405 nm). A four-color LED device was fabricated by combining TEtEuBr3 with green-emitting Eu(II) hybrids and the commercial red phosphor K2SiF6:Mn4+ on a 450 nm blue InGaN chip.

Characterization

Single-crystal X-ray diffraction was conducted using an XtaLAB Synergy R X-ray single-crystal diffractometer equipped with a hybrid pixel array detector and a Mo Kα radiation source (λ = 0.71073 Å) at 150 K. The structures were solved by direct methods and refined using full-matrix least squares on F2 using the SHELX-2018 program package. Crystallographic data and details of structural refinements are provided in Supplementary Tables 1, 2. The Cambridge Crystallographic Data Centre has assigned CCDC numbers 2405465 (ByEtEuBr3) [https://doi.org/0.5517/ccdc.csd.cc2lr2n3], 2405465 [https://doi.org/0.5517/ccdc.csd.cc2lr2n3], 2405466 (Pmim2EuBr4) [https://doi.org/0.5517/ccdc.csd.cc2lr2p4], 2405467 (MeEtEuBr3) [https://doi.org/0.5517/ccdc.csd.cc2lr2q5], 2405468 (TEtEuBr3) [https://doi.org/0.5517/ccdc.csd.cc2lr2r6], 2405469 (EmimEuBr3) [https://doi.org/0.5517/ccdc.csd.cc2lr2s7], these supplementary crystallographic data can be obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif.

Powder X-ray diffraction (XRD) data were collected using a D8 Advance diffractometer with Cu Kα radiation (λ = 1.541862 Å) at room temperature, operating at 40 kV and 15 mA.

Photoluminescence excitation (PLE), photoluminescence (PL), photoluminescence quantum yield, and PL decay spectra were measured using an FLS1000 fluorescence spectrophotometer (Edinburgh Instruments). The PL spectra, color-rendering index (Ra), and correlated color temperature (CCT) of the fabricated pc-LEDs were collected using an integrating sphere spectroradiometer system (ATA-1000, Everfine).

Computational methods

The TEtEuBr3 crystal was modeled by a periodic 1 × 1 × 2 supercell containing 528 atoms. The antiferromagnetic arrangement between adjacent Eu2+(4f7) ions along the 1D chain was adopted, which was found to be more energetically stable than the ferromagnetic arrangement. The structural and electronic properties of the supercell in the ground and excited electronic states were calculated by periodic DFT with the hybrid HSE06 functional33,34, while the phonon vibration eigenvalues and eigenvectors were calculated with the PBE + U method35,36,37, as implemented in the VASP package38,39. The electrons of Eu (5s25p64f76s2), Br(4s24p6), N(2s22p3), C(2s22p2), and H(1s1) were treated as valence electrons, and their interactions with the respective cores were described by the projected augmented wave (PAW) method40. The geometry optimizations were performed until the total energies and the forces on the atoms converged to 10−6 eV and 0.01 eV Å−1, respectively. Due to the large size of the supercell, one k-point Г was adopted to sample the Brillouin zone and the cutoff energy for the plane wave basis was set to 430 eV.

The emission bandshape for an electronic transition can be written as \(I\left(\omega \right)=C{\omega }^{3}A(\omega )\), where C is the normalization factor, ω is the angular frequency and \(A(\omega )\) is the lineshape function. The lineshape function can be calculated using the generating function approach under the Franck-Condon and parallel-mode approximations, provided that only the lowest vibrational level of the excited electronic state is occupied such as at low temperatures41,42,43,44. The lineshape function is expressed as

$$A\left({E}_{{ZPL}}-{{\hslash }}\omega \right)=\frac{1}{2\pi }{\int }_{-\infty }^{\infty }G\left(t\right){e}^{i\omega t-\gamma t}{dt}$$
(2)

where γ is a parameter representing homogeneous broadening and EZPL is the transition energy of the zero-phonon line. G(t) is the generating function given by G(t) = eS(t)–S(0), where S(t) is the Fourier transformation of the spectral function \(S\left(\omega \right)\), given by

$$S\left(\omega \right)={\sum}_{k}{S}_{k}\delta (\omega -{\omega }_{k})$$
(3)

where Sk is the partial Huang-Rhys (H-R) factor of the phonon mode with frequency ω. In the present study, the δ-function was approximated by a normalized Lorentzian function with width 240 cm–1. It can be expressed as \({S}_{k}=\frac{1}{2{{\hslash }}}{\omega }_{k}{\left(\triangle {Q}_{k}\right)}^{2}\), where the coordinate difference \({\triangle Q}_{k}\) is defined as

$${\triangle Q}_{k}={\sum}_{a}\sqrt{{m}_{a}}({R}_{a}^{e}-{R}_{a}^{g})\cdot {u}_{k,a}$$
(4)

ma is the mass of atom a, \({R}_{a}^{e}\) and \({R}_{a}^{g}\) are the equilibrium positions of atom a in the excited and ground electronic states, and \({u}_{k,a}\) is the displacement vector of atom a in the phonon mode k.