Tunable Cold/Warm White Light Obtained via Reversible Phase Transition of Antimony-Doped Indium Chlorides

Abstract

Metal halides with efficient, stable, and tunable white light emission are ideal for lighting applications, because their emission properties can be effectively optimized through rational doping and precise compositional engineering. However, the synthesis of such materials often requires strict conditions and complex procedures. In this work, we report a phase transition from Sb-doped Cs₂InCl₅·H₂O to Cs₂NaInCl₆, along with tunable white light emission. Partial substitution of Na⁺ enables the formation of high-energy multiple emission centers, resulting in efficient white light with an adjustable correlated color temperature ranging from 2500 K to 5000 K under 365 nm excitation. The photoluminescence quantum yield reaches up to 45.24%. Efficient energy transfer among emission centers and the doping concentration of Na⁺ are critical for achieving high-performance tunable white light. The synthesized Cs₂Na_xInCl_5+x:Sb composite exhibits excellent stability under ultraviolet irradiation and environmental conditions such as oxygen and humidity, even after 200 h of ultraviolet irradiation, the emission spectrum remains stable, with more than 80% of its initial efficiency being preserved. Our results show its potential for advanced lighting applications and provide valuable insight for a desirable emission-tunable metal halide design.

1. Introduction

Global lighting electricity consumption represents approximately 20% of the total societal electricity consumption. Substituting incandescent lamps with white light-emitting diodes (WLEDs) for lighting purposes constitutes an economically viable and efficient strategy to reduce electricity consumption [1]. WLEDs have become predominant in the field of solid-state lighting owing to their significant advantages, including energy efficiency, environmental friendliness, and ease of adjustment [2,3]. Lead halide perovskites (LHPs) have garnered significant attention owing to their exceptional optoelectronic properties, low fabrication costs, straightforward solution-based preparation processes, and substantial application potential in solar cells, LEDs, and scintillators [4,5,6,7,8]. However, the intrinsic toxicity and relatively poor chemical stability of LHPs have significantly impeded their practical applications [9]. Moreover, LHPs demonstrate high vulnerability to water, oxygen, heat, and light. Ion migration under intense light or electric field conditions leads to significant material degradation, which poses a critical challenge given that the operational environment of photovoltaic materials inherently involves exposure to light fields or electric fields [10,11,12]. Cryolite halides or halide double perovskites (A₂B′B″X₆) (HDPs) possess crystal structures that are analogous to those of lead-containing halide perovskites [13,14]. By replacing toxic lead ions with non-toxic metal cations, these materials emerge as promising substitutes for traditional lead-based halide perovskite materials [15,16,17,18]. However, the band gap of such materials is typically wide, and their photoluminescence (PL) is mainly attributed to parity-forbidden transitions, leading to relatively low efficiency. Doping has been demonstrated as one of the most effective and reliable strategies for modulating the optoelectronic properties of HDPs. By disrupting spin-confinement transitions and introducing additional emission centers, it is possible to simultaneously enhance the luminescence efficiency of the material and adjust its optical band gap [19,20,21]. By incorporating Sb³⁺ into Cs₂NaInCl₆ HDPs, bright blue emissions can be observed [22,23,24,25]. Furthermore, doping Cs₂InCl₅·H₂O with Ag⁺ or Bi³⁺ results in yellow-emissive materials exhibiting enhanced luminous efficiency [18,26,27].

Moreover, elastic deformation caused by excited electrons and holes lowers the system energy, thereby generating a pronounced Stokes shift that efficiently prevents self-absorption. Utilizing self-trapped exciton (STE) materials as a single light-emitting layer in electroluminescent devices can minimize blue light pollution at the source, eliminating the need for a blue excitation source. At the same time, this strategy circumvents self-absorption effects, reduces secondary energy losses inherent in traditional LED excitation processes, and simplifies the overall device architecture. In recent years, extensive research has been conducted on metal–organic frameworks and perovskites for full-color WLEDs. Among them, metal halide perovskites with broadband emission characteristics are promising candidate materials for addressing issues related to mixed-component photon reabsorption and color instability. Karunadasa [28] reported a two-dimensional organic–inorganic perovskite featuring undulating lead halide layers, which function as an efficient broadband white light emitter. Ma et al. [29] synthesized [(EDBE)PbBr₄] (where EDBE = 2,2′-(ethylenedioxy)bis(ethylammonium)), successfully achieving white light emission with a correlated color temperature (CCT) of 6519 K and Commission Internationale de l’Eclairage (CIE) coordinates of (0.30, 0.42). Tang et al. [30] successfully incorporated Na⁺ into Cs₂AgInCl₆, thereby modulating the parity of the wave function of STEs to suppress the parity-forbidden transition and effectively lowering the electronic dimensionality of the semiconductor. Following Bi³⁺ doping, a broadband white light emission with a fluorescence quantum efficiency of approximately 86% has been observed. Double perovskites based on In³⁺ hold great promise for white lighting applications. However, the majority of current studies focus on Ag-In mixed systems, where the emission originates from excitons trapped in [AgCl₆]⁵⁻ octahedra. A significant challenge lies in the susceptibility of Ag⁺ ions to reduction into elemental silver, which can compromise the structural integrity of the crystal lattice. Furthermore, the emission spectrum of this system lacks critical blue (400–480 nm) and red (610–700 nm) components, thereby limiting its ability to achieve a high color rendering index (CRI) and tunable CCT, both of which are crucial for advanced lighting applications. Shi et al. [15] successfully doped Sb³⁺ and Bi³⁺ ions into the Cs₂NaInCl₆ crystal, resulting in the observation of white light with a PL quantum yield (QY) of 77%. The blue emission originates from the Jahn–Teller effect induced by the incorporation of Sb³⁺ ions. Meanwhile, the introduction of Bi³⁺ ions, which possess a larger ionic radius, further deforms the [SbCl₆]³⁻ octahedron, thereby enhancing the emission intensity of the yellow STEs. Theoretical calculations demonstrate that the incorporation of Bi³⁺ doping introduces new subband energy levels, thereby resulting in yellow STE emissions. By precisely adjusting the Sb³⁺/Bi³⁺ doping ratio, it is feasible to modulate cold and warm white light emission. In the same year, a scalable aqueous synthesis strategy was established for the preparation of Sb³⁺-doped Cs₂InCl₅·H₂O/Cs₂NaInCl₆ systems under ambient conditions, culminating in the fabrication and characterization of a prototype white light-emitting diode [31]. Through precise stoichiometric control of Sb³⁺ (blue-emitting sensitizer), Tb³⁺ (green-emitting activator), and Sm³⁺ (red-emitting activator) ions, Li et al. successfully achieved high efficacy and spectrally tunable white light emission in the Cs₂NaInCl₆:Sb³⁺,Tb³⁺, and Sm³⁺ double perovskite system [32]. These results substantiate that this family of double perovskite phosphors demonstrates considerable potential for solid-state lighting applications, particularly in lighting devices with a high color rendering index.

In this study, we demonstrate a one-pot synthetic strategy for the fabrication of white light-emitting indium-based halides. Cs₂InCl₅·H₂O, a lead-free compound with excellent oxidation stability, was chosen as the host material and subsequently doped with Sb³⁺ ions. The incorporation of NaCl effectively modulates the bandgap configuration and energy transfer dynamics within the system, resulting in the formation of Sb-doped Cs₂InCl₅·H₂O and Cs₂NaInCl₆ white light-emitting complexes. These materials are capable of emitting highly efficient white light, with tunable CCT ranging from 2500 K to 5000 K. The proposed synthesis method is characterized by its simplicity, scalability, and reproducibility. This study provides valuable insights into the development of WLEDs that are highly efficient, chemically stable, and capable of color-temperature tuning under single-excitation conditions.

2. Materials and Methods

2.1. Materials

Cesium chloride (CsCl, 99.5%), indium(III) chloride tetrahydrate (InCl₃·4H₂O, 99.9%), antimony trichloride (SbCl₃, 99.9%), and sodium chloride (NaCl, 99.5%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Hydrochloric acid (HCl, 36.5% in water by weight) and ethanol anhydrous (CH₃CH₂OH, ≥99.7%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All materials were used as received.

2.2. Synthesis of Cs₂Na_xInCl_5+x:Sb

For Cs₂InCl₅·H₂O:Sb, 336.8 mg of CsCl (2 mmol), 234.6 mg of InCl₃·4H₂O (0.8 mmol), and 45.6 mg of SbCl₃ (0.2 mmol) were accurately weighed into a 20 mL round-bottomed flask. Subsequently, 5 mL of concentration 36.5% HCl was precisely measured and added to the flask. This resulted in the formation of suspensions containing the precursor materials of cesium chloride, indium chloride, and antimony chloride. The round-bottomed flask was placed in the oil bath and connected to a straight condenser, and a rubber tube was secured to the upper end of the condenser. The mixture was stirred and heated at 92 °C with a rotational speed of 600 rpm for 30 min to obtain the phosphor suspension. After the reaction mixture was rapidly cooled with 5 mL of ethanol, suction filtration was performed, followed by washing the solid with an additional 5 mL of ethanol. Suction filtration was repeated and the powder was subsequently vacuum-dried at 50 °C for 5 h to obtain the final product, yellow Cs₂InCl₅·H₂O:Sb. When synthesized samples with varying NaCl concentration, only the amount of introduced NaCl was adjusted. As a result, fluorescent materials that emit white and blue light (Excited at 365 nm) were successfully obtained.

2.3. Characteristics

XRD patterns of powders were collected using an X-ray diffractometer (Bruker-AXS D8 ADVANCE, Bruker, Karlsruhe, Germany.) with Cu Kα irradiation in the range of 10−60° (2θ), with a scan step of 0.02° and a step time of 0.4 s at room temperature. The UV−visible absorption spectra were recorded using an absorption spectrophotometer (Shimadzu UV-3600 UV-vis, Shimadzu Corporation, Kyoto, Japan). The PL and PLE spectra were measured on a fluorescence spectrophotometer (Edinburgh FLS1000, Edinburgh Instruments, Edinburgh, UK) using an excitation wavelength (λ_ex) of 320 nm. The PL stability of the sample was quantitatively assessed using a fluorescence spectrophotometer (Perkin-Elmer LS55, Waltham, MA, USA) with an excitation wavelength set at 365 nm. Temperature-dependent PL were conducted with the attachment of a temperature-controlling system (OptistatDN, Oxford Instruments, Abingdon, UK). The absolute PLQY of samples were determined using a Quantaurus-QY absolute PL quantum yield spectrometer (C11347-11, Hamamatsu Photonics, Hamamatsu, Japan) equipped with a calibrated integrating sphere under 365 nm excitation. Scanning electron microscopy (SEM) images were obtained via a field-emission scanning electron microscope (ZEISS Gemini 300, Oberkochen, Germany), and Energy Disperse Spectroscopy (EDS) was integrated as a built-in component of the instrument (OXFORD Xplore, Oberkochen, Germany). The electroluminescence characteristics of the fabricated white LEDs, based on a 1 W and 365 nm commercial blue light chip, were systematically evaluated using an OHSP-350M LED (Hangzhou, China) Fast-Scan Spectrophotometer equipped with an integrating sphere (HAAS-2000, Hangzhou, China).

3. Results

The undoped Cs₂NaInCl₆ and Cs₂InCl₅·H₂O exhibit weak STEs emissions at 530 nm and 580 nm, respectively. Figure 1a illustrates the formation process of STEs in Sb³⁺-doped Cs₂InCl₅·H₂O and Sb³⁺-doped Cs₂NaInCl₆ under 365 nm excitation. Prior research has demonstrated that the yellow and blue emissions originate from the excited states of [SbCl₆]³⁻ and [Sb(H₂O)Cl₅]²⁻, respectively [18,22]. Compared with [Sb(H₂O)Cl₅]²⁻, the octahedral deformation of [SbCl₆]³⁻ is less pronounced, resulting in lower energy consumption during the energy transfer process and a smaller band gap for luminescence. In addition, the strength of crystal filed of Sb³⁺, along with the orbital orientation, significantly influences the formation of the cation valence band. In Sb³⁺-doped Cs₂NaInCl₆, the moderate electron–phonon coupling energy and Stokes shift lead to a pronounced STEs narrowband blue emission [23]. In Sb³⁺-doped Cs₂InCl₅·H₂O, the [Sb(H₂O)Cl₅]²⁻ anion exhibits the greater structural deformation, which results in the higher energy consumption. Consequently, this leads to a smaller optical band gap and a larger Stokes shift. As depicted in Figure 1b, upon doping with Sb³⁺, Cs₂InCl₅·H₂O:Sb and Cs₂NaInCl₆:Sb exhibit yellow luminescence at 580 nm and blue luminescence at 447 nm, respectively, under excitation by 320 nm ultraviolet light, owing to the incorporation of discrete luminescent centers [33]. These results suggest a feasible strategy to realize tunable white light emission by combining Sb³⁺-doped Cs₂InCl₅·H₂O and Cs₂NaInCl₆.

In this work, guided by the literature, an Sb/In atomic ratio of 1:4 was selected to synthesize yellow light-emitting Cs₂InCl₅·H₂O:Sb [18]. As a result, the spectral characteristics of the material were modulated, enabling the targeted optical properties. The structural transformations occurring during synthesis are illustrated in Figure 2a. The XRD results indicate that upon adding NaCl (0.05 mmol) to the synthetic raw materials, new diffraction peaks corresponding to Cs₂NaInCl₆ are observed in the XRD at 23.9°, 34.1°, and 41.0°. As the amount of NaCl is further increased, the relative proportion of Cs₂NaInCl₆ progressively rises. When the added NaCl reaches 0.3 mmol, the pristine Cs₂InCl₅·H₂O phase is significantly suppressed, leading to the dominance of the Cs₂NaInCl₆ component. This observation can likely be attributed to differences in solubility among the various phases under the given chemical conditions. When NaCl amount reaches 1 mmol, the synthesized compound is assigned to pure Cs₂NaInCl₆ without any impurities (Figure 2b) [15,22,26].

Herein, we report the preparation of a highly efficient white-emitting phosphor comprising two components, Cs₂InCl₅·H₂O and Cs₂NaInCl₆. This was achieved by dissolving 2 mmol of cesium chloride, 0.8 mmol of indium chloride, 0.2 mmol of antimony chloride, and 0.12 mmol of sodium chloride in hydrochloric acid (named as Cs₂Na_0.12In_0.80Cl_5.12:Sb). The SEM results indicate that the prepared samples exhibit an average size of 5 μm, featuring relatively distinct rhombic corners. This morphology aligns well with the targeted cubic phase material, thereby confirming the excellent crystallinity of the material (Figure S1). The EDS mapping further showed that all constituent elements within the sample are relatively uniform, indicating effective bulk incorporation of Sb³⁺ rather than surface-limited doping, in agreement with XRD analysis (Figure 2c). Quantitative analysis of the elemental composition revealed that the ratios of Cs, Na, In, Cl, and Sb are 22.31, 0.89, 10.30, 66.17, and 0.33 (Figure S2, Table S1), respectively, which are consistent with the expected proportions based on the injected NaCl solution. This suggests that a minor amount of NaCl can be sufficient to trigger the phase transition from Cs₂InCl₅·H₂O host to Cs₂NaInCl₆, inducing sufficient blue light emission (Cs₂NaInCl₆:Sb) Cs₂NaInCl₆.

After doping Sb³⁺ into Cs₂NaInCl₆ and Cs₂InCl₅·H₂O halide double perovskites, pronounced changes in optical properties were observed in their optical properties. As shown in Figure 3a, the Cs₂InCl₅·H₂O:Sb (x = 0) and Cs₂NaInCl₆:Sb (x = 1) doped with Sb³⁺ exhibit strong emissions at wavelengths of 580 nm (yellow) and 447 nm (blue), respectively, upon 365 nm excitation. The emission centers of Cs₂NaInCl₆:Sb and Cs₂InCl₅·H₂O:Sb synthesized via this environmentally friendly method align with those obtained using high-concentration hydrochloric acid. To investigate the effect of NaCl addition on photoluminescence performance, a series of samples with varying Na/In molar ratios (the atomic content of In³⁺ remains unaltered (0.8 mmol)) were prepared under identical conditions. As shown in Figure 3a, upon the incorporation of NaCl to the Cs₂InCl₅·H₂O:Sb sample, an emission peak with a full width at half maximum (FWHM) of 80 nm emerged at 447 nm. As the molar ratio of the NaCl increased, the proportion of blue light luminescence gradually rose, enabling continuous spectral tunability from the yellow light region to the blue light region. By precisely controlling the NaCl content, cold-to-warm white light tuning was readily achieved, thereby facilitating the adjustment of the color temperature for the white LED fabricated using this sample (Figure S3).

By incorporating Na⁺ and Cl⁻ into the synthetic environment, the formation of the [Sb(H₂O)Cl₅]²⁻ octahedral structure was effectively suppressed, thereby increasing the proportion of [SbCl₆]³⁻. The PL of the sample exhibits no shift upon the addition of NaCl, thereby indicating that the two luminescent centers within the sample are independent and mutually unaffected. Absorption spectra indicate the absorption intensity at 252 nm for Sb³⁺-doped Cs₂NaInCl₆ is significantly enhanced compared to Sb³⁺-doped Cs₂InCl₅·H₂O. Additionally, the absorption spectrum exhibits a gradual redshift. This phenomenon can be attributed to the Jahn–Teller effect induced by the deformation of the water-coordinated [In(H₂O)Cl₅]²⁻ octahedron. Furthermore, the splitting distance between the two luminescent centers in Cs₂InCl₅·H₂O:Sb is considerably larger than that in [SbCl₆]³⁻ (Cs₂NaInCl₆:Sb) (as shown in Figure 2a). Moreover, the absorption peak at 334 nm exhibits a consistent trend, characterized by a decrease in intensity and a gradual redshift as the NaCl concentration increases. This behavior suggests that the lattice environment surrounding Sb³⁺ ions governs the configurational transition from the ground state (¹S₀) and the excited state (³P_n).

Analysis of the excitation processes in the samples reveals that the transitions in both Cs₂NaInCl₆:Sb and Cs₂InCl₅·H₂O:Sb originate from the ¹S₀→³P₁ transition. Notably, for Cs₂NaInCl₆:Sb (x = 1), two distinct and prominent excitation centers are observed near 317 nm and 334 nm, both of which correspond to the ¹S₀→³P₁ transition, as illustrated in Figure 3b. According to the selection rules, the transition is partially allowed. However, due to spin–orbit coupling, which leads to the mixing and enhancement of singlet (S = 0) and triplet (S = 1) spins, the spin-forbidden restriction can be effectively lifted. As reported in the literature, the splitting of the ¹S₀→³P₁ spectrum can be attributed to the pseudo-Jahn–Teller effect induced by lattice vibrations [34,35]. A relatively weak excitation center is observed at 280 nm, corresponding to the forbidden transition of ¹S₀→³P₂, which results from symmetry breaking. However, given that its absorption peak (approximately 260 nm) approaches the band edge of the host material, the excited-state electrons can readily enter the conduction band [36]. Consequently, it becomes challenging to detect spectral branches associated with this transition in the plotted spectrum [37]. In Cs₂InCl₅·H₂O:Sb, the excitation band within this region does not exhibit two distinct split excitation centers around 310 nm and 340 nm (Figure 3b). In Cs₂NaInCl₆:Sb, the edgeband absorption is attributed to the electronic transition from ¹S₀→³P₁, no distinct excitation peak is observable. From the excitation spectra of Cs₂Na_0.12In_0.80Cl_5.12:Sb, it is evident that the yellow light at 580 nm in the white light sample originates from the photophysical process associated with Cs₂InCl₅·H₂O:Sb emission (x = 0.12). In contrast, the blue light at 447 nm corresponds to the spectral characteristics of Cs₂NaInCl₆:Sb (x = 0.12–1).

As illustrated in Figure S4 and Table S2, within the temperature range of 87 K to 307 K, the FWHM of Peak 446 nm (Cs₂NaInCl₆:Sb) initially decreases as the temperature increases from 77 K to 127 K (57.4 → 52.8 nm), followed by a continuous increase from 127 K to 287 K (52.8 → 117.7 nm). This behavior is characteristic of thermally activated processes in luminescent materials, where the linewidth broadening at elevated temperatures is primarily governed by enhanced electron–phonon interactions. The accelerated broadening rate with increasing temperature suggests a dominant contribution from multi-phonon processes. In contrast, Peak 580 nm (Cs₂InCl₅·H₂O:Sb) maintains a constant FWHM (94.2 nm) across the temperature range of 87–307 K, demonstrating negligible thermal broadening. This temperature-independent emission profile indicates an exceptionally stable energy level configuration of the Sb³⁺ luminescent center, which exhibits remarkable resistance to thermal perturbations. Consequently, this center serves as the dominant emission source under elevated temperatures. The emission intensity reaches its maximum value of 1.17 × 10⁷ at 107 K, followed by a gradual decline to 0.685 × 10⁷ at 287 K, representing superior thermal stability compared to Peak 446 nm. The temperature-dependent PL intensity can be expressed as follows:

$$I\left(t\right) = I_0/(1+A{e}^{-E_b/T{K}_b})$$

where I₀ is the intensity at 0 K, A is a constant, E_b is the exciton binding energy, and K_b is the Boltzmann constant. The activation energy values derived from the fitting analysis of Peak 446 nm and Peak 580 nm in the luminescence spectrum are determined to be 0.05 eV and 0.10 eV, respectively, which is beneficial to radiative recombination.

The electron–phonon coupling constitutes the fundamental physical mechanism underlying the generation of STEs, and its strength can be quantitatively characterized by the Huang–Rhys factor (S). The electron–phonon interaction can be described approximately and quantitatively by the S and the phonon frequency (ℏω_phonon), which can be extracted by fitting the temperature-dependent FWHM curve using the following Equation:

$$FWHM(T)=2.36\sqrt{S}\mathrm{\hslash }{\mathsf{\omega }}_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{n}}\sqrt{\mathrm{c}\mathrm{o}\mathrm{t}\mathrm{h}{\displaystyle \frac{\mathrm{\hslash }{\mathsf{\omega }}_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{n}}}{2{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}}}$$

The fitting analysis of Peak 446 nm reveals that the S is 2.88 and the ℏω_phonon is 0.026 eV. The relatively large S value facilitates the formation of STEs, while the thermal broadening is predominantly attributed to optical phonon vibrations. These results indicate the presence of medium-to-strong electron–phonon coupling in the sample, which promotes lattice self-trapping of excited-state electrons and leads to STE formation. This electron–phonon interaction is the fundamental mechanism responsible for the observed STE-related photoluminescence characteristics. For Peak 580 nm, the FWHM remains largely unchanged, indicating that the parameter S is approximately constant. The coupling strength of the material is inherently fixed, with the recombination rate primarily governed by intrinsic factors such as the system’s band structure, phonon spectrum, and temperature, rather than by variations in coupling strength. This suggests that the material exhibits significantly enhanced thermal stability in its luminescent properties [18,23].

The time-resolved PL (TRPL) decay curves of Cs₂InCl₅·H₂O:Sb, Cs₂NaInCl₆:Sb, and Cs₂Na_0.12In_0.80Cl_5.12:Sb are shown in Figure 3c. The measurements of Cs₂InCl₅·H₂O:Sb and Cs₂NaInCl₆:Sb were conducted at 580 nm and 445 nm, respectively. The two luminescent components of Cs₂Na_0.12In_0.80Cl_5.12:Sb were fitted independently. The TRPL decay components for all the samples are listed in Table S3. Both Cs₂NaInCl₆:Sb and Cs₂InCl₅·H₂O:Sb exhibit two distinct emission behaviors. For Cs₂NaInCl₆:Sb, the dominant fluorescence decay lifetimes are 0.09 μs (22%) and 1.04 μs (78%), whereas for Cs₂InCl₅·H₂O:Sb, the corresponding decay lifetimes are 0.62 μs (17%) and 4.19 μs (83%). The presence of microsecond-scale decay lifetimes is characteristic of self-trapped exciton (STE) emission. As reported in previous studies, the shorter and longer decay components can be attributed to radiative transitions from the spin-triplet excited states ³P₁→¹S₀ and ³P₀→¹S₀, respectively [18,38]. The fitting results indicate that the average fluorescence lifetimes are 1.01 μs and 4.08 μs, respectively. Compared to isolated three-dimensional structures, fully connected three-dimensional frameworks exhibit a more rigid lattice. Upon the occurrence of STEs, the more constrained [SbCl₆]³⁻ octahedra experience reduced structural deformation, require lower energy dissipation, and display shorter fluorescence decay lifetimes, which are consistent with the material’s structural characteristics (Figure 2a). For the Cs₂Na_0.12In_0.80Cl_5.12:Sb sample, the fluorescence lifetime of the blue emission was measured as 1.06 μs, while that of the yellow emission was determined to be 4.13 μs. This finding confirms the presence of two distinct exciton recombination channels in the sample, which are attributed to Cs₂InCl₅·H₂O:Sb and Cs₂NaInCl₆:Sb, respectively (Table S3). Furthermore, the fluorescence lifetimes of the two components in the white light-emitting sample closely resemble those of the individually synthesized samples, indicating that the crystalline quality of the white light sample prepared via the one-pot method is comparable to that of the separately prepared samples. Under 365 nm excitation, all three samples demonstrated strong fluorescence emissions, with absolute PLQY over 50.20% for Cs₂InCl₅·H₂O:Sb, 44.59% for Cs₂Na_0.12In_0.80Cl_5.12:Sb, and 46.64% for Cs₂NaInCl₆:Sb, respectively (Figure S5). Owing to the presence of distinct photophysical processes, the excitation rules for the two materials exhibit inconsistencies. As illustrated in Figure 3d, the same white light sample can yield varying luminescence spectra when excited via different mechanisms. With increasing excitation energy, the intensity of the yellow component rises, resulting in warm white emission that gradually shifts to pure yellow. Conversely, with decreasing excitation energy, the blue component becomes more prominent, leading to cold white emission that ultimately evolves into pure blue. In addition, under identical excitation conditions, the fluorescence spectra of the phosphors can be modulated by varying the NaCl content (Figure S6). The Cs₂NaInCl₆:Sb and Cs₂InCl₅·H₂O:Sb exhibit superior luminescent performance, achieving broadband blue and yellow emissions under a comparable excitation wavelength range (295–365 nm), thereby meeting the requirements for high-efficiency WLEDs. The stability of phosphors is a critical factor that significantly influences the performance of WLEDs.

To investigate the stability of the prepared samples under ultraviolet light, the stability of the blue light-emitting Cs₂NaInCl₆:Sb, yellow light-emitting Cs₂InCl₅·H₂O:Sb, and white light-emitting Cs₂Na_0.12In_0.80Cl_5.12:Sb samples were evaluated under irradiation from a 365 nm ultraviolet lamp. The results indicate that both the blue-emitting Cs₂NaInCl₆:Sb and the yellow-emitting Cs₂InCl₅·H₂O:Sb exhibit comparable stability. After continuous irradiation for 200 h, the spectral intensities of both materials are maintained at approximately 80% of their initial values (Figure 4a,b); representative fluorescence spectra are provided in Figure S7. The PL spectra of the Cs₂Na_0.12In_0.80Cl_5.12:Sb, Cs₂NaInCl₆:Sb, and Cs₂InCl₅·H₂O:Sb exposed to air for 2 months remain almost unchanged (temperature: 293 K, humidity: <25%) compared to the freshly prepared powders (Figure S8). The temperature-dependent PL measurement of Cs₂Na_0.12In_0.80Cl_5.12:Sb is from 327 to 467 K in the air. The results indicate that the two emission peaks display comparable thermal stability, with the luminescence intensity at 385 K retaining approximately 60% of its initial value at 327 K (Figure S9 and Table S4). Subsequently, XRD analysis of Cs₂Na_0.12In_0.80Cl_5.12:Sb confirm that the components retain their original crystal structure after 200 h of storage, with no significant phase transformation observed (Figure 4c). The corresponding PL image of the powder under 365 nm is presented in Figure 4d, indicating that the sample maintains excellent luminescent performance.

Owing to its high fluorescence emission intensity and superior ambient stability, the one-pot synthesized Cs₂Na_xInCl_5+x:Sb is anticipated to serve as an efficient luminescent material for WLEDs. To demonstrate this concept, a WLED device was fabricated by employing the dual-component fluorescent material synthesized via the one-pot method in conjunction with a commercially available 365 nm chip. As depicted in Figure 5a, the WLED device demonstrates a bright electroluminescence (EL) at a driving current of 240 mA. The CIE 1931 chromaticity coordinates are measured to be (0.4288, 0.4034), with a color rendering index (Ra) of 88.5 and a CCT of 3136 K for white light emission. The emission spectra of the LED at various forward bias currents (ranging from 20 to 320 mA) under different current densities are presented and the output power of white light emission exhibits a monotonic increase with the rise in forward current. Under varying driving currents, the spectra demonstrate negligible emission shifts, and the color temperature of the LED remains relatively stable, fluctuating only slightly within the range of 3027 to 3225 K (Figure 5b). These results suggest that this type of 0D indium-based halide exhibits a significant potential for application in high-performance lighting-grade WLEDs.

4. Discussion

In this study, a facile and scalable synthetic strategy based on controlling the NaCl content was employed to synthesize luminescence-tunable Cs₂Na_xInCl_5+x:Sb phosphors. The incorporation of NaCl in this work introduces novel energy transfer pathways between Sb³⁺ luminescent centers with distinct coordination environments—specifically, the formation of [SbCl₆]³⁻ octahedra and distorted [SbCl₅]²⁻ pyramids induced by Na⁺ doping. This unique structural modulation allowed the facile preparation of phosphors with tunable correlated color temperatures ranging from 2500 to 5000 K under 365 nm excitation. The phosphors exhibit excitation-dependent fluorescence characteristics, with a PLQY as high as 45.24% at 365 nm. Mechanistic analysis reveals that efficient energy transfer among multiple Sb³⁺ luminescent centers (from high-energy [SbCl₅]²⁻ to low-energy [SbCl₆]³⁻) and an optimized Na⁺ doping concentration are critical to achieving highly efficient and tunable white emission. This energy transfer mechanism differs from the typical dipole–dipole interaction in rare-earth-doped phosphors, providing a new paradigm for spectrum tuning in halide-based luminescent materials [39]. Moreover, the synthesized Cs₂Na_xInCl_5+x:Sb composite shows excellent resistance to ultraviolet irradiation, thermal behavior, oxygen, and humidity, demonstrating stable physicochemical properties and long-term environmental stability. With high PLQY, spectrum tunability, and stable white light phosphor addressing the dual demands of environmental friendliness and performance in lighting applications, this material holds strong potential for indoor lighting and plant supplementary lighting. Moreover, this study provides a generalizable approach for designing luminescence-tunable halide phosphors, which can be extended to other Sb-doped or Bi-doped indium/tin halide systems, thereby advancing the development of next-generation solid-state lighting materials.