A New Zero-Dimensional (CsK₂)BiCl₆ Metal Halide: Boosting Emission via B-Site Mn-Doping

Abstract

The A site of zero-dimensional (0D) metal halides A₃BiCl₆ can be replaced by Cs and/or K, thus, four possible 0D A₃BiCl₆ forms exist, such as (Cs₂K)BiCl₆, (CsK₂)BiCl₆, K₃BiCl₆ and Cs₃BiCl₆. It is well known that Cs₃BiCl₆ has been reported. We predict that both (Cs₂K)BiCl₆ and K₃BiCl₆ do not have enough structural and thermodynamic stability, but (CsK₂)BiCl₆ should be a 0D stable A₃BiCl₆ candidate based on density functional theory (DFT). Furthermore, 0D (CsK₂)BiCl₆ metal halide was experimentally prepared by the solvothermal method. Though (CsK₂)BiCl₆ metal halide exhibits an indirect bandgap and poor luminescence properties, the emission can be boosted by B-site Mn-doping due to the efficient energy transfer from self-trapped excitons (STE) to the d-state of Mn ions. Our results enrich the family of 0D bi-based metal halides and provide guidance for the regulation of the structural and optical properties of metal halides.

1. Introduction

Lead halide perovskites with the chemical formula APbX₃ (A = Cs⁺, Rb⁺, X = Cl, Br, and I) have developed rapidly in recent years due to their unique optoelectronic properties, and they are widely used in various fields, such as solar cells, light-emitting diodes (LEDs), X-ray detectors, etc. [1,2,3,4,5,6,7,8,9,10,11,12,13]. At present, all-inorganic perovskites have been extensively studied, and different types of perovskites can exhibit different optical properties, which determine the direction of their application in devices [14,15,16]. For example, for solar cell devices, a high-dimensional perovskite structure is required as the emission layer of the cell. Highly interconnected octahedrons have a higher effective carrier mass and better electron space transport. This will directly affect the photoelectric conversion efficiency of the device [17,18,19,20,21,22]. Efficient emission of perovskite is an important requirement for the high external quantum efficiency of LED devices. If perovskite light-emitting layers are chosen for the preparation of LED devices, low-dimensional metal halides are a suitable choice [23,24,25,26,27]. Compared with three-dimensional (3D) perovskites, low-dimensional perovskites have better luminescence properties, especially zero-dimensional (0D) perovskites, whose isolated octahedron characteristics can effectively localize photogenerated carriers and further increase the probability of radiation recombination. As a result, most zero-dimensional metal halides exhibit ultra-high photoluminescence quantum efficiency and are candidate materials for a new generation of LED and display devices [28,29,30,31].

Compared with 3D perovskites, 0D perovskites exhibit unique crystal structures due to their metal halide, with regular octahedra being completely isolated by monovalent cations [32,33,34]. Therefore, it exhibits strong quantum confinement and exciton-phonon interaction, resulting in exciton localization and the formation of self-trapped exciton states. This unique electronic structure makes it exhibit excellent optoelectronic device performance and has a wide range of application prospects [35,36,37,38]. In 2016, M. Bakr et al. synthesized a 0D Cs₄PbBr₆ perovskite nanocrystal with a high PLQY of 45%, which is much higher than that of 3D perovskites [39]. However, the inherent toxicity and poor environmental stability of lead-based halide perovskites still hinder practical applications. The most common method is to use trivalent ions (Sb³⁺, In³⁺, and Bi³⁺) instead of divalent Pb ions to form low-dimensional nontoxic metal halides, such as Cs₃A₂X₉ or Cs₃AX₆ (A = Sb, In, Bi; X = Cl, Br, and I) [40,41,42]. In 2017, Tang et al. synthesized new all-inorganic 0D Cs₃Sb₂Br₉ blue light quantum dots by the ligand-assisted recrystallization method [43]. It has a high PLQY of 46%, and the mechanism of luminescence enhancement is mainly due to the quantum well band structure and its larger exciton binding energy, which effectively improves the radiative recombination efficiency. Additionally, the 0D In-based halide perovskite Cs₂InBr₅·H₂O shows red emission with a PLQY of 33% [44], and emits yellow emission after heating to dehydrate the material, showing the unique property of sensitive water detection. Regarding Bi-based halide perovskites, the research on 2D Bi-based halide perovskites has progressed rapidly, while the corresponding 0D materials have progressed slowly. In 2020, Chen et al. used benzoyl halides as halide precursors, and synthesized homogeneous Cs₃BiX₆ NCs by the heat injection method [1]. We have also recently reported that a moderate amount of Mn-doping can effectively enhance the orange broadband emission of Cs₃BiCl₆ by the breaking bandgap transition [40]. However, there are few reports on the A-site regulation of 0D Cs₃BiCl₆ components on the effects of the crystal structure and optical properties.

In this work, we have applied DFT calculations to study the structure and stability of different A sites of A₃BiCl₆, such as (Cs₂K)BiCl₆, (CsK₂)BiCl₆, K₃BiCl₆ and Cs₃BiCl₆. Our results show that (CsK₂)BiCl₆ has good structural and thermodynamic stability, but it exhibits an indirect bandgap and poor optical properties., while (Cs₂K)BiCl₆ and K₃BiCl₆ cannot form stable structures. Mn-doping in (CsK₂)BiCl₆ can change the energy band structure of (CsK₂)BiCl₆ and improve its optical properties. Furthermore, on the basis of theoretical calculations, we further synthesized (CsK₂)BiCl₆ metal halides by the solvothermal method, and further regulated the optical properties by Mn-doping. The results show that the PLQY of Mn-doped (CsK₂)BiCl₆ are greatly improved, and a broadband bright orange-yellow emission at 605 nm is achieved. Our DFT calculations provide a good prediction of the optical properties and structural stability of the 0D (CsK₂)BiCl₆, which enrich the structural composition of 0D Bi-based lead-free metal halides, and can help to accelerate the development of an environmentally stable 0D Bi-based halide perovskite derivative.

2. Result and Discussion

2.1. Theoretical Prediction of Crystallographic and Thermodynamic Stabilities of 0D A₃BiCl₆

The first-principle calculations were carried out using density functional theory with a plane-wave basis set and projector-augmented wave (PAW) method [45,46] as implemented in the Vienna ab initio simulation package (VASP). The geometries and electronic structures were relaxed by the generalized gradient approximation (GGA) in the modified Perdew–Burke–Ernzerhof (PBE) form [47,48,49]. A kinetic-energy cutoff of 350 eV was tested and found to be sufficient for plane-wave expansion to achieve good convergence. The conjugate gradient algorithm was used until the total energy was converged to 10⁻⁵ eV in the electronic self-consistency loop, and a force criterion of 0.01 eV Å was allowed. There were 40 atoms in the A₃BX₆ simulation unit and k points were set as 7 × 7 × 4.

First, we constructed the 0D structures of (Cs₂K)BiCl₆, (CsK₂)BiCl₆, and K₃BiCl₆ through the regulation of the A site (Figure 1). The 0D (Cs₂K)BiCl₆, (CsK₂)BiCl₆, and K₃BiCl₆ are monoclinic crystal structures with space group C/2c, consistent with 0D Cs₃BiCl₆. In (CsK₂)BiCl₆, the BiCl₆³⁻ octahedron is sterically isolated by surrounding Cs⁺ and K⁺ atoms. The distance between two adjacent Bi atoms in (CsK₂)BiCl₆ is approximately 7.71–7.76 Å, indicating almost no interaction between the two adjacent BiCl₆³⁻ octahedra and the nature of 0D electronic dimensionality [50].

The Goldschmidt empirical criterion was used to evaluate the crystal stability of perovskite structure materials [51,52]. Thus, two empirical quantities were obtained: the Goldschmidt tolerance factor (t) and the octahedral factor (μ). Previous statistical analysis of all existing halide perovskites has shown that stable perovskite structure formation requires 0.81 < t < 1.11 and 0.41 < μ < 0.90. Due to the existence of multiple cations, the formulas for t and μ in this paper can be defined as:

$$\mu =r_{\mathrm{Bi}}/r_{\mathrm{Cl}}$$

(1)

$$t_{{\mathrm{Cs}}_{3-x}{K}_x{\mathrm{BiCl}}_6}=\left\{\frac{\left(3-x\right)r_{\mathrm{Cs}}+x{r}_{\mathrm{K}}}{3}+r_{\mathrm{Cl}}\right\}/\sqrt{2}\left(r_{\mathrm{Bi}}+r_{\mathrm{Cl}}\right)$$

(2)

where r_Cs, r_K, r_Bi and r_Cl are the Shannon ionic radii of Cs⁺, K⁺, Bi³⁺ and Cl⁻ ions, respectively. The t and μ of the four structures are shown in Figure 2. Our calculations show that both t and μ of Cs₃BiCl₆, (Cs₂K)BiCl₆ and (CsK₂)BiCl₆ are within reasonable ranges, indicating that the Cs₃BiCl₆, (Cs₂K)BiCl₆ and (CsK₂)BiCl₆ compounds are stable perovskite structures. Although K₃BiCl₆ has a μ = 0.569, within the recommended range, its t-value of 0.794 suggests that it may form a distorted perovskite structure.

Then, to further evaluate the thermodynamic stabilities of these structures, we calculated their decomposition enthalpies (ΔH_d) with respect to possible decomposition pathways. ΔH_d is defined as the energy difference between the decomposed producers and the A₃BiCl₆ compound. The large positive values of ΔH_d (decomposed into corresponding binary materials) above 20 meV per atom indicate the thermodynamic stability of the A₃BiCl₆ compounds. In the stability assessment, including the possibility of decomposition pathways involving ternary compounds is quite important for quaternary compounds. Therefore, in our calculations, we considered the expected decomposition products reported experimentally, including selected ternary compounds. For (CsK₂)BiCl₆, we have investigated the following possible decomposition pathways:

(CsK₂)BiCl₆ → CsCl + BiCl₃ + 2KCl     82.23 meV/atom

(3)

(CsK₂)BiCl₆ → ⅓Cs₃BiCl₆ + ⅔BiCl₃ + 2KCl  65.61 meV/atom

(4)

(CsK₂)BiCl₆ → ⅓Cs₃Bi₂Cl₉ + ⅓BiCl₃ + 2KCl  21.96 meV/atom

(5)

The reaction pathways for Cs₃BiCl₆, (Cs₂K)BiCl₆ and K₃BiCl₆ are shown in the Supporting Information.

For the four structures, Cs₃BiCl₆, (Cs₂K)BiCl₆ and (CsK₂)BiCl₆ show positive ΔH_d values, while K₃BiCl₆ shows negative ΔH_d values in decomposition paths involving only binary compounds, indicating that K₃BiCl₆ cannot be synthesized stably (Figure 2). However, when considering ternary compounds, such as Cs₃Bi₂Cl₉, (Cs₂K)BiCl₆ shows negative values, indicating that even if (Cs₂K)BiCl₆ can be synthesized, it would spontaneously decompose into a mixture of Cs₃Bi₂Cl₉ and other binary compounds, so it is difficult to synthesize 0D (Cs₂K)BiCl₆ under thermodynamic equilibrium conditions. This explains why there have been no experimental reports on the successful synthesis of pure 0D (Cs₂K)BiCl₆ so far. For (CsK₂)BiCl₆, the decomposition enthalpy of (CsK₂)BiCl₆ is positive under the decomposition paths of all known compounds, indicating that this structure has good thermodynamic stability and may be synthesized experimentally.

2.2. Electronic Structure of (CsK₂)BiCl₆

The above studies show that (CsK₂)BiCl₆ is a stable compound, and the electronic properties of (CsK₂)BiCl₆ were investigated using DFT calculations. The energy band structure shows that the indirect bandgap of (CsK₂)BiCl₆ is 3.92 eV (Figure 3a). Combined with DOS, it can be seen that the valence band maximum (VBM) of (CsK₂)BiCl₆ mainly comes from the s orbital of Bi and the p orbital of Cl, while the conduction band minimum (CBM) is mainly contributed by the p orbital of Bi and Cl (Figure 3b). In the charge density of Figure 3c, whether it is HOMO or LUMO, the charge is mainly concentrated between the Bi atom and the Cl atom. This is consistent with our previously calculated results for Cs₃BiCl₆, indicating that the change of the A site has a very negligible effect on the electronic structure of Cs₃BiCl₆. However, it can effectively reduce the proportion of Cs in the compound and expand the compositional space of Bi-based halides.

Similar to Cs₃BiCl₆, the inherent indirect bandgap of (CsK₂)BiCl₆ may lead to its poor PL performance, limiting its further applications. Previous research results show that doping can effectively tune the electronic structure of materials, and experimentally reasonable doping can improve the PL performance of metal halides [27,53]. As a suitable metal ion dopant, Mn²⁺ has been widely used to tune the optical properties of semiconductors. Therefore, the electronic structure of (CsK₂)BiCl₆ after Mn doping was calculated. We constructed an Mn-doped (CsK₂)BiCl₆ structure by replacing Bi with Mn and optimized it to obtain the most stable crystal structure. Figure 3d–f shows the energy band structure, DOS and charge density of Mn-doped (CsK₂)BiCl₆, respectively. Compared with pristine (CsK₂)BiCl₆, Mn-doping produces two impurity states in the forbidden band. Combining DOS, it is found that these two impurity states are mainly due to the contribution of the 3D orbital of Mn. Figure 3f shows the charge density of Mn-doped (CsK₂)BiCl₆. The results show that the charge density around Mn is higher than that of Bi in both HOMO and LUMO. This is mainly due to the different valence states of Mn²⁺ and Bi³⁺, while the high charge density around Mn explains the charge balance of Mn-doped (CsK₂)BiCl₆.

When light passes through some solid materials, electrons, atoms or ions in the material will interact with the light, resulting in reflection, absorption and projection. The absorption of light by semiconductor materials is mainly divided into intrinsic absorption, lattice vibration absorption and carrier absorption. The absorption spectrum is calculated from the dielectric function equation [54]:

$$\alpha \left(\omega \right)=\left(\sqrt{2}\right)\omega {\left[\sqrt{{\epsilon }_1{\left(\omega \right)}^2+{\epsilon }_2{\left(\omega \right)}^2}-{\epsilon }_1{\left(\omega \right)}^2\right]}^{\frac{1}{2}}$$

(6)

where ε₁ and ε₂ correspond to the real and imaginary parts of the dielectric function, respectively. Usually, the interaction between the dopant atoms and the substrate leads to enhanced optical properties of the doped structure. Figure S1 (see in Supplementary Materials) calculates the optical absorption of undoped and Mn-doped (CsK₂)BiCl₆. Compared with pristine (CsK₂)BiCl₆, the absorption coefficient of Mn-doped (CsK₂)BiCl₆ moves in the lower energy direction, the absorption edge is red-shifted, and the energy corresponding to the peak of the absorption coefficient is consistent with the dielectric function (Figure S2). In the 1.2–4.4 eV region, the absorption coefficient of Mn-doped (CsK₂)BiCl₆ was significantly enhanced, indicating that Mn-doped (CsK₂)BiCl₆ has a better absorption effect on visible light.

2.3. Experimental Crystal Structure of (CsK₂)BiCl₆

The pristine 0D (CsK₂)BiCl₆ crystals were synthesized with a simple solvothermal method. The detailed experimental method is described in the supporting information. Figure S3 shows the powder X-ray diffraction (PXRD) pattern of the samples. The theoretical XRD results match well with simulated XRD patterns and show no impurity formation.

To understand the optical properties of 0D (CsK₂)BiCl₆, the absorption and PL spectra of the material were tested. The PL spectrum has no emission at room temperature, which may be due to the indirect bandgap properties of the material. The recombination process of the excited state band edge in the indirect bandgap material needs the assistance of phonons, making the nonradiative recombination dominant, thus, reducing the radiative recombination process, and in turn resulting in its poor PL performance [55]. Figure S4 shows the absorption spectrum and Tauc plot diagram of (CsK₂)BiCl₆. The absorption spectrum shows that 0D (CsK₂)BiCl₆ has absorption characteristics between 250 and 400 nm, especially at 260, 320 and 360 nm. These characteristic absorption peaks come from the 6s2 to 6s1p1 transition in the BiCl₆³⁻ octahedron. The Tauc plot shows that the material has two linear band edge intercepts of 3.29 and 4.02 eV, respectively, which is similar to that of Cs₂NaBiCl₆ [56]. The experimental bandgap value of the (CsK₂)BiCl₆ material is 4.02 eV, which is higher than the theoretical calculation result (3.8 eV). This computed bandgap is lower than the experimentally estimated bandgap because standard DFT calculations usually underestimate the bandgap of the material.

Mn²⁺ ion is widely used for improving and regulating the optical properties and band structure of materials [57,58,59]. For the poor optical properties of (CsK₂)BiCl₆, Mn²⁺ doping is expected to enhance its luminescence and make it a candidate material for optoelectronic device applications. A series of (CsK₂)BiCl₆ metal halides doped with different Mn²⁺ concentrations were synthesized. The schematic structure of Mn-doped (CsK₂)BiCl₆ is shown in Figure 4a. Mn²⁺ ions are doped into the lattice to replace Bi³⁺ to form an isolated MnCl₆^4- structure. The PXRD patterns of (CsK₂)BiCl₆ doped with different Mn²⁺ concentrations are shown in Figure 4b. The doping of Mn²⁺ ions do not show any new impurity peaks. The overall peak position is consistent with the pristine (CsK₂)BiCl₆. The results show that Mn²⁺ doping does not destroy the structure of the host (CsK₂)BiCl₆.

The X-ray photoelectron spectra (XPS) further confirmed that Mn²⁺ ions were successfully doped into (CsK₂)BiCl₆ and Mn²⁺ doping does not produce extra phase (Figure 4c). Figure 4d shows the high-resolution XPS spectra and peak fitting of Mn 2p. The peak at 641.0 eV is attributed to Mn 2p. Figure S5 shows the high-resolution XPS spectra of Cs, K, Bi and Cl in the samples. Compared with the pristine sample, the binding energies of Cs, K and Bi in the Mn-doped sample are slightly increased, which shows that the electron distribution of the Mn-doped samples is more compact. The composition and distribution of elements in a sample are usually determined using scanning electron microscope (SEM) images and an energy disperse spectrometer (EDS), Figures S6a and 4e are the SEM image and EDS spectra of 10%Mn: (CsK₂)BiCl₆, respectively. The results show that Cs, K, Bi, Cl and Mn elements are uniformly distributed in the lattice. Figure S6b shows the element content measured in selected local areas of 10%Mn: (CsK₂)BiCl₆ sample. The basic atomic ratio of (Cs + K):Bi:Cl is approximately 3:1:6. By using 9:1 of the nominal Bi/Mn ratio, but the actual measured Mn content is 7.5%.

2.4. Optical Properties of (CsK₂)BiCl₆

Previous studies have shown that the efficient luminescence center and energy transfer acceptor of Mn²⁺ ions can effectively improve the optical properties of materials. Therefore, the optical properties of the 10%Mn:(CsK₂)BiCl₆ sample were investigated. Figure 5a shows the absorption spectra of 10%Mn:(CsK₂)BiCl₆ and pristine (CsK₂)BiCl₆ samples, respectively. For the pristine (CsK₂)BiCl₆, the BiCl₆³⁻ octahedrons have a strong absorption at 260, 320 and 360 nm due to the 6s²→6s¹p¹ transition in Bi³⁺. The 10%Mn:(CsK₂)BiCl₆ sample has the similar absorption characteristics as the pristine (CsK₂)BiCl₆ sample before 380 nm, the overall absorption peak position and shape are consistent. The main reason is that the absorption peak in this interval comes from the characteristic absorption of BiCl₆³⁻ octahedrons. However, the 10%Mn:(CsK₂)BiCl₆ sample has an additional absorption peak at 430 and 520 nm, which is from the ⁶A_1g to ⁴T_1g absorption transition of Mn²⁺. Figure 5b shows the PLE and PL spectra of 10%Mn:(CsK₂)BiCl₆ sample. The excitation spectra show three main excitation peaks that correspond to the absorption spectra. The excitation peaks less than 400 nm come from the transition from the ground state ¹S₀ of Bi³⁺ to the excited states ³P₁ and ¹P₁. The excitation peaks larger than 400 nm come from Mn²⁺ characteristic transitions. The PL spectra show that, compared with pristine (CsK₂)BiCl₆, the material achieves bright orange emission at 605 nm through Mn²⁺ doping modification. Under the excitation of different wavelengths, such as 295, 365 and 430 nm, the peak position and peak shape of the PL spectrum are consistent. In addition, the peak shapes and positions of the excitation spectra measured with different emission wavelengths remain consistent, and the results are shown in Figure S7. The PL intensity is highest under the optimal excitation wavelength (365 nm), which suggests that the observed emission is actually from the intrinsic states of the samples, rather than nonradiative transitions, such as lattice defects and surface states.

The photophysical mechanism of 10%Mn:(CsK₂)BiCl₆ is shown in Figure 5c. Under the excitation of UV light, the electron absorbs energy and transitions from the valence band to the conduction band. For undoped samples, due to the nature of the indirect bandgap, most of the energy is lost through phonon vibration. The carriers in the conduction band cannot be transferred to self-trapped excitons or formed into self-trapped excitons, resulting in very poor luminescent performance. For Mn-doped samples, combined with the previous theoretical calculation, Mn doping can form an impurity state in the energy band structure, and the excited state carriers can effectively relax to the ⁴T₁ state of Mn²⁺, and transition from the ⁴T₁ state to the ⁶A₁ state, during this process, photons are emitted, and a bright orange light emission is produced. Figure S8 shows the PL spectra of pristine (CsK₂)BiCl₆ and x%Mn:(CsK₂)BiCl₆, and the pristine sample exhibits very poor optical properties. After Mn²⁺ doping, the sample exhibits a broadband emission at 605 nm, and the emission peak intensity gradually increases with increasing Mn-doping concentration. When the Mn²⁺ doping concentration is 10%, the PL intensity of the system reaches its highest. Figure 5d shows the time-resolved PL lifetimes of 5%, 10% and 20% of Mn²⁺ doped (CsK₂)BiCl₆ samples monitored at 605 nm. The curve can be fitted by a double exponential function A(t) = A₁exp(−t/τ₁₎ + A₂exp(−t/τ₂), where A(t) is the time change in PL intensity at time t, τ₁/τ₂ represents the lifetime of different recombination processes, and A₁/A₂ is the corresponding weight. The fitting data are shown in Table S1. The results show that the samples in the system exhibit a submillisecond lifetime, which is consistent with the typical characteristics of the PL lifetime of Mn²⁺ ions.

The intrinsic photophysical properties of (CsK₂)BiCl₆ are further revealed, and the temperature-dependent emission spectra of Mn-doped systems are measured. Figure 5e shows the PL spectrum of 10%Mn:(CsK₂)BiCl₆ in the temperature range of 80~360 K. Similar to common metal halides, the PL intensity increases significantly at low temperatures, which is due to the weakening of electron-phonon coupling with decreasing temperature, thereby suppressing nonradiative recombination. The PL intensity decreases as the temperature increases from 80 to 360 K, which is consistent with the model that more and more phonons participate in intensifying nonradiative recombination at high temperatures. It is worth noting that the PL peak position of the sample shifted slightly with temperature. As shown in Figure S9, with the temperature increasing from 80 to 360 K, the central peak position shifted from 630 to 605 nm. This is because the increase in temperature leads to the expansion of the lattice, thereby increasing the Mn-Mn spacing. The luminescence of Mn²⁺ is closely related to the Mn-Mn spacing. The increase in spacing makes the interaction between Mn-Mn weaken, resulting in an increase in the energy level difference of ⁴T₁—⁶A₁. Figure 5f shows a pseudo color plot of temperature-dependent PL intensity and wavelength. Compared with the PL intensity at low temperature, the luminescence quenching occurred at 300 K, indicating that the luminescence performance of the material at room temperature is poor. Figure S10 shows the full width at half maximum (FWHM) as a function of temperature, and the results show that FWHM increases significantly with increasing temperature. The electron-phonon coupling in a soft halide matrix leads to a larger FWHM with increasing temperature. However, excessive electron-phonon coupling can cause energy to be dissipated by vibrations of phonons, leading to the predominance of nonradiative recombination and degrading the luminescent properties of the material. In general, improving the optical properties of bismuth-based perovskite materials requires solving the strong electron-phonon coupling characteristics inside the material and enhancing its exciton binding energy. How to effectively improve these two factors is the solution and strategy to solve the poor optical properties of such materials, and the design and regulation of future materials should be broken through and improved in this direction.

3. Conclusions

In summary, we apply DFT calculations to study the structure and stability of different A sites in 0D A₃BiCl₆. Theoretical analysis shows that 0D (CsK₂)BiCl₆ has a stable structure. We further successfully synthesized (CsK₂)BiCl₆ using the solvothermal method to confirm our prediction. The pristine (CsK₂)BiCl₆ has poor luminescence properties due to the inherent indirect bandgap feature. The successful Mn doping helps to break the forbidden transition of (CsK₂)BiCl₆ while maintaining the 0D structure and boosts the efficient orange emission, which comes from the efficient energy transfer from STE to d-state of Mn ions. These results enrich the family of 0D metal halides, making (CsK₂)BiCl₆ a new candidate for future low-toxicity and low-dimensional optoelectronic materials.