Electron-Phonon Interaction in Te-Doped (NH₄)₂SnCl₆: Dual-Parameter Optical Thermometry (100–400 K)

Abstract

Lead-free perovskite variants have emerged as promising candidates due to their self-trapped exciton emission. However, in ASnX₃ systems, facile oxidation of Sn(II) to Sn(IV) yields A₂SnCl₆ vacancy-ordered derivatives. Paradoxically, despite possessing a direct bandgap, these variants exhibit diminished photoluminescence (PL). Doping engineering thus becomes essential for precise optical tailoring of A₂SnX₆ materials. Herein, through integrated first-principles calculations and spectroscopic analysis, we elucidate the luminescence mechanism in Te⁴⁺-doped (NH₄)₂SnCl₆ lead-free perovskites. Density functional theory, X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) confirm Te⁴⁺ substitution at Sn sites via favorable chemical potentials. Spectral interrogations, including absorption and emission profiles, reveal that the intense emission originates from the triplet STE recombination (³P₁ → ¹S₀) of Te centers. Temperature-dependent PL spectra further demonstrate strong electron–phonon coupling that induces symmetry-breaking distortions to stabilize STEs. Complementary electronic band structure and molecular orbital calculations unveil the underlying photophysical pathway. Leveraging these distinct thermal responses of PL intensity and peak position, 0.5%Te:(NH₄)₂SnCl₆ emerges as a highly promising candidate for non-contact, dual-parameter optical thermometry over an ultra-broad range (100–400 K). This work provides fundamental insights into the exciton dynamics and thermal engineering of optical properties in this material system, establishing its significant potential for advanced temperature-sensing applications.

1. Introduction

Luminescent all-inorganic metal halides have emerged as versatile optoelectronic platforms due to their tunable electronic structures and crystallographic flexibility [1,2]. Lead halide perovskites represent a prominent subset, achieving near-unity photoluminescence (PL) quantum yields and broad spectral tunability [3]. However, intrinsic lead toxicity and operational instability fundamentally constrain their device implementation. This has accelerated the development of lead-free alternatives, where ions with analogous electronic structures to Pb²⁺ (e.g., Ge²⁺, Sn²⁺, Sb³⁺ and Bi³⁺) [2,4,5,6,7,8] show promise as B-site substitutes in ABX₃ frameworks.

Tin-based perovskites attract significant interest because of their distinctive self-trapped exciton (STE) emission and environmental benignity [4,9,10,11,12,13,14,15]. However, the facile oxidation of Sn(II) to Sn(IV) leads to irreversible phase transformation into vacancy-ordered A₂SnX₆ derivatives. Although thermodynamically stable, these phases typically exhibit severely quenched photoluminescence—a critical limitation attributed to dominant non-radiative recombination pathways.

Doping engineering has proved to be an effective strategy to overcome these limitations. Although Sb³⁺/Bi³⁺ doping has been shown to enable broadband emission [16,17], recent advances demonstrate that tetravalent dopants (e.g., Te⁴⁺) are particularly promising for enhancing optoelectronic performance. For instance, Han et al. demonstrated that Te⁴⁺-doped Cs₂SnCl₆ serves as an efficient system for CO₂ photoreduction, expanding the materials library for photocatalytic applications [18]. Similarly, Tang et al. achieved intense yellow-green luminescence (580 nm) in Sn⁴⁺/Te⁴⁺ ion-exchanged materials via Jahn–Teller distortions, reaching a record PL quantum yield of 95.4% [13]. Despite these advances, Te-doped (NH₄)₂SnCl₆ remains underexplored, with systematic studies on its optical properties still lacking.

In this paper, undoped (NH₄)₂SnCl₆ and Te-doped (NH₄)₂SnCl₆ were synthesized and we systematically investigated their structure and photoluminescence properties to reveal the correlated photoluminescence mechanism. The morphology and structure of the samples were characterized using techniques such as TEM, XPS and XRD. UV–Vis absorption spectroscopy and temperature-dependent photoluminescence spectra further confirmed strong electron–phonon coupling, which induces symmetry-breaking distortions that stabilize self-trapped excitons (STEs). DFT calculations revealed linear thermochromic behavior in Te-doped (NH₄)₂SnCl₆ over a broad temperature range, thereby bridging theoretical and experimental insights into the structure–property relationship. Notably, the pronounced temperature dependence of the emission peak position enables 0.5%Te:(NH₄)₂SnCl₆ to achieve highly accurate temperature-sensing across a wide range. Compared with conventional ratiometric thermometry based on emission intensity or intensity ratios, this peak-shift-based approach offers higher measurement accuracy and reduced susceptibility to environmental interference [19,20].

2. Materials and Methods

Chemicals: All reagents and solvents were used without further purification. Ammonium chloride (NH₄Cl; 99.99%), stannic chloride hydrate (SnCl₄∙5H₂O; 99.995%) and tellurium tetrachloride (TeCl₄; 99.9%) were purchased from Aladdin. Hydrochloric acid (HCl; 36.0~38.0% in water) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China.

Synthesis of (NH₄)₂SnCl₆: (NH₄)₂SnCl₆ polycrystalline powders from small-batch production were prepared by dissolving 1 mmol SnCl₄∙5H₂O in a 3.5 mL HCl solution using constant stirring with a magnetic stir bar under ambient conditions. Then, 2 mmol NH₄Cl in 0.5 mL deionized water was immediately dropped into the above solution. Upon the addition of NH₄Cl, a white polycrystalline precipitate was formed. The product was then extracted from the crude solution via centrifugation (4500 rpm; 3 min, TG16-WS, Beijing North Tech Sichuang Scientific Instrument Co., Ltd., Beijing, China), washed with a HCl solution twice and dried under pressure overnight. The Te-doped (NH₄)₂SnCl₆ samples were prepared via precipitation reactions following the same procedure with the addition of a nominal amount of TeCl₄ in the HCl solution.

Characterization: The samples were characterized by transmission electron microscopy (TEM). TEM images were obtained using a JEM-2200FS (JEOL, Tokyo, Japan) machine equipped with an emission gun operated at 200 kV.

Optical Measurements: The optical absorption spectra measurements were carried out between 250 and 1000 nm using a deuterium–halogen light source and recorded with an optical fiber spectrometer (QE65000, Ocean Optics, Orlando, FL, USA) and a data collection time of 10 s. The PL experiments were carried out using a 355 nm excitation laser with 10 mW power.

Structural characterization: The structural characterization of the synthesized samples was performed using an X-ray diffractometer (XRD, D8 Advance Bruker, Berlin, Germany) with Cu Kα radiation (λ = 1.5406 Å), scanning over a range of 10–60° (2θ). The Raman spectra were recorded using a spectrometer equipped with liquid-nitrogen-cooled CCD (iHR 550, Symphony II, Horiba Jobin Yvon, Irvine, CA, USA). A 671 nm single-mode DPSS laser was utilized to excite the sample and the output power was 10 mW. The resolution of the system was 1 cm⁻¹. All the high-pressure experiments were performed at room temperature.

3. Results and Discussion

The (NH₄)₂SnCl₆ and Te:(NH₄)₂SnCl₆ samples were synthesized according to the report in [9]. The details were provided in the experimental section. The transmission electron microscopy (TEM) images reveal that the polycrystalline powders synthesized at room temperature consist of rod-shaped particles with a uniform morphology, having lengths of approximately 15 μm (Figure 1a). Such morphological uniformity plays a critical role in influencing device performance and operational stability. Enhanced structural homogeneity helps to reduce defects, thereby optimizing the functional response of the device (such as response sensitivity and operational stability). As depicted in Figure 1b, lead-free vacancy-ordered (NH₄)₂SnCl₆ perovskite crystallizes in the cubic phase (left panel). Notably, Te is the same periodic element as Sn and they have similar coordination characteristics with halogen, suppressing the doping. This results in a lattice defect and strain as far as possible. After doping the Te⁴⁺ ions, the [SnCl₆]²⁻ octahedron in (NH₄)₂SnCl₆ can be partly substituted by the formed [TeCl₆]²⁻ octahedron (right panel). The [Sn(Te)Cl₆]²⁻ octahedra are isolated from each other, similar to the typical structure of Cs₂SnX₆ (X = Cl, Br and I).

To confirm the successful incorporation of Te without inducing structural phase transitions, the XRD patterns were recorded. The XRD of the as-synthesized undoped (NH₄)₂SnCl₆ sample corresponded well with the standard pattern of (NH₄)₂SnCl₆. No impurity diffraction peaks were observed, indicating that the as-synthesized (NH₄)₂SnCl₆ was pure. No impurity phase was detected in 0.5% Te-doped (NH₄)₂SnCl₆, indicating the high purity of 0.5%Te:(NH₄)₂SnCl₆. The refinement profiles of the (NH₄)₂SnCl₆:xTe (x = 0 and 0.5%) samples showed well-refined simulated patterns (Figure 2a). Furthermore, as is well-established, the ion radius of Te⁴⁺ (VI, 0.97 Å) is bigger than that of Sn⁴⁺ (VI, 0.69 Å). With Te doping, the XRD peaks gradually shifted to a smaller diffraction angle, verifying that the partial Sn⁴⁺ was successfully replaced by the bigger Te⁴⁺ in the (NH₄)₂SnCl₆ lattice. Detailed Rietveld refinements are provided in Table S1. Furthermore, the stable oxidation states of Sn⁴⁺ and Te⁴⁺ ensure excellent ambient and thermal stability. As shown in Figure 2b, the absorption spectrum of pure (NH₄)₂SnCl₆ shows a bandgap of 4.26 eV and the strongest band edge absorption, located at 291 nm, is slightly smaller than the reported value of (NH₄)₂SnCl₆ (310 nm). The absorption tail in the long-wavelength region could be attributed to defect or surface states, similar to those observed in Cs₂SnCl₆ NCs. As Te⁴⁺ is incorporated, the absorption spectrum shows two distinct absorption humps at 300–360 nm and 370–500 nm. As shown in Figure S1, the bandgap of 0.5%Te: (NH₄)₂SnCl₆ significantly reduced to 2.66 eV compared with (NH₄)₂SnCl₆ (4.26 eV). The newly emerging absorption region between 350 and 500 nm could be assigned to localized Te 6s² → 6s¹p¹ transitions in [TeCl₆]²⁻ octahedra. The long-wavelength absorption feature centered at ~390 nm can be assigned to the ¹S₀ →³P₁ transition of Te⁴⁺, which is a doublet caused by the dynamic Jahn–Teller effect. Correspondingly, the peak of the short-wavelength band at ~330 nm is assigned to the ¹S₀ →¹P₁ transition.

At room temperature, Sn(IV)-based compounds are more stable in air than tin(II)-based counterparts. However, the absence of ns² active lone pairs cannot effectively promote the formation of Jahn–Teller-like STEs; thus, the pure Sn(IV)-based halides of the (NH₄)₂SnCl₆ white powder showed no emission under a 355 nm UV lamp. Upon the exchange of Sn/Te ions, the introduction of Te⁴⁺ ions with active 5s² lone pairs effectively promotes the formation of Jahn–Teller distortions in the lattice structure [13]. The Jahn–Teller distortions formed during the expansion process are conducive to the improvement in light emission performance. Hence, after doping the Te⁴⁺ ions, the formed 0.5%Te: (NH₄)₂SnCl₆ perovskite emits a strong yellow light under a 355 nm UV lamp. As shown in Figure 2c, the photoluminescence (PL) spectra show a bright broadband orange emission peak at 585 nm with a full width at half maximum (FWHM) at about 116 nm and a large Stokes shift. The broadband PL spectrum profile and large Stokes shift are due to the intense lattice distortion and robust Jahn–Teller effect. Hence, a significant improvement in photoluminescence was observed from the Te⁴⁺ substitution. Moreover, the exact element content was determined by XPS. In Figure 2d,e, the peaks located at 597.78 and 587.48 eV belong to Te⁴⁺ 3d_3/2 and 3d_5/2, respectively. These XPS profiles indicate that Te prefers to replace the Sn site, forming the TeCl₆ octahedron in 0.5%Te: (NH₄)₂SnCl₆ perovskite variants.

To further clarify the emission mechanism, the PL emission spectra at different temperatures were measured from 80 K to 420 K (Figure 3a,b). A 2D projection of the emission spectra with contour lines demonstrated the evolution of luminescence with temperature (Figure 3c). The emission intensity first increased as the temperature increased from 80 K to 280 K, but then decreased with temperatures from 280 K to 420 K (Figure 3d). Such behavior suggests that PL could originate from thermally activated delayed fluorescence, which involves reverse intersystem crossing (RISC) from long-lived triplet-excited states back to short-lived singlet-excited states [21] via the thermally activated non-radiative recombination. A comparison with the previous commonly observed PL shifts with temperature in ABX₃ and A₂BB’X₆ perovskites (X = Cl, Br and I) [22,23,24] revealed that this abnormal PL blue shift monotonically corresponds with increasing temperatures and is ascribed to important thermal lattice expansion [22]. As shown in Figure 3e, the variation in PL intensity with temperature can be expressed as follows:

$$I=I_0/(1+{\mathrm{A}\mathrm{e}\mathrm{x}\mathrm{p}}^{-E_a/k_B\mathrm{T}})$$

where I₀ and I are the emission intensities at 0 K and measurement temperature, respectively. E_a is the thermal activation energy, which is considered to evaluate the exciton binding energy [25]. k_B is the Boltzmann constant. The E_a of 0.5%Te:(NH₄)₂SnCl₆ was calculated to be 553 meV, which is much higher than Te:Cs₂SnCl₆ (270 meV) and Rb₂TeCl₆ (117 meV) [10,11]. The E_a of 0.5%Te:(NH₄)₂SnCl₆ is much higher than the thermal energy at room temperature and indicates the formation of stable STEs with highly localized behavior. The electron–phonon coupling degree can be effectively reflected by the variation in FWHM with temperature (Figure 3f). The temperature-dependent FWHM can be fitted as follows:

$$\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}(\mathrm{T})=2.36\sqrt{S}\mathrm{ħ}{\omega }_{Phonon}\sqrt{\mathit{coth}{\displaystyle \frac{\mathrm{ħ}{\omega }_{Phonon}}{2k_{B}T}}}$$

where S is the electron–phonon coupling parameter, ħω is the phonon frequency and k_B is the Boltzmann constant. The S of 7.5 and ħω of 46.2 meV were obtained by fitting. The ħω value is consistent with the Raman spectra (Figure S2). Such a value of S indicates robust electron–phonon coupling, implying prominent lattice distortion and the resulting Jahn–Teller effect in 0.5%Te: (NH₄)₂SnCl₆.

Generally, the DFT calculations provide an insight into the optical properties and energy band structure of undoped (NH₄)₂SnCl₆ and Te-doped(NH₄)₂SnCl₆ (Figure 4a,b). In the undoped compound, the VBM mainly consists of Cl-p orbitals, while the conduction band minimum (CBM) is dominated by Cl-p and Sn-s orbitals. The NH₄⁺ ions do not contribute near the band edges. Upon Te doping, the electronic density of states reveals a new VBM above the original valence band. The bandgap is abruptly reduced to 1.78 eV, which is mainly attributed to the newly formed VBM, consisting of states of the Cl 3p and filled pseudo-closed Te 5s orbitals. It is well-established that the STE emission of Te-doped perovskites is likely a result of the interplay between the 5s² lone pair of Te⁴⁺ [11]. As an ion whose outer electron configuration is s², the energy bands of Te⁴⁺ are generally composed of ground state ¹S₀, singlet-excited state ¹P₁ and triplet-excited state ³P_n (n = 0, 1 and 2) [26]. In combination with the above results, the luminescence mechanism diagram of 0.5%Te:(NH₄)₂SnCl₆ is illustrated in Figure 4d,e. Due to the soft lattice structure of 0.5%Te:(NH₄)₂SnCl₆, the excited carriers are readily localized to form bound excitons, which then relax to the trap state. Electrons are emitted through energy transfer to the Te⁴⁺ triplet STEs, followed by non-radiation relaxation, resulting in STE emission. For the Te⁴⁺ of s² electron configuration, ¹S₀ → ³P₁ and ¹S₀ → ¹P₁ are parity-allowed due to spin-orbit coupling. The electrons are transferred to the singlet state with high energy, then undergo the intersystem crossing (ISC) process from singlet STEs to triplet STEs. Finally, the compound from triplet STEs to the ground state results in a wide emission band with a large Stokes shift [24]. At low temperatures (80–280 K), the ³P₁ state is partially frozen because the thermal energy is insufficient to overcome its energy barrier. As the temperature rises from 80 K to 280 K, this state becomes thermally activated, enhancing the reverse intersystem crossing (RISC) and leading to an increase in the photoluminescence (PL) intensity. However, a further increase in temperature beyond 280 K enhances non-radiative recombination pathways, which subsequently quenches the PL emission. Therefore, the emission intensity of 0.5%Te:(NH₄)₂SnCl₆ reaches a maximum at 280 K. The emission blue shift monotonically corresponding with increasing temperature is ascribed to important thermal lattice expansion and enhanced electron–phonon coupling. The electron–phonon coupling coefficient is basically scaled to the temperature because of atom motion in the lattice. There is also a positive correlation between the electron–phonon coupling coefficient and temperature in perovskites. Previous studies prove that higher temperatures lead to a larger electron–phonon coupling constant [27], resulting in a blue shift of the emission. However, the influence of the electron–phonon coupling constant on emission intensity is non-linear: a strong coupling constant contributes to enhanced emission intensity, whereas an excessively large coupling constant promotes non-radiative recombination, leading to diminished emission intensity. With an increase in temperature from 280 K to 420 K, the excessively large electron–phonon coupling accounts for the crossover between the GS and the STEs, promoting phonon-assisted non-radiative recombination and thereby reducing the emission intensity of 0.5%T Te:(NH₄)₂SnCl₆.

Additionally, temperature-dependent luminescence data were processed to generate plots depicting the evolution of the peak position and full width at half maximum (FWHM) as functions of temperature (Figure 5a,b). The emission peak exhibits a linear blue shift with increasing temperature according to λ = −0.14 T +615.6 nm (R² = 0.99), where T denotes temperature in Kelvin. This temperature sensitivity corresponds with an absolute temperature sensitivity. Therefore, to quantify observed changes, the absolute temperature sensitivity can be determined as ${\mathrm{S}}_{\mathrm{A}}={\displaystyle \frac{∆\mathsf{\lambda }}{∆\mathrm{T}}}$, with Δλ representing the spectral shift per temperature increment of ΔT. The absolute sensitivity (S_A) is 0.14 nm/K and relative sensitivity (Sr) is 0.23% K⁻¹ at 100 K. Concurrently, the FWHM of the Te-doped emission broadens linearly as FWHM = 0.11T +73.5 meV (R² = 0.99). Based on the research findings, 0.5%Te:(NH₄)₂SnCl₆ demonstrates significant potential as a promising candidate for non-contact dual-parameter thermometry across a wide temperature range of 100–400 K. The chromaticity coordinates of 0.5%Te:(NH₄)₂SnCl₆ can effectively be controlled by temperature, ranging from orange-yellow to light yellow, which can be observed from the emission color changes throughout the temperature-increase process (Figure 5c). Moreover, the CIE coordinates are (0.5439, 0.4303) at 80 K and (0.3943, 0.5195) at 420 K. The CIE(x,y), CCT(K) and CRI of 0.5%Te:(NH₄)₂SnCl₆ at different temperature are presented in Table S2. In addition, the typical thermochromism behavior, linear variation in the emission wavelength and FWHM of the emission band with temperature make it a promising candidate for temperature sensors (Figure 5d). The linear thermochromic behavior of the material originates from temperature-dependent changes in the crystal structure. DFT calculations were performed to simulate the crystal structures at various temperatures and the lattice parameters were plotted as a function of temperature (Figure S4). The results indicate a nearly linear expansion of the lattice parameters with an increasing temperature. This structural expansion is identified as the primary mechanism responsible for the observed linear thermochromism. Temperature variations can alter the crystallinity and interfacial stress of a material, thereby affecting its performance. Given that perovskite materials possess a soft lattice structure, reversible structural and optical changes are achievable within a certain temperature range. The XRD patterns of (NH₄)₂SnCl₆ and 0.5%Te:(NH₄)₂SnCl₆ were observed before and after storing them under ambient conditions for 20 months. All patterns show a pure-phase crystal structure, indicating no structural evolution and, therefore, good stability of the materials under ambient conditions (Figure S3). Furthermore, the sample can withstand the extreme condition of immersion in water without any pretreatment and maintain a bright luminescence. This stability significantly broadens its application in the field of temperature-sensing. These results suggest that the luminescence behavior of 0.5%Te:(NH₄)₂SnCl₆ can be promoted by modulating the temperature, which makes it a potential candidate for modern highly efficient and visual (colorimetric) optical temperature sensors.

4. Conclusions

In summary, we investigated the structural evolution and optical properties of 0.5%Te:(NH₄)₂SnCl₆ as a function of temperature from 80 K to 400 K. At low temperatures (80–280 K), the ³P₁ state is partially frozen because the thermal energy is insufficient to overcome its energy barrier. As the temperature rises from 80 K to 280 K, this state becomes thermally activated, leading to an increase in the photoluminescence (PL) intensity. However, a further increase in temperature beyond 280 K enhances non-radiative recombination pathways, which subsequently quenches the PL emission. Therefore, the emission intensity of 0.5%Te:(NH₄)₂SnCl₆ reaches a maximum at 280 K. The emission blue shift monotonically corresponding with increasing temperature is ascribed to important thermal lattice expansion and enhanced electron–phonon coupling. When increasing the temperature from 280 to 420 K, the excessively large electron–phonon coupling accounts for the crossover between the GS and the STEs, promoting phonon-assisted non-radiative recombination and thereby reducing the emission intensity of 0.5%Te:(NH₄)₂SnCl₆. Based on the research findings, 0.5%Te:(NH₄)₂SnCl₆ demonstrates significant potential as a promising candidate for non-contact dual-parameter thermometry across a wide temperature range of 100–400 K. Our study delivers fundamental insights into the optical properties and band structure of 0.5%Te:(NH₄)₂SnCl₆ while establishing this material as a promising platform for non-contact thermometry across an ultra-broad temperature range (100–400 K).