K₆Sn₄F₁₂I₂•0.5H₂O: a zero-dimensional alkali metal tin mixed halide compound exhibiting color change due to crystal water loss

INTRODUCTION

Metal halides have been researched in detail as a result of their structural and functional diversity, which has resulted in many important and interesting optoelectronic applications, such as photovoltaics, photoelectricity, nonlinear optics and photoluminescence^[1-11]. According to the structure-property relationship, these optoelectronic properties are determined by their microstructures. For example, by reducing the structural dimensions from three dimensions to zero dimensions to promote the recombination of photogenerated carriers, (TTA)₂SbCl₅ and (bmpy)₉[ZnCl₄]₂[Pb₃Cl₁₁] have realized ultrahigh photoluminescence quantum yield^[12,13]. Moreover, through halogen element hybridization, metal halides, such as Cs₂HgI₂Cl₂, Rb₂CdBr₂I₂ and K₂SbF₂Cl₃, [NH₂(CH₂CH₃)₂]₃Bi(Cl_1-xBr_x)₆, have significantly improved the laser damaged threshold and can maintain their nonlinear optical effects simultaneously, which make them good candidates as nonlinear optical materials in the infrared spectral region^[14-17]. Considering their excellent optoelectronic properties and important applications, it is necessary to continue the research of structure-property relationships in metal halides.

Metal halides are normally obtained by solution or hydrothermal methods, where water can play an important role in determining their structures and optoelectronic properties. For example, by introducing water molecules to BiF₃, Liu et al.^[18] successfully regulated the optical birefringence by changing the coordination environment of Bi³⁺. Yang et al.^[19] changed the luminescence color from orange to blue by recrystallizing Cs₂InCl₅ in water. Cordero et al.^[20] investigated the roles of interstitial water in the structural transitions and stability of FAPbI₃ and MaPbI₃. Therefore, further investigation into the role of water molecules in the modulation of metal halide properties is essential.

Our group has focused on materials explorations of metal halides^[21-24]. In our previous work, we reported dozens of metal halides with nonlinear optical effects, photodegradation effects and photoluminescence properties^[25,26]. By introducing water, we successfully promoted the generation of more local photoelectrons and increased the photoluminescence quantum yield in metal halides^[27]. In this work, a new zero-dimensional alkali-metal tin halide, K₆Sn₄F₁₂I₂•0.5H₂O, is synthesized by a hydrothermal method. Its structure consists of crystal water molecules and fundamental structural blocks of [Sn₄F₁₂I₄] trapped in [K₁₈] cages. When exposed to air, its color slowly changes from colorless to orange. Experimental measurements combined with theoretical calculations reveal that it is crystal water loss that leads to the color change.

MATERIALS AND METHODS

RESULTS AND DISCUSSION

Single crystals of K₆Sn₄F₁₂I₂•0.5H₂O were obtained by a hydrothermal method using the raw materials of 1 g (0.027 mol) of NH₄F, 1 g (0.006 mol) of KI and 1 g (0.004 mol) of SnCl₂•2H₂O dissolved in 4 mL of deionized water. The mixture was reacted at 493 K and further slowly cooled to room temperature. Colorless transparent crystals of K₆Sn₄F₁₂I₂•0.5H₂O were obtained [Figure 1A]. The crystal structure was solved and refined on the basis of single-crystal data. The powder XRD patterns of obtained crystals show good agreement with the calculated results derived from the single-crystal data [Figure 1B], indicating the purity of the obtained samples (crystal data, atomic coordinates, atomic displacement parameters and bond distances and angles are given in Supplementary Tables 1-4).

Figure 1. (A) Photographs of obtained single crystals with color change. (B) Powder XRD patterns of K₆Sn₄F₁₂I₂•0.5H₂O. XRD: X-ray diffraction.

K₆Sn₄F₁₂I₂•0.5H₂O crystallizes in the cubic centrosymmetric space group of Fd-3m (No. 227) with unit cell parameters of a = b = c = 17.1860 Å. All the atoms are occupied in one crystallographically unique position in the symmetric unit. The K, Sn, F, I, O and H ions are at Wyckoff positions 48f, 32e, 96g, 16c, 8b and 32e. In the structure, the Sn²⁺ cations are coordinated with three F^- anions and form [SnF₃] triangular pyramids for the presence of stereochemically active lone-pair electrons on Sn²⁺ cations. The Sn-F bonds length are 2.0709 Å and the F-Sn-F angles are 84.156°, both of which are comparable with those in ASnF₃ (A = Na, K, Rb or Cs)^[28,29], indicating the rationality of our structural solution.

In order to illuminate the structure clearly, the [Sn₄F₁₂] supertetrahedra, [I₄] tetrahedra and [K₁₈] cage are displayed in Figure 2A. The [Sn₄F₁₂] supertetrahedra are composed of four [SnF₃] triangular pyramids with the Sn²⁺ cations located at the vertex. Every four isolated I^- anions can construct the [I₄] tetrahedra, which are staggered with the [Sn₄F₁₂] supertetrahedra and formed the cubic [Sn₄F₁₂I₄] fundamental structural blocks. In this cubic [Sn₄F₁₂I₄] fundamental structural block, the [SnF₃] triangular pyramids and I^- anions occupy the eight vertexes, respectively. The K⁺ cations are inserted into the space between the [Sn₄F₁₂I₄] fundamental structural blocks and every eighteen K⁺ cations construct a [K₁₈] cage, as shown in Figure 2A. These [K₁₈] cages further trap the cubic [Sn₄F₁₂I₄] fundamental structural blocks, as shown in Figure 2B. The distance between the neighboring K⁺ cations in the [K₁₈] cages is 4.4637 Å, which is large enough to hold the cubic [Sn₄F₁₂I₄] fundamental structural blocks. All K⁺ cations are six-coordinated to F^- anions with two short K-F bonds of 2.7735 Å and four long bonds of 2.8792 Å. The F-K-F angles vary from 57.632° to 123.107°, resulting in a planar arrangement of the four farther F^- anions, which makes the [KF₆] polyhedra adopt a saddle-like shape. These saddle-like polyhedra further construct the three-dimensional framework by sharing the edges, as shown in Supplementary Figure 1. After adding the crystal water molecules, the fundamental structural blocks construct the crystal structure of K₆Sn₄F₁₂I₂•0.5H₂O shown in Figure 2C.

Figure 2. (A) [Sn₄F₁₂] supertetrahedra, [I₄] tetrahedra and [K₁₈] cage. (B) [Sn₄F₁₂I₄] fundamental structural block in [K₁₈] cage. Upper panel, view along the [210] direction, and lower panel, view along the a-axis. (C) Structure of K₆Sn₄F₁₂I₂•0.5H₂O.

As shown in Figure 1A, when exposed to air, K₆Sn₄F₁₂I₂•0.5H₂O exhibits a color change from colorless to orange within 24 h. It should be noted that this phenomenon only occurs on the surface, with the internal color unchanged, even after exposure in air for one month. As a result, a single-crystal structure solution is not available for the microscopic structure of the compound after the color change. In order to investigate the color change mechanism, experimental characterization of the powder samples (obtained by grinding the single crystals and exposing them to air for more than 24 h) and theoretical calculations are performed.

The powder XRD patterns of the samples after the color change are shown in Figure 1B. As shown, the patterns are highly consistent with those before the color change, except for the decrease in crystallinity and two peaks from unknown compounds. This means that the color change does not change the powder XRD patterns. UV-vis-NIR diffuse-reflectance spectra for the samples before and after the color change were measured and the results are exhibited in Figure 3A. As the results show, the shortest absorption edges are 350 and 565 nm for K₆Sn₄F₁₂I₂•0.5H₂O before and after the color change, corresponding to 3.5 and 2.2 eV, respectively. The results also agree well with the sample colors before (colorless) and after (orange) the change. Thermogravimetric measurements were also carried out to evaluate the stability of K₆Sn₄F₁₂I₂•0.5H₂O and the results are shown in Figure 3B. Clearly, K₆Sn₄F₁₂I₂•0.5H₂O started losing weight at the beginning and finally remained at ~45% after the thermal measurement. Even close to room temperature, the weight of K₆Sn₄F₁₂I₂•0.5H₂O decreased. This means that K₆Sn₄F₁₂I₂•0.5H₂O possesses relatively low stability and its color change happens with weight loss at room temperature, which is consistent with the observations from the UV-vis-NIR diffuse-reflectance experiments. Considering the slow weight loss and relatively low transition temperature, K₆Sn₄F₁₂I₂•0.5H₂O is most likely to lose crystal water during the color change.

Figure 3. (A) UV-vis-NIR diffuse-reflectance spectra of K₆Sn₄F₁₂I₂•0.5H₂O before (blue) and after (red) color change. (B) Thermogravimetric analysis curve of K₆Sn₄F₁₂I₂•0.5H₂O.

As the crystal structure after the color change is unavailable, first-principles calculations^[30] on K₆Sn₄F₁₂I₂•0.5H₂O and its anhydrous structure K₆Sn₄F₁₂I₂ (hypothetical) were performed to investigate the color change mechanism. The hypothetical structure K₆Sn₄F₁₂I₂ was built by removing the crystal water and its structure was optimized. After geometry optimization, the obtained structure of K₆Sn₄F₁₂I₂ possesses well-matching powder XRD patterns [Figure 1B] with the experimental ones after the color change, which proves the rationality of our hypothesis. As shown in Figure 4A, the calculated band gap of 2.24 eV for K₆Sn₄F₁₂I₂•0.5H₂O is smaller than the experimental value of 3.5 eV, owing to the discontinuity of the exchange-correlation energy^[31]. When the crystal water molecules are removed from the structure, K₆Sn₄F₁₂I₂ exhibits a smaller band gap of 1.96 eV [Figure 4C]. This trend agrees well with the experimental observations and can be accounted for the color change that the water loss makes the band gap smaller. In addition, we further calculated the partial density of states between the bandgap projected on the constituent elements of K₆Sn₄F₁₂I₂•0.5H₂O [Figure 4B] and the hypothetical K₆Sn₄F₁₂I₂ [Figure 4D]. Clearly, for both compounds, the electronic states close to band gaps are mainly composed of Sn and I orbitals, indicating that their shortest absorption edges are both determined by the heavy element atoms, i.e., Sn and I. However, the presence of crystal water molecules changes the local coordinating environments of Sn and I, which affect their band structure. As shown, the orbitals of I are more localized at the top of valence bands in K₆Sn₄F₁₂I₂•0.5H₂O. In comparison, the orbitals of I at the top of valence bands in K₆Sn₄F₁₂I₂ are more disperse for the absence of water, which leads to the reduction of the band gap and further results in the color change.

Figure 4. Calculated electronic band structure and PDOS projected on the constituent elements of K₆Sn₄F₁₂I₂•0.5H₂O and hypothetical K₆Sn₄F₁₂I₂. (A) and (B) for the former and (C) and (D) for the latter. PDOS: Partial density of states.

CONCLUSIONS

In summary, a new alkali-metal tin halide, K₆Sn₄F₁₂I₂•0.5H₂O, was synthesized by a hydrothermal method. It possesses a zero-dimensional structure consisting of crystal water molecules and [Sn₄F₁₂I₄] fundamental structural blocks trapped in [K₁₈] cages. Interestingly, K₆Sn₄F₁₂I₂•0.5H₂O exhibits a color change from colorless to orange when exposed to air. Experimental measurements and theoretical calculations were performed and the results revealed that this color change is attributed to the loss of crystal water in the microstructure.