Effect of Na–Sn Flux on the Growth of Type I Na₈Si₄₆ Clathrate Crystals

Abstract

In the crystal growth of Na–Si clathrate (type I, Na₈Si₄₆) during Na evaporation from a Na–Si–Sn solution at 723 K, the composition of a Na–Sn flux in the starting material strongly influences the morphology and size of the formed clathrate crystals. In this study, the crystals obtained using this flux were larger than the crystals prepared without a flux, and some of them had faceted surfaces. At the Na₄Si₄ (precursor):4Na–Sn (flux) = 1:4 ratio, multiple dents were observed on crystal surfaces, indicating that the precipitation of a Na₉Sn₄ solid phase prevented the growth of Na–Si clathrate crystals. In addition, synthesis conditions, under which type I crystals could be obtained by conventional thermal decomposition in vacuum, were established. The results of this work suggest that type I Na–Si clathrate crystals are stable even at temperatures as high as 723 K due to the suppressed evaporation of Na.

1. Introduction

Si clathrates are cage-like compounds composed of Si atoms that exhibit a unique spatial structure surrounded by five-membered and six-membered rings (similar to that of fullerenes). Most Si clathrates contain guest atoms such as alkali or alkaline earth metals in Si cages, and their properties strongly depend on the relationship between these atoms and the cages [1,2,3]. Therefore, Si clathrates containing guest atoms have a wide range of potential applications, including storage materials, thermoelectric devices, and solar cells. On the other hand, guest-free Si clathrate is promising as a next-generation solar cell material because it has about a 0.7 eV larger band-gap than a cubic diamond-type silicon [4]. This clathrate was prepared by extracting Na from a Na-Si clathrate which contains Na atoms in Si cages [5,6]. Among various Si clathrates, the Na–Si clathrates have been actively studied as the precursor of guest-free Si clathrate as described in the literature [1,2,3]. The reported Na–Si clathrates are classified into type I (Na₈Si₄₆) and type II (Na_xSi₁₃₆, 0 < x < 24) compounds. Type I clathrates belong to the Pm$\overline{3}$n space group and consist of two types of Si polyhedra: dodecahedra (Si₂₀) and tetrakaidecahedra (Si₂₄). The frameworks of type II clathrates with the Fd$\overline{3}$m space group comprise dodecahedra (Si₂₀) and hexacaidecahedra (Si₂₈). In these compounds, Si polyhedral cages encapsulating Na atoms share faces with tetrahedrally coordinated bonds.

Na–Si clathrates are generally synthesized from Na₄Si₄ Zintl compounds [2,3]. However, because clathrates are prepared by the thermal decomposition of solid precursors, the obtained samples are miniaturized to powders containing micro-sized grains. Note that bulk crystals are indispensable for property evaluation and applications. Although powder sintering is widely used to prepare bulk samples, an Na–Si clathrate decomposed after heating above 923 K in a previous study [7], while the covalently bonded framework prevented low-temperature sintering. To prepare bulk Na–Si clathrate crystals, type II crystal growth was realized via thermal decomposition under the stress generated by spark plasma sintering [8]. Furthermore, a novel thermal decomposition method was proposed, in which Na₄Si₄ was heated inside a closed space surrounded by NaCl and graphite to suppress Na evaporation, and Na atoms were gradually extracted from Na₄Si₄ by their reaction with graphite [9]. As a result, type I and type II single crystals with sizes of hundreds of micrometers have been successfully synthesized.

In our research group, the solution growth of Na–Si clathrate single crystals was realized by evaporating Na from a Na–Si–Sn solution [10]. In addition, type I and type II clathrate crystals could be selectively prepared by controlling the synthesis temperature [11]. Recently, type II single crystals with sizes of several millimeters were obtained from seed crystals [12]. In the described methods, Na₄Si₄ was used as a precursor for the Si source, and an Na₁₅Sn₄ or Na₉Sn₄ compound melt was employed as a flux for precursor dissolution. In this previous study, a starting composite material consisting of Na₄Si₄, Na₁₅Sn₄, and Na with a molar ratio of Na: Si: Sn = 6:2:1 was heated to 723 K to prepare a Na–Si–Sn solution by dissolving Na₄Si₄ in a Na–Sn flux. Na–Si clathrate crystallized during Na evaporation from this solution. The compositional changes induced by heating are described by the following equation:

2Na₄Si₄ + Na₁₅Sn₄ + Na → 4/23Na₈Si₄₆ + Na₉Sn₄ + 313/23Na (vapor)

(1)

Because this process involves flux-assisted solution growth, the composition of the Na–Sn flux in the starting material strongly influences the morphology and size of the produced clathrate crystals. Therefore, in the present work, we examined the effect of Na–Sn flux on the growth of type I Na–Si clathrate crystals under various flux conditions. In addition, a synthesis condition, under which an almost single phase of type I could be obtained even by conventional thermal decomposition in vacuum, was identified.

2. Experimental

Metal Na pieces (Nippon Soda Co., Ltd., Tokyo, Japan, purity: 99.95%), Si powder (High Purity Chemical Research Institute Co., Ltd., Saitama, Japan, purity: 5 N), and granular Sn (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan, purity: 99.99%) were used as the starting materials. Na, Si, and Sn were weighed in a glove-box filled with Ar gas to achieve the ratio Na₄Si₄:4Na–Sn = 1:4x (x = 0, 0.1, 0.25, 0.5, or 1.0) and then placed into a BN crucible (Denka Co., Ltd., Tokyo, Japan, purity: 99.5%, outer diameter: 8.5 mm, inner diameter: 6.5 mm, depth: 18 mm). The crucible was subsequently sealed inside a stainless-steel container (SUS316: outer diameter: 12.7 mm, inner diameter: 10.7 mm, height: 80 mm) filled with Ar gas. The reaction container was heated at 1173 K for 12 h and then slowly cooled to room temperature. After heating, the container was opened in a glove box under an Ar atmosphere, and the BN crucible containing the sample was removed and placed inside another stainless-steel reaction container connected to a vacuum system (SUS316: outer diameter: 12.7 mm, inner diameter: 10.7 mm, height: 300 mm). After the sample transfer, the pressure inside the container filled with Ar gas was reduced to approximately 10⁴ Pa using a dry pump. A schematic diagram of the reaction container and detailed experimental procedure are described elsewhere [9]. The upper part of the reaction container with the sample was heated in an electric furnace.

In this study, crystals were grown at 723 K, and a type I clathrate single phase was obtained. Assuming that all the produced crystals are of type I, at a molar ratio between the solute Na₄Si₄ and solvent 4Na–Sn equal to 1:4x mol, the chemical reaction occurring during Na evaporation can be described as follows:

Na₄Si₄ + 4x(4Na–Sn) → 2/23Na₈Si₄₆ + xNa₉Sn₄ + (76/23 + 7x)Na ↑

(2)

In this experiment, the evaporation rate of Na was equal to (76/23 + 7x)/(4 + 16x) × 100 (%).

Table 1. Experimental conditions, ideal and experimental Na evaporation rates, and reaction products obtained under these conditions.

Flux Component, x	Heating Temperature (K)	Heating Time (h)	Pressure inside the Container (Pa)	Ideal Evaporation Rate of Na (%)	Experimental Evaporation Rate of Na (%)	Product
0	723	15	~104	82.6	85.3	type I + d-Si
0.1	723	14	~104	71.5	70.2	type I
0.25	723	17	~104	63.2	64.2	type I
0.5	723	24	~104	56.7	54.9	type I
1.0	723	28	~104	51.5	48.7	type I
0	723	15	~10	82.6	87.1	d-Si
0	673	15	~10	82.6	88.9	type I + type II + d-Si

lists the Na evaporation rates calculated at x = 0, 0.1, 0.25, 0.5, and 1.0. The evaporation velocity of Na in the utilized container was about 4.0 mg/h, the value of which does not really depend on the amount of Na. Therefore, the heating time required to completely convert Na₄Si₄ to Na₈Si₄₆ was calculated from the ideal Na evaporation rate and Na evaporation velocity at each flux composition. For example, in the case of x = 0.5, the amount of Na in the starting materials was about 175 mg in the present experiment. Therefore, about 99 mg of Na should be evaporated to convert Na₄Si₄ to Na₈Si₄₆ because the ideal evaporation rate of Na is 56.7% in this case. The heating time required to evaporate the 99 mg of Na is calculated to 24.8 h from the evaporation velocity of Na. The heating time was shortened to 24 h because excessive heating causes the decomposition of Na-Si clathrate. According to Table 1, the heating times at the flux compositions corresponding to x = 0.1, 0.25, 0.5, and 1.0 are similarly set to 14, 17, 24, and 28 h, respectively. When Sn is not used (x = 0), Na is extracted from the solid phase Na₄Si₄; therefore, the Na evaporation velocity decreases to about 3.4 mg/h. In addition, Na was used in excess (Na:Si = 1.1:1) to ensure that unreacted Si did not remain in the raw materials. The heating time was calculated at the excessive Na content and set to 15 h for x = 0.

After heating, the sample was removed from the reaction container in the glove box and washed sequentially with ethanol and pure water in air. The Sn traces remaining after washing were removed by dissolving them in dilute nitric acid with a concentration of 10% or less. In addition, to investigate the effect produced by the flux on the crystal growth, the synthesis procedure was also conducted under high vacuum without using a Na−Sn flux, as was done in a previous work [7]. The inside of the container was depressurized to approximately 10 Pa using a rotary pump, and heated at 673 and 723 K for 15 h. The obtained sample was washed with ethanol and pure water as described above.

Scanning electron microscopy (SEM; JEOL, Tokyo, Japan, S4800) was performed to observe the produced single crystals. The crystal phase was identified by powder X-ray diffraction (XRD, Rigaku, Tokyo, Japan, RINT 2200).

3. Results and Discussion

To clarify the effect of Na–Sn flux on the growth of clathrate crystals, clathrates were prepared under vacuum without a flux (x = 0). After heating Na₄Si₄ at 723 K for 15 h at a reduced pressure of 10⁴ Pa, Na evaporated from the sample, and agglomerates of fine particles with sizes of 10 µm or less were obtained (Figure 1a–c). According to XRD data, the resulting sample contained a mixture of type I clathrate Na₈Si₄₆ and diamond-type Si (d-Si) crystals (Figure 2a). In the absence of a flux (x = 0), the Na₄Si₄ precursor with a melting point of 798 K [13] is in a solid state at 723 K, and as a result, the obtained sample consists of fine particles.

During synthesis at the flux composition x = 0.1, particle agglomerates were obtained (Figure 1d–f). Their sizes were larger than those of the crystals prepared without a flux (x = 0), and some of these particles had faceted surfaces. Note that d-Si was not detected in the corresponding XRD pattern (Figure 2b), and the sample contained a single phase of type I Na₈Si₄₆ crystals. At x = 0.1, although large crystals were not obtained, some crystals had faceted faces, which confirmed crystal growth in the liquid phase. In this case, the Na₄Si₄ precursor was slightly dissolved in the flux and crystallized during Na evaporation. Furthermore, the utilized solution acted as a binder that facilitated crystal aggregation. Note that d-Si was not detected under this condition because the Na–Sn flux prevented its precipitation.

At the flux composition x = 0.25, multiple granular crystals were observed (Figure 1g). According to their SEM images, they consisted of single crystals with faceted surfaces (Figure 1h,i). Similar to our previous study, diamond-shaped faceted surfaces were produced, confirming the presence of single crystals with {110} facets [10,11]. In addition, the obtained XRD patterns revealed that all the prepared crystals were type I clathrate Na₈Si₄₆ (Figure 2c). Compared to the crystals grown at x = 0.25, many large crystals with a maximum length of 1.5 mm were obtained at x = 0.5. Even under these conditions, the fabricated single crystals were of type I and included {110} facets (Figure 2d). At x = 0.25 and 0.5, a sufficient amount of Na₄Si₄ was dissolved in the flux, and Na–Si single crystals grew during the evaporation of Na from the Na–Si–Sn solution. Although no significant differences between the crystals obtained at x = 0.25 and x = 0.5 were observed, the crystal size was slightly increased by using the Na–Sn flux.

For the crystals prepared at x = 1.0, multiple dents were observed on their surfaces as shown in Figure 1o. In addition, although these crystals had faceted surfaces, many of them were irregular, while a lot of crystals were smaller than those obtained at x = 0.5. It was confirmed that all crystals were of type I from XRD measurements (Figure 2e). Figure 3a,b shows the schematic diagrams of the crystal growth processes conducted at x = 0.5 and 1.0, respectively. The Na concentration in solution decreased during growth, and Na₉Sn₄ with a melting point of 751 K crystallized at 723 K. According to reaction equation (2), a mixed sample with a molar ratio of Na₈Si₄₆: Na₉Sn₄ = 2/23: 1 was obtained at x = 1.0, and the amount of Na₉Sn₄ generated under these conditions was twice as large as that obtained at x = 0.5. At x = 1.0, a large amount of Na₉Sn₄ crystals precipitated at the stage with a small Na evaporation rate, which prevented the crystal growth of Na₈Si₄₆ clathrate (Figure 3b). In the case of the crystal growth over 773 K, Na₉Sn₄ with a melting point of 751 K dissolves; therefore, its subphase does not contribute to the growth of clathrate crystals. However, type II clathrate precipitated at temperatures above 773 K; hence, to obtain a single phase of type I crystals, it is necessary to use an appropriate flux composition at 723 K. Furthermore, when the flux amount is large, the time required for Na evaporation also becomes long; therefore, an appropriate amount of Na is important for the successful growth of Na–Si clathrate crystals.

In addition to discussing the effects of flux, the previously unreported results of Na-Si clathrate without flux must be discussed. In the present study, a type I of Na-Si clathrate could be obtained without Sn flux at 723 K under 10⁴ Pa as shown in Figure 1a–c and Figure 2a. When the inside of the container was depressurized to 10 Pa and Na₄Si₄ was heated at 723 K, only the d-Si phase was produced (Figure 4a). In this case, the Na evaporation rate was 97.7%, indicating that almost all Na in the starting material was evaporated from the sample by heating. These results indicate that type I clathrate is unstable at 723 K under high-vacuum conditions; however, it can be obtained even at this temperature by suppressing the decomposition of the synthesis atmosphere.

In conventional clathrate synthesis, a mixture of type I and type II clathrates is often generated. In this study, although the type II clathrate was not generated without a flux (x = 0) at 723 K under 10⁴ Pa, a mixture of type I and type II crystals was also obtained at a temperature of 673 K (Figure 4b). Therefore, type I clathrate was stable at 723 K when Na evaporation was suppressed. Horie et al. synthesized a mixture of type I and type II clathrates by heating Na₄Si₄ at 723 K under 10⁻⁴ Torr [7]. In this study, to suppress the evaporation of Na, a cap with a pinhole was placed on the crucible, which increased the proportion of type I crystals in the obtained sample. This result suggests that type I clathrates can be stabilized even at 723 K by suppressing Na evaporation.

In the present study, d-Si was detected when Na₄Si₄ was heated at 723 K without a flux. Moreover, a large amount of Si was formed in the upper part of the sample after heating. During this process, Na evaporated from the upper open part of the crucible; therefore, it was concluded that Na was removed from the upper part of the sample. It is assumed that the Na–Si clathrate generated at the initial stage of this process was decomposed to Si because the clathrate produced in the upper part of the sample was heated for a longer time than that in the lower part of the sample. To clarify the effect of Na evaporation from the sample, the bulk Na₄Si₄ precursor was crushed into a powder and then heated at 723 K under 10⁴ Pa. Consequently, while a small amount of d-Si was precipitated, as shown in the XRD pattern depicted in Figure 4c, the decomposition of the obtained Na–Si clathrate was suppressed because Na was uniformly extracted from the sample. From these results, it was concluded that the reduced pressure condition during heating affected the generated phase. Furthermore, it was shown that type I clathrate crystals could exist stably even at a temperature of 723 K. However, Na evaporation tends to be uneven, and only a powder sample can be obtained using the previously described method.

From these results, high-vacuum synthesis and non-uniform evaporation of Na cause the decomposition of Na-Si clathrate to d-Si at 723 K. In the present study, type I crystals grow slowly in a solution rich in Na, which is not easily affected by the evaporation of Na. Hence, the flux-assisted method developed in this work is suitable for the reliable preparation of a type I Na–Si clathrate single phase.

4. Conclusions

In this study, we investigated the effect of Na–Sn flux on the growth of type I Na–Si clathrate crystals under various flux conditions and in the absence of a flux at 723 K. As a result, agglomerates of fine particles with sizes of 10 µm or less were obtained. Compared with the crystals prepared without a flux, larger crystals were produced using a Na–Sn flux, and some crystals had faceted surfaces. At x = 0.25 and 0.5 (Na₄Si₄ (precursor):4Na–Sn (flux) = 1:4x), diamond-shaped faceted surfaces were observed, confirming the successful synthesis of single crystals with {110} facets. Although the crystal size generally increased with increasing flux component, it noticeably decreased at x = 1.0, and numerous dents were formed on crystal surfaces at this condition. This indicates that the precipitation of the Na₉Sn₄ solid phase prevented the growth of Na–Si clathrate crystals. In addition, the optimal synthesis conditions under which type I crystals could be obtained even by the conventional thermal decomposition method in vacuum, were identified. From these results, it was concluded that the reduced pressure during heating strongly affected the final product. However, a single phase of type I crystals could not be obtained using the previously developed method. Hence, the flux-assisted method proposed in this study is suitable for the stable generation of a single phase of type I Na–Si clathrate crystals.