Crystal Growth Conditions of Types i and II Na-Si Clathrates by Evaporation of Na from a Na-Si-Sn Solution

Article abstract of DOI:10.1021/acs.cgd.7b01342

Single crystals of the type I clathrate Na₈Si₄₆ were synthesized by the evaporation of Na from a Na-Si-Sn solution (Na:Si:Sn = 6:2:1 molar ratio) at 723-873 K under an Ar atmosphere of 10⁵ Pa. In the mixture of type I and type II clathrates prepared by heating at 773 K for 72 h, type I single crystals with sizes of up to 5 mm were found. A single phase of single crystals of the Na₂₄Si₁₃₆ type-II clathrate having a {111} crystal facet of about 2 mm on one side was obtained by heating at 873 K for 9 h.

Full text of DOI:10.1021/acs.cgd.7b01342

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Article
Crystal growth conditions of type I and II Na-Si
clathrates by evaporation of Na from a Na-Si-Sn solution
Haruhiko Morito, Masahi Shimoda, Hisanori Yamane, and Kozo Fujiwara
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01342 • Publication Date (Web): 28 Nov 2017
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Crystal growth conditions of type I and II NaꢀSi
clathrates by evaporation of Na from a NaꢀSiꢀSn
solution
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Haruhiko Morito1, Masashi Shimoda2, Hisanori Yamane2, and Kozo Fujiwara1
1 Institute for Materials Research, Tohoku University, 2ꢀ1ꢀ1 Katahira, Aobaꢀku, Sendai 980ꢀ
8577, Japan.
2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2ꢀ1ꢀ1
Katahira, Aobaꢀku, Sendai 980ꢀ8577, Japan.
KEYWORDS clathrate, single crystal, silicon, sodium
ABSTRACT
Single crystals of the type I clathrate Na8Si46 were synthesized by the evaporation of Na from a
NaꢀSiꢀSn solution (Na:Si:Sn = 6:2:1 molar ratio) at 723–873 K under an Ar atmosphere of 105
Pa. In the mixture of type I and type II clathrates prepared by heating at 773 K for 72 h, type I
single crystals with sizes of up to 5 mm were found. A single phase of single crystals of the
Na24Si136 typeꢀII clathrate having a {111} crystal habit of about 2 mm on one side was obtained
by heating at 873 K for 9 h.
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1. INTRODUCTION
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Sodium–silicon (NaꢀSi) clathrates, Na8Si46 (type I) and NaxSi136 0 < x ≤ 24 (type II), are
openꢀframework compounds composed of polyhedral Si cages encapsulating Na atoms.1, 2 Type I
NaꢀSi clathrate exhibits various characteristics, such as superconductivity,3 hydrogen storage,4
and thermoelectricity with a glasslike thermal conductitity,5 due to the unique openꢀframework
structure. The electrical characteristics of stoichiometric type I clathrate Na8Si46 shows a metallic
behavior, whereas those of type II clathrate NaxSi136 can be drastically changed from metallic to
semiconductor as a function of x.6–9 The bandgap of the Naꢀfree Si clathrate (Si136) has been
estimated to be 1.9 eV by first principles calculation, and this bandgap is larger than that of the
diamondꢀtype Si crystal.10, 11 Type II NaꢀSi clathrate could be of use in solar cell and
optoelectronic devices.
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The NaꢀSi clathrates have conventionally been synthesized by the thermal decomposition
of a Zintl compound (Na4Si4) at temperatures of 673–823 K under a high vacuum (< 10−2 Pa).1, 2
Using this method, the obtained samples are powdery solids with a grain size of micrometers
because Na4Si4 (melting point 1071 K)12 is solid at these temperatures. In addition, type I and II
clathrates often coexist in the obtained samples, and it is difficult to prepare singleꢀphase
samples of each phase.13 For the study and practical application of the NaꢀSi clathrates, single
crystals have been required.
Beekman et al. reported that type II clathrate Na24Si136 could be grown by heating Na4Si4
at 873 K under a load of 100 MPa using a sparkꢀplasma sintering (SPS) apparatus.14
Furthermore, single crystals of type I and type II could be selectively prepared by a kinetically
controlled thermal decomposition method in which Na4Si4 was sandwiched between graphite
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flakes with NaCl layers.15, 16 In this method, Na4Si4 was surrounded by a NaCl layer in a graphite
mold to reduce Na evaporation. Na was gradually extracted from Na4Si4 via the reaction between
the NaCl layer and graphite flakes, leading to slow crystal growth of the NaꢀSi clathrates. Type I
and type II single crystals with sizes of about 200 and 250 µm, respectively, were formed by
heating at 858 and 938 K, respectively.16
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Our group has succeeded in growing single crystals of the type I clathrate Na8Si46 with a
size up to about 1.5 mm by the evaporation of Na from a NaꢀSiꢀSn solution prepared by heating
Na, Na4Si4, and Na15Sn4 at 723 K for 24 h.17 The crystal growth was performed under a reduced
argon pressure of 104 Pa, which is higher than the pressures (< 10−2 Pa) used in conventional
methods. A supersaturated condition for the flux growth of single crystal growth was realized by
the reduction of the Na content in the solution. A mixture of polycrystalline type I and type II
clathrates and Si was prepared by Na evaporation at 773 K for 12 h, but a single phase of the
type II clathrate could not be obtained.
In the present study, the growth of larger crystals of the Na8Si46 type I clathrate was
attempted by reducing the evaporation rate of Na from the NaꢀSiꢀSn solution. To reduce the Na
evaporation rate, we heated the solution at an Ar pressure of 105 Pa, which is close to
atmospheric pressure and higher than that of the previous study (104 Pa). The higher pressure
enabled us to grow type II single crystals at 873 K.
2. EXPERIMENTAL SECTION
A piece of Na metal (Nippon Soda, purity 99.95%, 171–187 mg), Si powder (Kojundo
Chemical Laboratory, 4N, 70–76 mg), and Sn granules (Wako Pure Chemical Industries,
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99.99%, 147–161 mg) were weighed in a mole ratio of Na:Si:Sn = 6:2:1 in an Arꢀfilled glove
box (O2 < 1 ppm, H2O < 1 ppm). These starting materials were charged in a boron nitride
crucible (Showa Denko, 99.5%, 6.5 mm inner diameter, 18 mm depth) and sealed in a stainlessꢀ
steel container (SUS316, 12.7 mm outer diameter, 80 mm height) with Ar gas. The container was
heated in an electric furnace at 1173 K for 12 h and cooled to room temperature. After heating, a
mixture of Na15Sn4, Na4Si4, and Na was obtained in the BN crucible. The BN crucible containing
the mixture was transferred to another long stainless steel container with a different size (45 mm
outer diameter, 200 mm height) in the Arꢀfilled glove box. A schematic of the container is shown
in the Supplemental Data of Ref. 18. The sample in the crucible was placed in the lower part of
the container, and this part was heated with an electric heater at 723–923 K. Na metal was
evaporated from the sample and condensed at the upper cooler upper part of the container, which
had a temperature gradient. After heating, the crucible was taken out from the container in the
Arꢀfilled glove box. Airꢀ and moistureꢀsensitive compounds such as Na4Si4 and NaꢀSn binary
compounds in the products were reacted with 2ꢀpropanol and ethanol in air, and the waterꢀ
soluble reactants were removed by washing with water (alcohol treatment). Sn formed by this
treatment was removed from the residues by dissolution in a dilute nitric acid aqueous solution
(acid treatment).
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The morphologies of the crystals in the residues were observed with an optical microscope
(Olympus, SZX16) and a scanning electron microscope (SEM, Keyence, VEꢀ8800). Some of the
residues from the products were powdered in an agate mortar, and the powder Xꢀray diffraction
(XRD) patterns were measured using an Xꢀray diffractometer (XRD, Rigaku, RINT 2200, Cu
radiation) to identify the crystalline phases.
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The XRD intensity data of a type II clathrate single crystal were measured with a single
crystal XRD diffractometer (Bruker AXS, D8QUEST/CP, Mo radiation) and analyzed with the
APEXꢀIII program.19 The structure refinement was performed using SHELXL97.20 The facet
planes of the type I and II single crystals were indexed with the XRD intensity data. The density
of the type II single crystals was measured by the Archimedes method. Quantitative analysis of
type II single crystals was carried out using an electron probe microanalyzer (EPMA, JEOL,
JXAꢀ8200). The software program VESTA21 was used for drawing the crystal structure.
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3. RESULTS AND DISCUSSION
3.1 Crystal growth conditions of the Na-Si clathrates
Table 1 shows the synthesis conditions (heating temperature and time), the crystalline
phases and weight of residues after the alcohol/acid treatments. The evaporated Na ratio (RNa =
(amount of Na evaporated)/(amount of Na in the starting sample) × 100 (%)) was calculated by
postulating that the weight loss of the sample after heating corresponded to the amount of the
evaporated Na. Figure 1 shows the relationship between the heating time, t (h), and the
evaporated Na ratio, RNa (%), together with the crystalline phases of the residues identified by
powder XRD. Photographs of the whole residues obtained after the alcohol/acid treatments of the
samples prepared at different conditions are shown in Fig. 2.
When the starting NaꢀSiꢀSn solution was heated at 723 K for 48 h, about 18.5% of the Na
was evaporated (RNa = 18.5%). The product was completely dissolved by the alcohol/acid
treatments, and there was no crystal. The Si in the NaꢀSiꢀSn solution crystallized as Na4Si4,
which was eluted during the alcohol treatment. RNa increased with increasing heating time,
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reaching 31.6% after heating for 96 h and to 57.7% after heating for 216 h, as shown in Fig. 1.
The residues of 2.4 and 68 mg obtained by heating for 96 and 216 h, respectively, were single
phases of the type I clathrate Na8Si46 (Table 1, Figs. 2 (a) and (b)). If all the amount of Si (71
mg) in the starting sample with Na:Si:Sn = 6:2:1 is used for clathrate formation, 81.1 mg of type
I Na8Si46 would be formed. Thus, the yields of type I crystals prepared by heating at 723 K for
216 h were 84% based on the amount of Si in the starting sample. The size of the type I single
crystals prepared at 723 K for 216 h was about several hundred micrometers, although the
maximum was 1.5 mm (Fig. 2 (b)). In a previous study of type I single crystal formation,17 a Naꢀ
rich starting sample with a composition of Na:Si:Sn = 10:2:1 was heated at 723 K and 104 Pa for
12 h, and 61% of the Na was evaporated. The Na concentration of the melt corresponded to that
of the melt obtained after 35% Na evaporation from the starting melt with Na:Si:Sn = 6:2:1 by
heating at 723 K and 105 Pa for 216 h in the present study. Although the Na evaporation rate of
the previous study was faster than that of the present study, the sizes of the type I crystals
obtained by heating at 723 K are similar in both studies.
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RNa was 23.9% after heating at 773 K and 105 Pa for 24 h, and there was no crystal after
the alcohol/acid treatment, and this was also the case after heating at 723 K and 105 Pa for 48 h.
After heating for 48 h, RNa increased to 41.7% at 773 K, and 47 mg of the residue, including
granular type I single crystals with a size of more than 2 mm, was obtained, as shown in Fig. 2
(c). RNa was increased to 53.3% by heating for 72 h, and a type I single crystal of 5 × 3 × 3 mm3
with {110} habit planes and aggregates of a few millimeters of type I (Na8Si46) and type II
(NaxSi136) single crystals were obtained (Fig. 2 (d)). A mixture of type II microcrystals (several
micrometers to several tens of micrometers) and Si powder was formed by heating for 120 h with
RNa = 72.9%.
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Photographs of the whole residues after the alcohol/acid treatments of the samples
prepared by heating at 823 K for 12 h (RNa = 34.8%) and 24 h (RNa = 65.5%) are shown in Fig. 2
(e) and (f), respectively. The 33ꢀmg residue formed after heating for 12 h comprised granular
single crystals of the type I clathrate with a size of up to ~1 mm. Most of the single crystals with
about 1 mm in size have {110} habits as shown in Fig. 3(a). The residue after 24 h heating was
composed of a few granular single crystals of type I (size: < 1 mm) and type II (size: 0.5–1.5
mm). The total weight of these crystals was 65 mg. The triangular {111} and quadrangular
{100} planes were developed for most of the type II crystals (Fig 3(b)). After heating for 36 h
(RNa = 81.1%), the residue was a mixture of type II single crystals and Si grains.
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Granular type I single crystals (maximum size: ~ 2 mm, total weight of the residue: 27 mg)
were obtained by heating at 873 K for 6 h (RNa = 31.8%), as shown in Fig. 2 (g). Type II single
crystals weighing 62 mg in total with a size of 2–3 mm and {111} triangular facets were
obtained by heating for 9 h (RNa = 51.2%) (Fig. 2 (h)). The formation yields of type II crystals
prepared at 873 K for 9 h were 76 % based on the amount of Si in the starting sample. The
residue became a mixture of type II and Si after heating for 12 h (RNa = 65.3%).
A residue of 18 mg was obtained from the sample prepared by heating at 923 K for 3 h
(RNa = 32.8%), and this was the single phase of diamondꢀtype Si. Figures 4 (a) and (b) show
SEM images of a fragment of the residue, which was covered with aggregated Si spheres of 20–
50 µm in diameter. The surface of the spheres consisted of petalꢀlike streaky grains. No pores
were observed on the fracture surface of the fragment. This result indicates that the fragment
under the aggregates of spheres was a bulk Si crystal.
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3.2 Discussion of the crystal growth of Na-Si clathrates in the Na-Si-Sn solution
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In the present study, the single phase of type I clathrate was obtained at 723 K, and the
type II clathrate was formed at 773–873 K. Type I crystals were also formed by heating at 823 K
for 12 h and 873 K for 6 h, and the single phase product of type II clathrate was prepared by
heating at 973 K for 9 h. In the sample prepared at 923 K, the only diamondꢀtype Si was formed
and the crystallization of clathrate was not confirmed. Horie et al. reported that multiphase
samples of type I and type II clathrates could be synthesized by the general method in which
Na4Si4 compound was heated at 723–793 K under a high vacuum (10−2 Pa).13 In this synthesis,
the ratio of type I contained in the sample was increased with increasing synthesis temperature.
However, type I crystals thermally decomposed into Si and the sample changed to a mixture of
type II and Si above 823 K.13 In the kinetic thermal decomposition method using SPS, a single
phase of type I, a mixture of type I and type II, a single phase of type II, and diamondꢀtype Si
were obtained at 723, 823, 873, and 923 K, respectively.16 As similar to the case of these
previous studies, the type II clathrate crystallizes at temperature higher than that of the type I
clathrate crystallization, and the clathrates could not be formed over 923 K in the present study.
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The evaporation rate of Na from the NaꢀSiꢀSn solution and supersaturation of the clathrate
crystals could be reduced by increasing the Ar pressure from 104 Pa in the previous study17 to 105
Pa. As a result, the NaꢀSi clathrates crystals could grow from a smaller number of crystal nuclei
in the NaꢀSiꢀSn solution and crystal growth of type I clathrate could continue for 72 h at 773 K,
resulting in growing a type I single crystal with a size over 3 mm. Furthermore, the reducing of
Na evaporation rate realized the crystal growth of the NaꢀSi clathrates at higher temperatures of
823–873 K.
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The amounts of singleꢀphase type I crystals obtained at 723 K for 96 h (RNa = 31.6%), 823
K for 12 h (RNa = 34.8%), and 873 K for 6 h (RNa = 31.8%) were 2.4, 33 and 27 mg, respectively.
Under these conditions, RNa was around 32–35%, but the number of crystals obtained at 723 K
was about oneꢀtenth of those prepared under other conditions. The size of the crystals increased
from less than 1 mm to about 2 mm with increasing temperature. These results may give an
explanation of the crystal growth during cooling, based on the temperature dependence of
solubility. However, diamondꢀtype Si crystals (18 mg) were formed by heating at 923 K for 3 h
(RNa = 32.8%), and no clathrate crystallized. The formation of Si at 923 K indicates that the Naꢀ
Si clathrate crystals did not grow during the present cooling process.
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Form the present results, the type I clathrate was first crystallized by Na evaporation, and,
then, the type II clathrate was crystallized at 773–873 K and RNa ≥ ~50%. From the morphology
and the developed habit planes of the single crystals as shown Fig 3 (a) and (b), it is conceivable
that the type I clathrates dissolved into the solution again and the type II clathrates crystallized
during the process. Further heating led to the decomposition of the clathrates to Si.
3.3 Crystal structure analysis of type II single crystal
The composition of type II single crystals obtained by heating at 823 K for 24 h was
analyzed by EPMA, revealing a Na: Si ratio of (15.0 ± 0.2): (85.0 ± 0.2) (at.%), which agrees
with the composition x = 24 of NaxSi136 (0 < x ≤ 24), indicating the full occupation of the Na
atoms in the Si28 and Si20 cages in the structure. Other impurity elements such as Sn were not
detected. The density of the type II single crystals measured by the Archimedes method was 2.28
± 0.02 Mg/m3.
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The single crystal of Na24Si136 prepared by the kinetic thermal decomposition method was
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analyzed by XRD (cubic, space group Fd3m (No. 227) a = 1.47121(1) nm). The atomic
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displacement parameter of the Na2 site in the Si28 cage of NaxSi136 was extremely large
compared to that of the Na1 site in the Si20 cage. Similar results have been reported in previous
studies of other researchers.6, 7, 9, 11 In the structural model of Na24Si136 determined in the
previous study, the Na2 site was divided to an offꢀcenter site (48f).15 In the present study, the
crystal structure of a single crystal of the type II clathrate obtained after heating at 823 K for 24 h
was analyzed with this model, and the R1 value of the refinement was 0.8% for all data (R1 =
Σ||Fo|−|Fc|| / Σ|Fo|, where Fo and Fc are observed and calculated structure factors, respectively).
The results of the analysis are shown in Tables S1–S3 and Fig. S1. The cubic lattice constant a =
1.47088(3) nm is consistent with the literature value, and the refinement of the occupancies for
the two Na sites showed no deficiency. The density of 2.281 Mg/m3 calculated with the formula
of Na24Si136 agreed with that from measurement.
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4. Conclusions
Single crystals of type I clathrate Na8Si46 were obtained by Na evaporation from a NaꢀSiꢀ
Sn solution (Na:Si:Sn = 6:2:1 molar ratio) at 723–873 K under an Ar atmosphere of 105 Pa,
where the evaporated Na ratio (RNa) was over 30%. The type II clathrate crystallized with RNa of
about 50% or more at 773–873 K. In a mixture of type I and type II clathrates prepared by
heating at 773 K for 72 h, type I single crystals with a size up to about 5 mm were found. A
single phase of type II crystals having a {111} crystal habit of about 2 mm on one side was
obtained by heating at 873 K for 9 h. Single crystal Xꢀray analysis revealed that the type II
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clathrate crystallized at 823 K was Na24Si136, where the Na sites in the cages of Si atoms were
fully occupied.
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FIGURES
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Figure 1. Heating time dependence of the evaporated Na ratio, RNa, and the crystalline phases at
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Figure 2. Photographs of the crystals prepared by heating a NaꢀSiꢀSn solution at 723 K for 96 h
(a) and 216 h (b), at 773 K for 48 h (c) and 72 h (d), at 823 K for 12 h (e) and 24 h (f), and at 873
K for 6 h (g) and 9 h (h).
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Figure 3. SEM images of the type I single crystal prepared at 823 K for 12 h (a) and type II sigle
crystal prepared at 823 K for 24 h (b).
Figure 4. Photographs of the Si crystals prepared by heating a NaꢀSiꢀSn solution at 923 K for 3
h.
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Figure S1. Crystal structure of Si28 and Si20 cages in Type II clathrate prepared by heating a
NaꢀSiꢀSn solution at 823 K for 24 h.
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Table 1. The heating temperature and time, Na evaporation ratio against the initial Na in the
starting materials, RNa (%), and the products obtained by the alcohol/acid treatments.
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Heating conditions
Na evaporation
ratio, RNa (%)
Products
Mass of
products (mg)
Temp. (K) Time (h)
723
773
48
96
216
24
48
72
120
12
24
36
6
18.5
31.6
57.7
23.9
41.7
53.3
72.9
34.8
65.5
81.1
31.8
51.2
65.3
32.8
ꢀ
0
2.4
68
0
type I
type I
ꢀ
type I
47
65
72
33
65
70
27
62
65
18
type I, II
type II, Si
type I
823
873
923
type I, II
type II, Si
type I
9
type II
type II, Si
Si
12
3
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ASSOCIATED CONTENT
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Supporting Information.
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Experimental details, crystallographic data; Figures S1; Table S1ꢀS3. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
morito@imr.tohoku.ac.jp
Present Addresses
†If an author’s address is different than the one given in the affiliation line, this information may
be included here.
Author Contributions
‡These authors contributed equally.
ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI Grant Number JP16H06123.
REFERENCES
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Kauzlarich, S. M. J. Am. Chem. Soc. 2007, 129, 13857.
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For Table of Contents Use Only
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Crystal growth conditions of type I and II NaꢀSi clathrates by evaporation of Na from a NaꢀSiꢀSn
solution
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Haruhiko Morito, Masashi Shimoda, Hisanori Yamane, and Kozo Fujiwara
Single crystals of type I clathrate Na8Si46 and type II clathrate Na24Si136 were obtained by Na
evaporation from a NaꢀSiꢀSn solution.
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