Oxidation of M4Si4 (M = Na, K) to clathrates by HCl or H2O

Article abstract of DOI:10.1021/ja0705691

The synthesis of clathrate-II germanium □₂₄Ge₁₃₆ in an ionic liquid was recently discovered.¹ The key step in this process, the controlled oxidation of reactive precursor phases by a protic acid, was successfully applied to silicides M₄Si₄ (M = Na, K). Through this method, clathrate-I compounds of composition M_8-xSi₄₆ were obtained as crystalline products. The synthetic method demonstrates a mild oxidative route to a large class of metastable intermetallic compounds. Copyright

Full text of DOI:10.1021/ja0705691

Published on Web 04/04/2007
Oxidation of M4Si4 (M ) Na, K) to Clathrates by HCl or H2O
Bodo Bo¨hme,† Arnold Guloy,‡ Zhongjia Tang,‡ Walter Schnelle,† Ulrich Burkhardt,†
Michael Baitinger,† and Yuri Grin*,†
Max-Planck-Institut fu¨r Chemische Physik fester Stoffe, Dresden, Germany, and Department of Chemistry and the
Texas Center for SuperconductiVity, UniVersity of Houston, Houston, Texas 77204
Received February 1, 2007; E-mail: juri.grin@cpfs.mpg.de
An intriguing question related to the chemistry of Zintl phases
is whether their ionic description is truly reflected in their chemical
behavior and not merely a nominal structural description. It had
been presumed that the oxidation of these compounds in protic acids
would simply lead to stable elemental and oxide/hydroxide phases.
Our investigations on the redox reactions of typical Zintl phases,
the alkali metal monosilicides, with gaseous protic agents such as
HCl or H2O has resulted in the synthesis of clathrate-I silicides,
Na8-xSi46 and K8-xSi46. This is the first report of a high-yield
scalable synthesis of clathrate silicides based on a controlled
oxidation of the monosilicides, Na4Si4 or K4Si4. The key step in
this process is related to the recently reported synthesis of clathrate-
Figure 1. Crystal structure of M8-xSi46; Si24 polyhedra (green); Si20
II germanium 024Ge136 in an ionic liquid.1
polyhedra (blue), M atoms (gray, inside the cages), and Si atoms (brown).
Clathrate-I compounds based on group 14 elements are expected
to be promising materials for a variety of applications such as in
clathrate-I silicides requires an oxidizing agent that would react in
thermoelectricity and optoelectronics.2-4 Generally, difficulties in
a controlled manner at the appropriate temperatures. High yields
preparing clathrates in high yields are still an important drawback
of crystalline M8-xSi46 were subsequently achieved by the oxidation
for any expected application. So far, Si clathrates could only be
of M4Si4 with gaseous HCl:
obtained by high-pressure synthesis5 or thermal decomposition of
the alkali metal monosilicides.6,7 Current thermal decomposition
11.5M4Si4 + 38HCl f M8Si46 + 38MCl + 19H2
routes of precursor silicides exhibit low yields of the desired
product. Furthermore, R-Si is usually formed as a byproduct of
the decomposition. A simple direct synthesis of binary clathrates,
The oxidation reactions were performed via two different experi-
mental setups:10
M
8-xSi46, from the elements has not yet been reported. The reason
(1) The finely powdered precursors Na4Si4 (≈0.13 mmol) or K4-
Si4 (≈0.08 mmol) were sealed under argon with a 20-fold molar
excess of NH4Cl spatially separated from each other into a Duran-
glass tube. The tube was heated for 1 h at 400 °C (Na4Si4) or 450
may be that these clathrates already decompose in closed Ta
ampoules under argon atmosphere at around 600 °C, and at lower
reaction temperatures it is difficult to achieve equilibrium within a
reasonable time. The preparation route introduced in this contribu-
tion is therefore a promising alternative for the synthesis of
intermetallic clathrates.
The clathrate-I structure of M8-xSi46 consists of an open
framework formed by tetrahedral-bonded silicon atoms (Figure 1).
In contrast to R-Si, which forms the diamond structure having
hexagonal Si6 rings with chair configuration, the clathrate-I structure
features Si5 and planar Si6 rings that cause substantial deviations
from ideal tetrahedral bond angles. This allows an arrangement with
two different types of cages in the Si framework: Si20 formed by
12 pentagons and Si24 formed by 12 pentagons and 2 hexagons.
The “guest” alkali metal atoms are located within the cages of the
framework.
A new low-temperature approach for obtaining a clathrate-like
germanium framework was recently reported.1 Na4Ge4 or Na12Ge17
were reacted in a 1:1 mixture of AlCl3 and n-dodecyltrimethylam-
moniumchloride (DTAC) at 300 °C to 024Ge136. The silicide
precursors Na4Si4 or K4Si4 transform into clathrates, M8-xSi46,
under similar conditions. The product also contains R-Si and
unknown amorphous phases. Syntheses at higher reaction temper-
atures using AlCl3/DTAC are difficult because of uncontrollable
decomposition of the ionic liquid. The successful synthesis of
°C (K4Si4). From 160 °C on, NH4Cl dissociates completely to HCl
11,12
and NH3
(psat(HCl, 400 °C) ≈ 2.1 bar,11 ptot ≈ 6 bar).
(2) Gaseous HCl is produced by reaction of NaHSO4 and NaCl
in a glass vessel under argon atmosphere at 180 °C. The gaseous
HCl is directed via a glass bridge to the reactor containing the
monosilicide. The reactor (equipped with a blow-off valve) is
heated, for 20 h, at 400 °C in the case of Na4Si4 and 450 °C with
K4Si4. Both setups allow a wider range of synthesis temperature
without the limitations originated from the ionic liquid route as
previously reported. It is also possible to use larger amounts of the
precursors. However, the dependence of the oxidation process on
the HCl pressure and the formation of clathrate-II NaxSi136 as a
byproduct have not yet been optimized. Furthermore, the reaction
is not limited to HCl as the protic agent. Through the use of setup
2 Na4Si4 could be oxidized within 20 h at 300 °C with water vapor
(psat(25 °C) ≈ 0.03 bar) to a mixture of silicides with clathrate-I
and clathrate-II crystal structures.
8
9
The products from both syntheses 1 and 2 were washed with
water to remove the metal salts. The dark gray powders were
subsequently dried under vacuum at room temperature.
Scanning electron microscopy investigations (Philips XL30, LaB6
cathode) showed that in both reactions sharp-edged particles and
agglomerates with sizes up to 20 µm were formed (Figure 2; see
also Supporting Information S1-2). Energy dispersive X-ray
† Max-Planck-Institut fu¨r Chemische Physik fester Stoffe.
‡ University of Houston.
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10.1021/ja0705691 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
with traces of a ferromagnetic impurity. After taking the impurities
into account, the molar susceptibilities were calculated to be ø(300
K) ) -5 × 10-4 emu mol-1 for Na8-xSi46 and ø(300 K) ) -3 ×
10-4 emu mol-1 for K8-xSi46. The electrical resistivity F(T) of a
cold pressed sample of K8-xSi46 powder is almost temperature-
independent with F ≈ 0.025 Ω m.19 No phase transition or
superconductivity is observed above 1.8 K. For the composition
M8-xSi46, a bad-metallic behavior and a Pauli-paramagnetic con-
tribution to ø(T) is expected. The observed diamagnetic behavior
of the filled clathrates can be attributed to large additional
diamagnetic contributions from ring currents.20
We have demonstrated a viable controlled oxidation of Zintl
phases that results in metastable compounds with open-framework
structures. The synthesis route to silicon clathrates is a general route
and can therefore be applied to Zintl-phases of other elements, as
well as, other saltlike intermetallics.
Figure 2. SEM image of Na8-xSi46 agglomerates obtained from oxidation
of Na4Si4 by HCl after 1 h, setup 1. (Au sputtered sample, SE contrast.)
spectroscopy (EDXS; EDAX Si(Li) detector) confirms the presence
of solely Na (K) and Si in the products.13
Acknowledgment. The authors thank P. Scheppan, Dr. S.
Hoffmann, R. Koban and the Kompetenzgruppe Struktur. A.M.G.
and Z.T. acknowledge support from the NSF and the Welch
Foundation.
After 1 h reaction of Na4Si4, through the use of setup 1, the
XRPD data revealed a mixture of two crystalline products,
clathrate-I Na8-xSi46 (≈70 mass %) and clathrate-II Na24-xSi136
(≈30 mass %). No R-Si was detected in the products (page S3).
Full-profile Rietveld refinement of the clathrate-I crystal structure
showed full occupancies at the three Si sites.14 Both Na positions
were found to be partially occupied giving a final composition of
Na6.2(1)Si46 (page S4). The observed lattice parameter of a )
10.199(1) Å is close to that reported in the literature for Na8Si46
(page S6).7,15 The crystal structure of the minor byproduct, clathrate-
II NaxSi136, was refined only as a first approximation. Its silicon
framework seems to be fully occupied, and of the two possible
Supporting Information Available: SEM images, XRPD patterns
and crystallographic data of Na6.2(1)Si46 and K7.0(1)Si46, comparison and
evaluation of lattice parameters with literature data. The material is
available at http://pubs.acs.org.
References
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Y. Nature 2006, 443, 320-323.
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Phys. ReV. Lett. 1999, 82, 779-782.
cation positions only one is occupied (partially) leading to a
(3) Moriguchi, K.; Munetoh, S.; Shintani, A. Phys. ReV. B 2000, 62, 7138-
16
composition of Na0.6Si136
,
thus revealing a viable route to the
7143.
formation of the empty silicon clathrate-II. Through the use of setup
2, Na4Si4 was reacted for 20 h to form a mixture of clathrate-I
Na8-xSi46, a ) 10.198(1) Å, and clathrate-II NaxSi136, a )
14.645(1) Å. XRPD indicated that crystalline silicon is not formed.
The 1 h treatment of K4Si4, using setup 1, results in only one
crystalline product (page S3), clathrate-I K8-xSi46 with a )
10.278(1) Å and an amorphous phase. The occupancy of the cation
positions in this case is higher compared with the Na compound,
and yields a composition of K7.0(1)Si46 (page S5).17 Through the
use of setup 2, K4Si4 was reacted to form clathrate-I K8-xSi46 with
a ) 10.280(1) Å (page S6).
To distinguish the effects of thermal decomposition from the
oxidation with HCl, the stability of the precursors was investigated
for all reaction temperatures without an oxidizing agent. After 1 h
treatment (Na4Si4 at 400 °C; K4Si4 at 450 °C), XRPD indicated
that both precursors remain unchanged. Only thermal treatments
for 20 h result in significant amounts of clathrate-I with residues
of M4Si4. Moreover, the lattice parameter of K8-xSi46 obtained from
thermal decomposition of K4Si4, a ) 10.267(1) Å, is smaller than
that of the product obtained from the reaction with HCl (a )
10.280(1) Å). These results prove that the M8-xSi46 phases obtained
with both experimental setups result from the reaction of M4Si4
with HCl.
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(10) For safety reasons all heat treatments under HCl pressure were performed
in a laboratory hood.
(11) Smits, A.; de Lange, W. J. Chem. Soc. 1928, 2944-2952.
(12) Oppermann, H.; Huong, D. Q. Z. Anorg. Allg. Chem. 1995, 621, 665-
670.
(13) A low intensity Cl K line, the only significant minority line beside small
contribution of O or N K line, indicates small residues of NH4Cl or MCl.
(14) Crystallographic data of Na6.2(1)Si46: space group Pm-3n, a ) 10.199(1)
Å; 1.70(4) Na in 2a 000; 4.50(5) Na in 6c 1/4 1/2 0; 6 Si in 6d 1/4 0 1/2,
16 Si in 16i xxx, x ) 0.1840(3), 24 Si in 24k 0yz, y ) 0.3053(3), z )
0.1184(4).
(15) Ramachandran, G. K.; Dong, J.; Diefenbacher, J.; Gryko, J.; Marzke, R.
F.; Sankey, O. F.; McMillan, P. F. J. Solid State Chem. 1999, 145, 716-
730.
(16) Crystallographic data of Na0.6Si136: space group Fd3hm, a ) 14.645(1)
Å; 0.6(3) Na in 8b 3/8 3/8 3/8; 8 Si in 8a 7/8 7/8 7/8; 32 Si in 32e xxx,
x ) 0.7832(6), 96 Si in 96h xxz, x ) 0.8200(4), z ) 0.6324(5).
(17) Crystallographic data of K7.0(1)Si46: space group Pm3hn, a ) 10.278(1)
Å; 1.60(2) K in 2a 000; 5.40(4) K in 6c 1/4 1/2 0; 6 Si in 6d 1/4 0 1/2,
16 Si in 16i xxx, x ) 0.1842(3), 24 Si in 24k 0yz, y ) 0.3061(4), z )
0.1180(4).
(18) Magnetization was measured at various external fields between 100 Oe
and 70 kOe (1.8-400 K) in a SQUID magnetometer (MPMS XL-7,
Quantum Design) on polycrystalline samples (m ≈ 4.5 mg). The
contribution of the glass sample holder was subtracted.
(19) The electrical resistivity measurements were performed on a cuboid-shaped
cold-pressed sample, annealed at 300 °C. Conventional dc four-point
method, 4-320 K; inaccuracy of absolute values are (20 %.
(20) Paschen, S.; Baenitz, M.; Tran, V. H.; Rabis, A.; Steglich, F.; Carrillo-
Cabrera, W.; Grin, Yu.; Strydom, A. M.; Du Plessis, P. de V. J. Phys.
Chem. Solids 2002, 63, 1183-1188.
Differential scanning calorimetry measurements in welded Nb
ampoules exhibit an exothermic effect at 594 °C for Na8-xSi46 and
at 716 °C for K8-xSi46 (obtained from setup 1), indicating the
metastable character of both clathrates. Annealing Na8-xSi46 at 600
°C for 18 h results in a partial decomposition of the clathrate-I
compound to R-Si (XRPD data). K8-xSi46 is completely decomposed
to R-Si within 18 h at 720 °C.
Magnetic susceptibility measurements18 ø(T) ) M/H on Na8-xSi46
and K8-xSi46 samples consistently showed a diamagnetic substance
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