J O U R N A L O F C H E M I S T R Y Materials Structural characterisations of the NaxSi136 and Na8Si46 silicon clathrates using the Rietveld method Edouard Reny,* Pierre Gravereau, Christian Cros and Michel Pouchard† Institut de Chimie de la Matie`re Condense�e de Bordeaux, UPR CNRS 9048, 87, Avenue du Docteur Albert Schweitzer, 33608 Pessac Cedex, France. E-mail: Reny@chimsol.icmcb.u-bordeaux.fr Received 16th June 1998, Accepted 16th September 1998 The crystal structure of the non-stoichiometric NaxSi136 silicon clathrate has been refined using the Rietveld method, in order to determine accurately the distribution of the sodium atoms within the two available sites.In agreement with the previous data, it was found that for x8, the alkali atoms occupy exclusively, and not only preferentially the eight larger Si28 sites.For 8<x<24, the filling of the sixteen smaller Si20 cages occurs gradually with increasing x, and a slight increase of the unit cell parameter is then observed. The crystal structure of the stoichiometric Na8Si46 clathrate, which is present as impurity in the studied samples, has also been refined. alkali metal is sodium or potassium, the silicon cages seem to Introduction be fully occupied leading to the stoichiometric compound Thermal decomposition of the Zintl phase MSi (M=Na, K, M8Si46.Rb, Cs) under vacuum or inert atmosphere leads to the The silicon host lattice of the MxSi136 structure (Fig. 2) is formation of clathrate type alkali metal silicides.1–6 Depending composed of 16 pentagonal dodecahedra and 8 hexakaý�deon the alkali metal and the experimental conditions, two types cahedra (12 pentagonal and 4 hexagonal faces), Si28.The unit of structures are formed, corresponding to the formula MxSi46 cell is also cubic (a#14.62 A° ) with the space group Fd39m. The (x#8 for M=Na and K, x#6 for M=Rb) and MxSi136 (M= silicon lattice oVers 16 sites with a 39m symmetry and 8 sites Na, Cs).The two structures were found to be respectively with a 439m symmetry located respectively at the (0 0 0) and isostructural to the clathrate hydrates of type I [or gas hydrate, (3/8 3/8 3/8) positions, consequently, the maximum authorised such as (Cl2)8(H2O)46] and type II [or liquid hydrate, such as value for x is 24. Unlike the Na8Si46 compound, MxSi136 is a (CHCl3)8(H2O)136 or (H2S)16(CCl4)8(H2O)136].2,3,7 In both non-stoichiometric phase, all the cages are not necessarily structures, the silicon host lattice is formed by a combination occupied. NaxSi136 can be obtained within a very large range of two types of polyhedra of fullerene type, i.e.having only of compositions depending on the experimental conditions: pentagonal and hexagonal faces.The basic polyhedron, which 1<x<23. is common to the two structures, is the pentagonal dodeca- The presence of alkali metals trapped in open host lattices hedra (12 pentagonal faces), Si20; it is the smallest possible made of tetrahedrally bonded covalent silicon atoms induces fullerene type cage. interesting physical properties for these compounds.3,8,9 The silicon host lattice of the MxSi46 structure (Fig. 1) is Following the discovery of the fullerene forms of carbon and composed of two pentagonal dodecahedra, Si20 and six tetraka- the superconducting behaviour of the intercalation compounds ý� decahedra (12 pentagonal and 2 hexagonal faces), Si24.The M3C60 or MM¾2C60 (M, M¾=Na, K, Rb, Cs), the silicon corresponding unit cell is cubic (a#10.19 A° ) with the space clathrates have been intensively reinvestigated on the group Pm39n.This silicon lattice oVers two sites with a 39m theoretical and experimental viewpoints.10–22 symmetry and six sites with a 4m2 symmetry located respectively at the (0 0 0) and (1/4 1/2 0) positions. When the trapped †Member of the Institut Universitaire de France. Fig. 2 Representation of the MxSi136 structure.Fig. 1 Representation of the M8Si46 structure. J. Mater. Chem., 1998, 8, 2839–2844 2839One of the major new results was the observation of General information on the Rietveld refinements superconductivity (Tc#4K) in the two clathrates Na2Ba6Si46 Rietveld refinements have been performed on each XRD and K2Ba6Si46, where the six larger Si24 cages are occupied pattern using the FULLPROF program.24 For every by barium instead of sodium and potassium respectively.14,15,18 diVractogram, the following parameters have been refined: the Other results concerning the non-stoichiometric NaxSi136 phase zero point, one asymmetry parameter, the six background have been obtained.For instance, a noticeable variation of polynomial parameters, the three full width at half maximum the electric properties has been measured depending on the (Hk) parameters of the Cagliotti law: Hk2=U composition, from semiconductor, NaxSi136 becomes progresstan2 h+V tan h+W, the g parameter of the pseudo–Voigt ively metallic when x increases.7–9,19 Moreover theoretical fonction, i.e.representing the combination of a Lorentzian calculation on pure Si136 (no alkali atoms trapped in the silicon and a Gaussian type of peak [PV=gL+(1-g)G], the scale cages) shows that the band gap opens by 0.7 eV comparatively factor, the atomic positions, the isotropic thermal agitation to diamond silicon.10–13,20 The 1.9 eV broad bandgap of this factors of the silicon and sodium atoms.compound is very close to that of porous silicon.This could For every diVractogram, the observed peaks are very close be of great interest in new electronic applications. More to the Lorentzian type (0.8<g<0.9). Consequently, the contri- recently, a 23Na NMR study performed on various NaxSi136 bution of a peak at 2hp is taken into account between samples revealed interesting information about the ionisation 2hp-20Hk and 2hp+20Hk.degree of the Na atoms encapsulated in the silicon cages. Due to correlation between the rate of occupancy and the According to this technique, the electronic state of the trapped thermal agitation parameters of the sodium atoms, it was atoms is intermediate between metallic and atomic.21–23 necessary to fix the sodium concentration x to the values The crystal structures of NaxSi46 and NaxSi136 have been found in analysis.The reliability factors used are defined in already investigated. In the first refinement of the structure of Table 2. Standard deviation was calculated taking into account NaxSi136 with x=9.5, a preferential but not exclusive occuthe Berar factor to correct local correlations.25 pancy of the eight large Si28 cages was observed, the occupancy rates being respectively 0.79 and 0.21 for the large Si28 and Determination of the sodium ratio in NaxSi136 the small Si20 sites.2 Another calculation performed by Cros on two compositions x=3 and x=10 led to more equilibrated The global sodium content for each sample has been occupancy rates.7 In a more recent investigation by Sim of a determined by X-ray microprobe using an E.P.M.A.Cameca series of ten compositions, it was observed that the sodium SX-100 apparatus. The values obtained have been confirmed atoms occupy almost exclusively the large Si28 cages for x8, by flame emission technique with a Perkin Elmer 306 double and for 8x24, the smaller Si20 sites are progressively beam spectrometer. occupied with increasing x.9 All the above reported structural To obtain a reliable value of x in NaxSi136, it is necessary studies by XRD were performed without the use of the most to determine the relative amount of the Na8Si46 impurity advanced refinement methods, which enable to get more phase.The mass ratio wj of each phase has been approached accurate data than previously. Furthermore, the preferential by quantitative phase analysis using the Rietveld method: site occupancy of the large Si28 cages for x8 was called into question in recent work.22 wj (%)=100× SjZjMjVj .i n (SiZiMiVi) These two reasons led us to undertake a careful investigation of the crystal structure of a series of samples of NaxSi136, with well characterised x values, by the Rietveld method.As far as where Sj is the scale factor for phase j, Mj the mass of the the Na8Si46 clathrs present as an impurity in our formula unit, Zj the number of formula units per unit cell and samples, this latter compound was also investigated. The Vj the volume of the unit cell.This allowed a correction on results of this study are reported in the present paper. the experimental value of x which was fixed in the final Rietveld refinements (Table 4).The similar isotropic thermal parameters found for the Na atoms in the diVerent Rietveld Experimental refinements constitute an indirect confirmation of the x fixed values. Preparation The clathrates NaxSi136 with x<14 are synthesised by thermal Structural characterisation of the Na8Si46 compound decomposition of NaSi under vacuum (10-4 Pa) at tempera- The XRD pattern of Na8Si46 (Fig. 3) revealed the existence tures between 340 and 440 °C. Those with larger values of x of a minor impurity phase NaxSi136 (ca. 7.7% in weight). A can be prepared in a closed steel reactor, according to the 23Na NMR spectrum obtained on this sample showed that the reaction: NaxSi136+Na (vapour)�Nax¾Si136 (x¾>x), in the impurity compound was very rich in sodium: thus x has been temperature range 370–400 °C.Six samples of NaxSi136 were considered equal to 24.21 Due to a very weak quantity of prepared, the concentration in sodium depending on the amorphous phase in the range 10–40° range (2h), a fifth order pyrolysis temperature. Samples have been numbered with polynomial could not fit well the background of all the XRD increasing values of x (Table 4).The concentration in sodium pattern. Consequently a background file was generated in the of sample VI has been raised to x=20.5 by a thermal treatment following way: in the 10–40° (2h) angular range, background under sodium vapour. All these samples contained some points were manually determined from the XRD pattern; for amounts of the second phase, Na8Si46. 2h>40°, the corresponding background points file was The clathrate Na8Si46 has been synthesised by thermal generated from the fifth order polynomial refined for this decomposition of NaSi under argon at 410 °C. It contained range, with the ‘pattern-matching’ option of the small amounts of the other phase, NaxSi136 with x>20. FULLPROF program. In a 10–120° (2h) angular range, 161 reflections were X-Ray diVraction patterns acquisition obtained for the major compound Na8Si46 and 141 for the minor Na24Si136 phase.The powder diVraction data extracted The powder diVraction patterns were collected on a X’PERT MPD (h-h) Philips diVractometer (Cu-Ka, graphite mono- for Na8Si46 are listed in Table 1. The final results (Table 2) obtained from the refinement of 29 parameters led to atomic chromator, 40 kV, 40 mA, receiving slit: 50 mm, angular range: 10–120° (2h), counting time: 30 s by steps of 0.02° (2h), positions of the silicon atoms with similar thermal parameters.These values are very close to those obtained previously by sample rotation, room temperature). 2840 J. Mater. Chem., 1998, 8, 2839–2844some of us.7 No preferred orientation correction was applied.One asymmetry correction parameter has been refined for values of 2h<40°. A list of the interatomic distances and angles is presented in Table 3 and visualised in Fig. 4. The average interatomic SiKSi distance is 2.369 A° and is close to the value in diamond-type silicon (2.352 A° ). The observed SiKSiKSi bond angles range from ca. 105 to ca. 125° and the average value is close to 109.54°, which is characteristic of an sp3 hybridisation. The calculated ‘free radius’ of the Si20 cages, based on the eight shortest Na(1)KSi(2) distances (r146= dNa(1)Si(2)-rSi), is 2.08 A° . The ‘free radius’ of the Si24 larger cages (r246) is 2.241 A° . The volume per formula unit of the clathrate type silicon host lattice is V/Z=23.058 A° 3, cf. 20.023 A° 3 in diamond-type silicon. Consequently, the clathrate type silicon network is 15.2% more open. Structural characterisation of the NaxSi136 compound Fourier diVerence functions have been calculated using the SHELXL 93 program.26 The diVerence between Fourier trans- Fig. 3 Final Rietveld plot to the X-ray diVraction for Na8Si46. The formation of the structural factors observed for NaxSi136 crosses represent the experimental data points and the upper continuous line the calculated spectra.The upper tick marks indicate (‘Fobs’) obtained via FULLPROF and the calculated structural the calculated reflection positions for the minor impurity phase factors of the empty silicon lattice Si136, (Fcalc), provides us NaxSi136 and the lower tick marks the calculated reflection position with a map of electronic densities attributed to the sodium of Na8Si46.The lower continuous line represents the diVerence. atoms. This study, performed with the diVraction pattern of sample II (x=3) revealed the two important following points: (i) there is no sodium in the pentagonal dodecahedric sites for Table 1 Powder diVraction data of Na8Si46 (Cu-Ka; l=1.540 60 A° ) x8 and (ii) the residual electronic density appears clearly in h k l dcalc lcalc h k l dcalc lcalc the centre of the Si28 cage (3/8 3/8 3/8).This rules out the hypothesis of a decentering of the sodium atoms in the silicon 1 1 0 7.211 30.9 5 1 0 2.000 10.6 cages, that could have been envisaged considering the relatively 2 0 0 5.099 27.2 4 3 1 2.000 <1 high value of the isotropic atomic displacement parameter 2 1 0 4.561 152.3 5 2 0 1.894 14.8 (B#8 A° 2). 2 1 1 4.163 85.7 4 3 2 1.894 47.2 In the case of sample V, where the value of x in NaxSi136 is 2 2 0 3.606 <1 5 2 1 1.862 <1 3 1 0 3.225 11.0 4 4 0 1.803 4.2 higher than the number of available Si28 sites (x=13.6), this 2 2 2 2.944 231.9 5 3 0 1.749 250.7 study shows a full occupation of the Si28 cages, the remaining 3 2 0 2.829 239.3 4 3 3 1.749 110.0 sodium atoms being perfectly centred in the pentagonal 3 2 1 2.726 692.3 5 3 1 1.724 124.1 dodecahedric cages (16c sites). 4 0 0 2.550 64.1 6 0 0 1.700 89.7 It is now possible to fix the atomic positions of the sodium 4 1 0 2.473 157.5 4 4 2 1.700 45.5 atoms and, knowing the global composition of NaxSi136, to 3 3 0 2.404 79.8 6 1 0 1.677 28.0 4 1 1 2.404 1.1 6 1 1 1.654 34.0 define the rate of occupancy of the two types of sodium sites 4 2 0 2.280 8.9 5 3 2 1.654 255.1 in all samples.In sample VI, a weak amorphous contribution, 4 2 1 2.225 52.4 6 2 0 1.613 42.8 probably linked to a high concentration in Na8Si46 (#13%) 3 3 2 2.174 23.4 5 4 0 1.593 28.9 compound, led to a background determination in two steps, 4 2 2 2.082 7.8 6 2 1 1.593 12.1 as seen previously for Na8Si46 and allowed us to release atomic 4 3 0 2.040 21.5 5 4 1 1.574 1.2 Table 2 Atomic parameters and R factorsa for Na8Si46 in space group Pm39n Atom Site x y z Biso/A° 2 Si(1) 6c 0.25 0 0.5 1.13(11) Si(2) 16i 0.1847(2) 0.1847(2) 0.1847(2) 1.08(6) Si(3) 24k 0 0.3088(2) 0.1173(2) 1.03(5) Na(1) 2a 0 0 0 2.5(3) Na(2) 6d 0.25 0.5 0 3.6(2) Cell parameter/A° 10.1983(2) Volume/A° 3 1060.67(5) Dx/g cm-3 2.311 g 0.43(1) Profile parameters U1=0.000(1) V1=0.013(2) W1=0.040(5) Rietveld reliability cRp=0.145 cRwp=0.177 x2=1.56 factors: Rp=0.0889 Rwp=0.127 RI=0.0426 Rf=0.0438 The R factors are defined as cRp=.i |yio-yic|/.i |yio-yib|, cRwp=(.i wi( yio-yic)2/.i wi( yio-yib)2)1/2, x2=.i wi( yio-yic)2/ (N-P+C), Rp=. i |yio-yic|/.i yio, Rwp=(.i wi( yio-yic)2/.i wiyio2)1/2, Fig. 4 Representation of two connected cages in the Na8Si46 structure. RI=. k |Iko-Ikc|/.k Iko, RF=. k |Fko-Fkc|/.k Fko. The eight non-equivalent bonding angles and the four SiKSi distances are indicated. J. Mater. Chem., 1998, 8, 2839–2844 2841Table 3 List of refined interatomic distances (A° ) and angles (°) for Na8Si46 SiKSi d1=Si(1)KSi(3) 2.373(2) NaKSi Na(1)KSi(2) 3.263(2) d2=Si(2)KSi(3) 2.371(2) Na(1)KSi(3) 3.369(2) d3=Si(3)KSi(3) 2.393(3) Na(2)KSi(1) 3.606(1) d4=Si(2)KSi(2) 2.306(2) Na(2)KSi(2) 3.786(2) Na(2)KSi(3) 3.425(2), 3.948(2) angles on Si(1) c=Si(3)KSi(1)KSi(3) 110.5(1) ales on Si(3) Q=Si(1)KSi(3)KSi(2) 105.9(1) e=Si(3)KSi(1)KSi(3) 109.0 d=Si(1)KSi(3)KSi(3) 124.8(1) angles on Si(2) h=Si(2)KSi(2)KSi(3) 108.5(1) j=Si(2)KSi(3)KSi(2) 105.2(1) b=Si(3)KSi(2)KSi(3) 110.4 a=Si(3)KSi(3)KSi(2) 106.8(1) positions and average isotropic thermal factors for this work.2,7,9 They confirm that the free radius of the Si20 cages is slightly smaller in the Si136 clathrate than in the Si46 one compound.Results of the Rietveld refinements for all samples are (r1136#2.00 A° instead of r146#2.08 A° ).No noticeable structural evolution, i.e. variation in the presented in Table 5 and 6. Fig. 5(a) and (b) present two examples of the refined XRD patterns (sample II and VI). lattice parameter and the atomic positions, occurs in sample I, II and III. Sample IV–VI see their lattice parameters Interatomic distances and bond angles for these two samples are presented in Table 7.They are visualised on Fig. 6. increasing slightly but significantly with the sodium concentration, i.e. when the Si20 cages start to fill up (Fig. 7) and is In the highly non-stoichiometric Na3Si136 compound, the average SiKSi distance is 2.360 A° (2.352 A° in diamond-type another point that is in favour of the preferential occupancy of the Si28 sites. 23Na NMR spectra acquired on various silicon). The SiKSiKSi bonding angles range from ca. 105.7 to ca. 120°. The calculated ‘free radius’ of the Si28 cage (r2136= NaxSi136 samples showed that the trapped Na atoms tend to conserve their 3s electron density and consequently can be dNa(1)Si(3)-rSi), based on the shortest Na(1)KSi(3) distance, is 2.722 A° . The ‘free radius’ of the empty Si20 cages is r1136= described as in a state between metallic and neutral, i.e.their radius is situated between 1.54 and 2.30 A° .27 As the free radius 1.990 A° (r1136=dNa(2)Si(1)-rSi). In the almost stoichiometric Na20.5Si136 clathrate, the average SiKSi distance is 2.371 A° and of the Si20 cages (r1136) is ca. 1.99 A° , sodium atoms would be less likely to intercalate in these cages than in the wider Si28 the values of r2136 and r1136 are 2.724 and 1.998 A° respectively. All these data are consistent with the results of previous cages (r2136#2.72 A° ).When the pentagonal dodecahedra start Table 4 List of the NaxSi136 samples studied Sample I II III IV V VI Pyrolysis temp. °C 440 400 370 340 340 Navap a x in NaxSi136 1.0±0.5 3.0±0.8 3.8±0.4 10.5±0.5 13.6±1.2 20.5±1.5 Si20 occupancy 0.0 0.0 0.0 0.15 0.35 0.78 Si28 occupancy 0.125 0.375 0.475 1.0 1.0 1.0 Weight ratio in 1.8 4.7 9.5 2.9 6.9 12.8 Na8Si46 (%) aSample VI has been obtained by submitting Na6Si136 (previously synthesised ) under a sodium vapor atmosphere for 30 h at 320 °C.Table 5 Refined atomic positions of NaxSi136 in samples I–VI Atom [site] x y z Biso/A° 2 Sample I Si(1) [2a] 0.125 0.125 0.125 0.49(18) (Na1Si136) Si(2) [32c] 0.2173(2) 0.2173(2) 0.2173(2) 0.45(10) Si(3) [96g] 0.1831(1) 0.1831(1) 0.3712(2) 0.50(6) Na(1) [8b] 0.375 0.375 0.375 6.9(2.7) Sample II Si(1) [2a] 0.125 0.125 0.125 0.29(16) (Na3Si136) Si(2) [32c] 0.2174(2) 0.2174(2) 0.2174(2) 0.26(9) Si(3) [96g] 0.1830(1) 0.1830(1) 0.3714(1) 0.35(5) Na(1) [8b] 0.375 0.375 0.375 5.6(1.1) Sample III Si(1) [2a] 0.125 0.125 0.125 0.27(12) (Na3.8Si136) Si(2) [32c] 0.2174(1) 0.2174(1) 0.2174(1) 0.28(7) Si(3) [96g] 0.1830(1) 0.1830(1) 0.3712(1) 0.49(4) Na(1) [8b] 0.375 0.375 0.375 8.2(9) Sample IV Si(1) [2a] 0.125 0.125 0.125 0.49(16) (Na10.4Si136) Si(2) [32c] 0.2175(1) 0.2175(1) 0.2175(1) 0.53(10) Si(3) [96g] 0.1831(1) 0.1831(1) 0.3712(2) 0.59(5) Na(1) [8b] 0.375 0.375 0.375 2.2(1.3) Na(2) [16c] 0 0 0 9.6(5) Sample V Si(1) [2a] 0.125 0.125 0.125 0.41(12) (Na13.6Si136) Si(2) [32c] 0.2178(1) 0.2178(1) 0.2178(1) 0.38(7) Si(3) [96g] 0.1831(1) 0.1831(1) 0.3715(1) 0.39(4) Na(1) [8b] 0.375 0.375 0.375 1.9(4) Na(2) [16c] 0 0 0 8.9(4) Sample VI Si(1) [2a] 0.125 0.125 0.125 0.35(18) (Na20.5Si136) Si(2) [32c] 0.2186(2) 0.2186(2) 0.2186(2) 0.43(10) Si(3) [96g] 0.1832(1) 0.1832(1) 0.1722(2) 0.45(5) Na(1) [8b] 0.375 0.375 0.375 0.7(2) Na(2) [16c] 0 0 0 7.8(6) 2842 J.Mater. Chem., 1998, 8, 2839–2844Table 6 Results from the refinements of NaxSi136 in samples I–VI Sample I Sample II Sample III Sample IV Sample V Sample VI Formula Na1Si136 Na3Si136 Na3.8Si136 Na10.4Si136 Na13.6Si136 Na20.5Si136 a/A° 14.6428(8) 14.6410(6) 14.6426(5) 14.6449(8) 14.6607(6) 14.7030(5) Cell volume/A° 3 3139.5(2) 3138.4(1) 3139.4(1) 3140.9(2) 3151.1(1) 3178.5(1) Dx/g cm-3 2.032 2.057 2.066 2.146 2.178 2.242 g 0.81(3) 0.89(2) 0.94(2) 0.87(2) 0.76(2) 0.84(3) Caglioti coeV.U 0.13(2) 0.106(10) 0.119(10) 0.136(16) 0.20(2) 0.19(1) V -0.033(12) -0.035(7) -0.033(7) -0.031(12) -0.035(11) -0.022(8) W 0.024(2) 0.016(1) 0.020(1) 0.021(2) 0.022(2) 0.015(2) Rietveld factors cRp 0.115 0.110 0.108 0.117 0.099 0.107 cRwp 0.148 0.146 0.138 0.148 0.127 0.127 x2 2.43 2.54 1.85 1.97 1.85 3.31 Rp 0.0883 0.0872 0.0820 0.0832 0.0764 0.0712 Rwp 0.120 0.123 0.111 0.114 0.104 0.0941 RI 0.0488 0.0484 0.0385 0.0398 0.0328 0.0562 RF 0.0296 0.0287 0.0241 0.0250 0.0220 0.0355 Fig. 6 Representation of two connected cages in the NaxSi136 structure.The seven non-equivalent bonding angles and the four SiKSi distances are indicated. occupied by sodium atoms, involving the value x=8 and the formulation Na8Si46. In the case of NaxSi136, our results show unambiguously that the sodium atoms are exclusively, and not only preferentially, located in the eight large Si28 sites for Fig. 5 Final Rietveld plot of the X-ray diVraction data for (a) sample x8, and that for 8<x24, the smaller Si20 sites are progress- II and (b) sample VI.The crosses represent the experimental data ively occupied with increasing x. These results are consistent points and the upper continuous line the calculated spectra. The upper tick marks indicate the calculated reflection positions for the with those of our study by 23Na NMR spectroscopy of the major phase NaxSi136 and the lower ticks marks the calculated two clathrates.21 In Na8Si46, two sharp lines with a shift of reflection position of the impurity phase Na8Si46.The lower continuous 1766 and 2019 ppm are observed, which have been identified line represents the diVerence. to correspond to sodium atoms in the Si20 and Si24 cages, respectively.In the case of NaxSi136, a broad line, centred at ca. 1800 ppm is observed in the composition range x8. Then, to fill up, strain caused by the guest atoms seems to have a steric influence on the host atomic positions and extends the with increasing x, this broad line exhibits two components which are finally resolved into two sharp lines located at 1608 lattice parameter.and 1812 ppm for x>20. These two sharp lines, which have been related to the appearance of metallic-like conductivity, Conclusion have been attributed to sodium atoms in the eight Si28 and sixteen Si20 sites, respectively. The present study confirms the previously reported data on the crystal structure of the two clathrates NaxSi46 and NaxSi136. Similar results, as to the position of the lines for the two clathrates have been recently reported by other authors.23 In NaxSi46, both the two Si20 and six Si24 sites are fully J.Mater. Chem., 1998, 8, 2839–2844 2843Table 7 List of refined interatomic distances (A° ) and angles (°) for NaxSi136 (sample II and VI) Sample II SiKSi d1=Si(1)KSi(2) 2.343(2) NaKSi Na(1)KSi(2) 3.997(2) d2=Si(2)KSi(3) 2.365(3) Na(1)KSi(3) 3.975(2) d3=Si(3)KSi(3) 2.339(3) Na(1)KSi(3) 3.902(3) d4=Si(3)KSi(3) 2.403(2) angles on Si(1) a=Si(2)KSi(1)KSi(2) 109.5(1) angles on Si(3) e=Si(2)KSi(3)KSi(3) 105.7(2) h=Si(2)KSi(3)KSi(3) 107.5(1) angles on Si(2) b=Si(1)KSi(2)KSi(3) 107.8(1) g=Si(3)KSi(3)KSi(3) 119.9(1) d=Si(3)KSi(2)KSi(3) 111.1(2) w=Si(3)KSi(3)KSi(3) 108.7(1) Sample VI SiKSi d1=Si(1)KSi(2) 2.383(3) NaKSi Na(1)KSi(2) 3.983(2) d2=Si(2)KSi(3) 2.376(3) Na(1)KSi(3) 3.989(2) d3=Si(3)KSi(3) 2.338(3) Na(1)KSi(3) 3.909(3) d4=Si(3)KSi(3) 2.420(3) Na(2)KSi(1) 3.183(1) Na(2)KSi(2) 3.280(3) Na(2)KSi(3) 3.384(3) angles on Si(1) a=Si(2)KSi(1)KSi(2) 109.5(2) angles on Si(3) e=Si(2)KSi(3)KSi(3) 105.3(2) h=Si(2)KSi(3)KSi(3) 108.1(2) angles on Si(2) b=Si(1)KSi(2)KSi(3) 107.2 g=Si(3)KSi(3)KSi(3) 119.8(2) d=Si(3)KSi(2)KSi(3) 111.6(2) w=Si(3)KSi(3)KSi(3) 108.8(2) 4 C.Cros and J. 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