Binary lithium silicides play a crucial role in high energy density anode materials for rechargeable batteries. During charging processes of Si anodes Li15Si4 is formed as a metastable phase which has been stabilized through Li by Mg, Zn and Al substitution. Here we investigate Li by Zn substitution in the lithium-richest phase Li17Si4 and report on the particular site preference of Zn atoms since Zn is substituting Li atoms only on one out of 13 possible lithium sites. This site preference shows an interesting relation to the closely related phase Li21Si5 and thus Li17−ε−δZnεSi4 with ε = 0.025(1) and δ = 0.033(1) can be considered as the missing link between the structures of Li21Si5 (= Li4.20Si) and Li17Si4 (= Li4.25Si).
Nowadays lithium-ion batteries (LIBs) are widely used as rechargeable power cell for electronic devices. Lithium intercalation of the anode is one of the important processes in lithium ion batteries. Due to its rather high theoretical lithium capacity, silicon is one of the promising candidates for anode materials in LIBs. Silicon anodes exhibit a more than one order of magnitude larger capacity (4200 mA h g−1 for Li17Si4) than currently used graphite anodes (372 mA h g−1) , .
Several phases are observed at the lithium-rich side of the binary phase diagram. These are Li17Si4, Li21Si5, the high-temperature phase Li16.44Si4 and meta-stable Li15Si4. None adapts to a composition appropriate for an electron-precise Zintl phase. For a valence-precise compound with isolated Si atoms a ratio of Li to Si of 4:1 is expected according to the formal charge transfer (Li+)4(Si4−) or Li16Si4. Most intriguing is meta-stable Li15Si4 which is, according to the Zintl formalism, short by one cation to charge balance 16 negative charges of four Si4−. However, meta-stable Li15Si4 transforms to thermodynamically stable phases by substituting Li by more electron rich electropositive elements. For Li15Si4 we found a stabilization through Li by Mg, Zn, and Al substitution and formation of Li14MgSi4, Li14.05Zn0.95Si4 and Li14.25Al0.75Si4, respectively , .
Recently Li17Si4 has been established as the lithium-richest phase in course of a detailed investigation of the Li-rich side of the binary phase diagram and allowed for a very accurate structure determination . This revealed significant differences to previously reported Li21Si5  to which it is very close in composition (Li17Si4=Li4.25Si, Li21Si5=Li4.20Si). Interestingly, no substantial phase width is reported for any of these phases.
In this context we also investigated the partial substitution of Li by Zn in Li17Si4 and report here on the particular site preference of the Zn atoms.
2 Experimental section
Li flux syntheses were performed as described in the literature , . Stainless-steel ampoules with a filter placed inside were also adapted from the literature . A total mass batch of 0.5 g contained the molar ratios of the elements Li:Zn:Si=17:1:3. Si and Zn powder were pressed into a pellet, which was loaded into the ampoule between Li pieces. The ampoules were sealed by arc welding and placed in a steel cylinder with silica wool inside. The cylinder was located in a muffle furnace (Nabertherm, Controller P330/B180), heated to T=750°C with a rate of 3 K min−1 and held at this temperature for 1 h. Afterwards the sample was cooled to 450°C at a rate of 5 K h−1 and kept at this temperature for 4 days. Excess Li was removed at this temperature by isothermal centrifugation. After cooling to room temperature, the ampoule was opened inside a glove box and the product was recovered as shiny crystals reaching a few millimeters in size. Most of the crystals were located on the top of the filter and some stacked to the wall at the bottom of the ampoule.
2.2 Energy-dispersive X-Ray spectroscopy (EDX)
To confirm the composition of Li17−ε−δZnεSi4 a JEOL-JSM 7500F scanning electron microscope equipped with an Oxford X-Max EDX analyzer was used with Mn as internal standard. Si was detected as the main element. The Zn signals were clearly observed, and the content was below 1 at.% (see Figure S2, Supporting Information available online).
2.3 X-ray crystallography
The PXRD pattern of Li17−ε−δZnεSi4 obtained from grinded single crystals was indexed with an F-centered cubic cell and perfectly matches with the recently discovered Li17Si4 structure model  (see Figure S1, Supporting Information available online). The slightly but significantly enlarged unit cell parameter a=18.7441(3) Å indicates the partial substitution of Li by Zn, if compared to the binary system Li17Si4 (18.7259(2) Å, Li17Sn4 structure type) . Single crystals were selected and cut in an Ar-filled glove box and placed inside a glass capillary which was subsequently sealed. Intensity data was collected at T=123 K and at room temperature. Low-temperature measurements were performed on a Bruker APEX II X-ray diffractometer equipped with a CCD detector, a fine focused sealed tube with MoKα radiation (λ=0.71073 Å) and a graphite monochromator. The data collection process was controlled with the Bruker Apex Software suite . Integration, data reduction, and absorption correction were performed with the programs Saint and Sadabs , .
The crystal structure was solved by Direct Methods (Shelxs-97)  and refined with full-matrix least-squares calculations on F2 (Shelxl-2014/7) . Fractional atomic coordinates and atomic displacement parameters for mixed Li/Zn sites were set equal. Details of the single-crystal structure investigation are given in Tables 1–3.
|Formula weight, g mol−1||231.54|
|Radiation; wavelength, Å||MoKα, 0.71073|
|Unit cell dimension a, Å||18.637(2)|
|ρcalcd., g cm−3||1.19|
|Crystal size, mm3||0.2×0.15×0.15|
|Absorption correction||Multi scan|
|θ range, °||1.89–41.76|
|Reflections collected||40 780|
|Reflection with I>2 σ(I)||1985|
|Final indices R1/wR2 [I>2 σ(I)]||0.023/0.054|
|Indices R1/wR2 (all data)||0.030/0.057|
|Goodness-of-fit on F2||1.083|
|Weight parameters a/b a||0.0238/3.6014|
|Largest diff. peak/hole, e Å−3||0.89/−0.33|
aw=[σ2(Fo2)+(aP)2+bP]−1, where P=[Max(Fo2,0)+2Fc2]/3.
Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: (+49) 7247-808-666; e-mail: email@example.com) on quoting the depository number CSD-1788919 (Li16.942Zn0.025Si4).
3 Results and discussion
Single crystals of Li17−ε−δZnεSi4 were obtained through Li flux syntheses by using the atomic ratio Zn:Si of 1:3 and Li removal at high temperature by isothermal centrifugation. Diffraction intensities were indexed in space group F4̅3m which is non-centrosymmetric, and therefore inversion twinning of two enantiomorphic species was considered for the structure refinement of a single crystal. All Li atoms could be refined with anisotropic displacement parameters. In the unit cell there are 80 silicon atoms located at 24g, 24f and two 16e sites, and 340 Li atoms located at 13 independent atomic sites (four 48h, 24g, 24f, six 16e, and 4a). Only the Li13 site (4a) shows a significant statistically mixed Li/Zn occupation (see Table 2). Careful inspection of the residual electron density revealed some density in the vicinity of Li7 (16i). Applying a split model for this position (Li7A and Li7B) led to unreasonably short Li7B–Li13/Zn13 distances of 1.366(9) Å. Consequently the statistically mixed Li/Zn position which is not fully occupied remains empty in correlation with the occupation of Li7B (Fig. 1). The total s.o.f. for the Li13/Zn13 site was restricted to 0.832 (Table 2). The only short interatomic distance of 2.23(1) Å remains between two Li7B atoms (Fig. 1e). The Li7A tetrahedron is occupied by a statistical Li/Zn mixture. The distances between the atoms Li13/Zn13 and Li7A are 2.388(3) Å (Fig. 1d) and thus slightly shorter than those reported for Li17Si4 (2.395(2) Å) . This small difference can be explained by the fact that Zn has a smaller metallic radius than Li (1.57 Å for Li and 1.37 Å for Zn) . The distances between the Li7A atoms are similar to those observed in Li17Si4 (3.900(4) Å comparing to 3.9101(16) Å). One could speculate that the 4a site is occupied only by Zn atoms, but this would lead to an even bigger vacancy at 4a and does not correlate with the split of the Li7A–Li7B atoms.
|Atom||Wyckoff position||x/a||y/b||z/c||s.o.f.||Ueq (Å2)a|
aUeq is defined as one third of the trace of the orthogonalized Uij tensor. bSum of s.o.f. is restricted to 0.832.
The crystallographically determined composition is Li16.942(1)Zn0.025(1)Si4 where the Zn content is at the level of an impurity. However, 0.12 at.% of Zn in the compound does appear as a significant value on the respective atomic site and qualitative EDX measurements confirmed the presence of Zn in the compound (see Figure S2, Supporting Information available online), but it was not possible to determine the Zn amount precisely by this method. The anisotropic displacement parameters for the Li13/Zn13 site are about 2–3 times smaller than for other Li sites (Table 3) which also supports the idea of the presence of the heavier Zn atom at this site. The results are in line with the observation that Li17Ge4 also incorporates Zn under the formation of Li17−εZnεGe4 (ε=0.0052(12)) , with a statistical Li/Zn mixture at the 4a atomic site (around 2.6% of Zn comparing to 12.4% for Li17−ε−δZnεSi4). However, this site was fully occupied in the Ge compound, and a splitting of the neighboring atoms around the 4a site has not been observed.
Currently Li17Si4 is the Li-richest phase in the binary Li-Si phase diagram, and it has been shown that its crystal structure is closely related to another Li-rich phase, Li21Si5 . In both compounds there are four sites of higher symmetry 4a (0, 0, 0), 4b (1/2, 1/2, 1/2), 4c (1/4, 1/4, 1/4), and 4d (3/4, 3/4, 3/4), which are all unoccupied for Li21Si5, but in which exclusively 4a is occupied in the model for Li17Si4. These special sites have a characteristic surrounding of neighboring atoms (Fig. 2a–d). The first shell consists of 4 Li atoms which form a tetrahedron. The next shell contains either 4 Si atoms or 4 Li atoms which in both cases form tetrahedra. These two tetrahedra interpenetrate each other to give a star, which is enclosed by an octahedron of either Li or Si atoms.
As mentioned above, the site 4a in Li17−ε−δZnεSi4 is occupied by a statistical Li/Zn mixture, and the positions are not fully occupied. The first shell of Li atoms around 4a (Fig. 2) is presented by tetrahedra formed by Li7A and Li7B atoms. The split model and the under-occupancy of 4a lead to a rather straightforward model: when the Li7B atom site is occupied, the central 4a site remains empty as in Nesper’s model for Li21Si5. When the Li7A atom site is occupied, the 4a site is statistically occupied by Li/Zn. This reflects the Li17Si4 model and is analogous to the situation in Li17−εZnεGe4.
After the undisputable structure determination of Li17Si4 some years ago, the question arises whether the closely related Li21Si5 exists as well. The difference between the two structures originates from the occupation of the specific site 4a. Whereas Li21Si5, which an occupation of the 4a site with Li was not found in recent detailed investigations of the Li-rich side of the Li-Si phase diagram, we found now that in Li17−ε−δZnεSi4 the site 4a is occupied by a mixture of Li and Zn and the occupation correlates with the occupation of neighboring atom sites. Thus the new ternary silicide Li17−σ−εZnεSi4 shows the characteristics of Li21Si5 and can be considered as the missing link between the structures of Li21Si5 and Li17Si4.
5 Supporting information
Indexing of the X-ray powder pattern collected for Li17−ε−δZnεSi4 and data of EDX measurements on single crystals of Li17−ε−δZnεSi4 are given as supplementary material available online.
Dedicated to: Professor Arndt Simon on the occasion of his 80th birthday.
The authors thank Katia Rodewald for the EDX measurements of Li17−ε−δZnεSi4.
 Apex suite of crystallographic software, Bruker AXS Inc., Madison, WI (USA) 2008.Search in Google Scholar
 Sadabs, Bruker AXS Inc., Madison, WI (USA) 2008.Search in Google Scholar
 Saint, Bruker AXS Inc., Madison, WI (USA) 2008.Search in Google Scholar
 G. M. Sheldrick, Shelxs-97, Program for the Solutionof Crystal Structures, University of Göttingen, Göttingen (Germany) 1997.Search in Google Scholar
 G. M. Sheldrick, Shelxl-2014, Program for Crystal Structure Refinement, University of Göttingen, Göttingen (Germany) 2014.Search in Google Scholar
 A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 1984.Search in Google Scholar
The online version of this article offers supplementary material (https://doi.org/10.1515/znb-2019-0157).
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