Copper and Zinc Substitutions in Clathrates of Tin: Synthesis

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Copper and Zinc Substitutions in Clathrates of Tin: Synthesis, Structural Characterization, and Physical Properties of A8Cu2.67Sn43.33 and A8Zn4Sn42 (A = K, Rb, Cs) with the Type‑I Structure Marion C. Schaf̈ er and Svilen Bobev* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Reported are two series of tin-based clathratesA8Cu2.67Sn43.33 and A8Zn4Sn42 (A = K, Rb, Cs)crystallizing in the cubic type-I structure. The six new compounds have been obtained in quantitative yields and have been characterized by single-crystal X-ray diffraction. Copper and zinc randomly substitute tin on only one of the three framework sites, 6c, while the remaining two sites 16i and 24k are exclusively occupied by tin. The alkali metals reside in the two types of cages in the structurethe 20-atom dodecahedra and the 24-atom tetrakaidecahedra. According to the structure refinements, both cages are completely filled; in the bigger tetrakaidecahedron, the smaller K and Rb atoms are displaced off its center, whereas the larger Cs atoms are located at the center. These findings are presented and discussed; preliminary results from measurements of the Seebeck coefficients (300−500 K) and the electrical resistivity (100−400 K) are presented as well. KEYWORDS: tin, clathrates, crystal structure, thermoelectrics

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INTRODUCTION Naturally occurring gas hydrates with the so-called clathrate structures are well-known.1 Examples like G8(H2O)46 and G24(H2O)136 (G = CO2, Cl2, CH4, Xe, etc.) were structurally characterized decades ago and shown to have structures in which the water molecules form open frameworks (host), with the corresponding gas molecules (guest) taking up the available empty space.1 Much later, in 1965, Cros et al.2 discovered the first Si-based clathrates Na8Si46 and NaxSi136 (x < 11), which adopt the same structures, now based on a network of tetrahedrally coordinated Si atoms and Na atoms residing in the resultant cages.2 After being a “laboratory curiosity” for years, clathrates have recently gained recognition due to their potential for thermoelectric applications.3 As stipulated by Slack, who first coined the term “phonon-glass, electron crystal,”3a clathrate-based thermoelectric materials can allow for guest atom vibrations (aka rattling) in the oversized cages, thereby lowering the lattice thermal conductivity while the rigidity of the framework helps maintain the electron mobility at a high level. In general, clathrates encompass a total of 10 different structure types,1,4 but the clathrates based on group 14 elements (Si, Ge, or Sn) are most commonly found in two structures: (i) type-I with nominal composition A8M46 (A = guest atom, M = framework atom) and (ii) type-II with nominal composition A24M136 (Figure 1). The typical guest atoms are the alkali metals and the alkaline-earth metals, including Eu from the lanthanides.4 The guest atoms partially or fully occupy dodecahedral M20 and tetrakaidecahedral M24 cages in the type-I structure (i.e., up to eight guests per formula © 2013 American Chemical Society

Figure 1. (a) Polyhedral representation of the crystal structure of typeI clathrate with the cubic space group Pm3̅n. (b) Polyhedral representation of the crystal structure of type-II clathrate with the cubic space group Fd3̅m. Pentagonal dodecahedra M20, red; tetrakaidecahedra M24, blue; hexakaidecahedra M28, green.

G8M46) and dodecahedral M20 and hexakaidecahedral M28 cages in the type-II structure (i.e., up to 24 guest atoms per formula G 24M136). In many group 14 based clathrates, partial substitutions of the framework-building atoms with elements from groups 13, 12, or even groups 10 and 11 are possible. Such modifications of the crystal and electronic structure appear to follow the Zintl concept,5 accounting for the prevalence of chemical formulas like A8X8M38 and AE8X16M30 (A = alkali metals; AE = alkaline-earth metals; X = group 13 Received: July 9, 2013 Revised: August 23, 2013 Published: September 11, 2013 3737

dx.doi.org/10.1021/cm402276n | Chem. Mater. 2013, 25, 3737−3744

Chemistry of Materials

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Table 1. Selected Crystallographic Data for the Type-I Clathrates A8Cu2.67Sn43.33 and A8Zn4Sn42 (Cubic Space Group Pm3̅n (No. 223); Z = 1)a refined formula formula weight temperature (K) radiation a (Å) V (Å3) ρcal (g cm−3) μ (cm−1) data/restraints/parameters GOF on F2 R1 (I > 2σI)b wR2 (I > 2σI)b largest difference peak/hole (e− Å−3)

K8Cu2.77Sn43.23(6)

Rb8Cu2.8Sn43.2(1)

5619.77

5991.29

12.0769(3) 1761.43(8) 5.30 162.9 341/0/21 1.157 0.0155 0.0319 0.60/−0.97

12.0868(8) 1765.8(2) 5.63 212.5 369/0/23c 1.083 0.0145 0.0324 0.54/−1.11

Cs8Cu2.69Sn43.31(5) 6374.67

K8Zn3.78Sn42.22(5)

5570.99 200(2) Mo Kα, λ = 0.71073 Å 12.1234(6) 12.0671(4) 1781.86(15) 1757.15(10) 5.94 5.27 196.7 164.2 342/0/17 342/0/21 1.208 1.315 0.0140 0.0138 0.0285 0.0268 0.74/−0.96 0.48/−0.47

Rb8Zn3.52Sn42.48(7)

Cs8Zn3.44Sn42.56(5)

5955.81

6340.13

12.0913(5) 1767.74(13) 5.60 213.3 338/0/21 1.119 0.0176 0.0278 0.95/−0.73

12.1239(4) 1782.08(10) 5.91 197.6 342/0/17 1.279 0.0102 0.0212 0.44/−0.48

The presented refinements are with K and Rb atoms in off-centered position in the tetrakaidecahedral cages. bR1 = ∑ ||Fo| − |Fc||/∑ |Fo|. wR2 = {∑[w(Fo2 − Fc2)2]/ ∑[w(Fo2)2]}1/2, where w = 1/[σ2Fo2 + (A·P)2 + B·P], and P = (Fo2 + 2Fc2)/3. A and B are the appropriate weight coefficients. c Increased number of variables due to a split-site refinement for Sn1 (see Supporting Information). a

was removed at 400 °C (centrifuged). The resultant silver crystals with metallic luster had sizes up to 3 × 2 × 2 mm and were often intergrown together. The obtained clathrates’ crystals are stable in air and moisture for a couple of months, and the samples can be cleaned from byproducts with diluted HCl solution. Some residual Sn and the air sensitive A4Sn420 were the typical impurities found in all samples; very small amounts of unreacted Cu pieces with characteristic color can be picked out with tweezers. The reactions between K, Cu, and Sn also contained bulky, lath-shaped crystals of Cu1+xSn.21 An additional problem in the A−Cu−Sn systems must also be explicitly mentioned herethe occurrence of the binary clathrates A8Sn44□2 (A = Rb, Cs; □ designates vacancies in the tin framework).22−25 These problems persisted even after changes in the nominal elemental ratios or the reaction temperature and cooling rates, which seemed to result in higher yields of the side products. The traditional solid-state method of fusing together stoichiometric mixtures was also tried but did not lead to better control over the reactions’ outcome. Instead, the high amount of residual Sn and the small size of the crystallites caused more problems with regard to the crystallography and the property measurements. While the reasons for the poor homogeneity in these systems are yet to be determined, we draw attention to the fact that Cs8Sn44□2 was only a minor contaminant to the targeted ternary phase, but Rb8Sn44□2 and Rb8Cu2.67Sn43.33 were found to be in an approximately 50:50 ratio. Notice that the differentiation between these phases is not easily done and required very careful crystallographic workthe average structure of Rb8Sn44□2 in space group Pm3̅n has lattice parameter a = 12.054(1) Å, whereas Rb8Cu2.67Sn43.33 has lattice parameter a = 12.087(1) Å (Table 1). Apparently, the differences in the peak intensities are subtle and perhaps not sufficient for unambiguous phase-identification (Supporting Information). Powder X-Ray Diffraction. X-ray powder diffraction patterns for all studied samples were collected at room temperature on a Rigaku MiniFlex powder diffractometer using filtered Cu Kα radiation. The typical runs involved θ−θ scans (2θmax = 70°) with scan steps of 0.05° and 2 s/step counting time. The data were analyzed for phase-purity with the JADE 6.5 software package. The intensities and the positions of the experimentally observed peaks and those calculated based on the corresponding single-crystal structures matched very well to one another (see Supporting Information). Single-Crystal X-Ray Diffraction. Single-crystal X-ray diffraction data were collected on a Bruker Smart Apex II duo diffractometer at 200 K using graphite-monochromated Mo Kα radiation (α = 0.71073 Å). Suitable single crystals of each compound were selected and cut to smaller dimensions (less than 0.1 mm) under mineral oil. The SMART26 and SAINTplus27 programs were used for the data collection, integration, and the global unit cell refinement from all

element). As such, the above formulations should be regarded as valence (electron precise) compounds. Of course, small deviations from the “ideal” formulas are possible and desirable for the optimization of transport properties. Several previous studies have already reported significant effects on the Seebeck coefficients in, e.g., K8Ga8−xSn38+x,6 and even more dramatic change from n- to p-type semiconducting behavior in Ba8Ga8−xSn38+x7 or Ba8Ga8−xGe38+x.8 We were intrigued by these discoveries, and since our group has had a long-lasting interest in clathrates’ chemistry,9−12 we set out to search for new clathrates in related systems. Having just completed extensive investigations on clathrates of tin with Ga substitutions,12 we decided to focus our efforts on additional exploratory work on Sn-based clathrates with metals from groups 12 and 11 as framework substituents. After all, the reports on such compounds appeared to be limited to a few known examples such as A8Zn4Sn42 (A = Rb, Cs),13,14 Cs8Cd4Sn42,15 and A8Hg4Sn42 (A = K, Rb, Cs).16 In contrast, Ge-based clathrates show much greater variety with examples encompassing many d elements such as Ba8Zn8Ge38,17 Ba8Au5.3Ge40.7,18 and Ba8(X,Ge)46 (X = Ni, Pd, Pt, Cu, Ag, Au),19 among others. Here, we present the initial results from our studies in the systems A−Cu−Sn and A−Zn−Sn (A = K, Rb, and Cs), by reporting the crystal chemistry and properties of the type-I clathrates A8Cu2.67Sn43.33 and A8Zn4Sn42. The substitution patterns are discussed and compared with the findings from previous work on A8Zn4Sn42 (A = Rb, Cs).13,14

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EXPERIMENTAL SECTION

Synthesis. Due to the high reactivity of alkali metals in air, all manipulations involving elemental K, Rb, and Cs were carried out in a glovebox with an O2/H2O level below 1 ppm or under a vacuum. All chemicals were purchased from Alfa or Sigma-Aldrich with a purity higher than 99.9%. Over the course of this study, various adjustments to the reaction conditions were made, and the best route toward the A8Cu2.67Sn43.33 and A8Zn4Sn42 clathrates was by the use of the self-flux method (i.e., for the reactions, Sn was used as both a reagent and a “solvent”). For this purpose, the starting materials were loaded in Nb tubes (length 70 mm, inner diameter 10 mm), and round Nb sheets with drilled small holes were inserted as “filter” parts. Elemental ratios were A/Cu/Sn = 8:3:43−50 (A = K, Rb, Cs) and A/Zn/Sn = 8:4:42− 50, respectively. After arc sealing and enclosing the tubes in evacuated fused silica jackets, the samples were heated in programmable muffle furnaces to 600 °C at a rate of 100 °C/h, kept for 15 h, and then slowly cooled down (rate −1 °C/h). In all cases, the excess molten Sn 3738



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data. Semiempirical absorption correction was applied with SADABS.28 The structures were refined to convergence by full matrix least-squares methods on F2, as implemented in SHELXTL.29 All sites were refined with anisotropic displacement parameters. Before the last refinement cycles, the atomic coordinates were set to conform to the literature.1 All refinements proceeded smoothly and easily converged to very low conventional R-values; the final difference Fourier maps were featureless.30 Selected details of the data collections and structure refinement parameters are listed in Table 1. Some noteworthy details of the structure refinements are described in the following paragraphs. The gathered intensity data were readily sorted by XPREP, confirming the cubic symmetry and the systematic absence conditions for the cubic space group Pm3̅n (No. 223), adopted by almost all clathrates with the type-I structure. The solution by direct methods in the chosen space group proceeded readily, and only after a few cycles, the refinements smoothly converged, confirming the validity of the basic model. Initially, the type-I clathrate structure was refined with Sn atoms solely occupying the framework sites and alkali metal atoms filling the cages. In the next step (for all cases), the occupancies of all three framework sites were refined by freeing the occupation factor of an individual site, while the remaining ones were kept fixed. This showed deviations from full Sn occupancy only at the framework site 6c. This is similar to the case of the A8Sn44□2 structures refined in Pm3̅n (No. 223), where the vacancies (□) in the tin framework are also located at this position.22,23a,25a For our structures, however, the unit cells are actually larger than those for the aforementioned compounds. This attests to a difference, which is puzzling at a first glanceafter all, both Cu and Zn have smaller elemental radii than Sn.31 However, Cu or Zn take “more space” than a framework vacancy, which could mean that in the studied type-I clathrates, the decreased electron density at site 6c is due to a partial substitution of Sn by the lighter Cu and Zn. Following this hypothesis, Cu/Zn were refined as randomly substituting Sn (occupancies given in Table 2 and in Supporting

Figure 2. Schematic representation of the polyhedral cages in (a) K8Cu2.67Sn43.33 (with the K atom in a four-way split at site 24k) and (b) Cs8Cu2.67Sn43.33 (with the Cs atom at the center of the tetrakaidecahedral cage) with anisotropic displacement parameters, drawn at the 95% probability level. anisotropic displacement parameters for the K and the Rb atoms in the tetrakaidecahedral cages. This was modeled as if these atoms are shifted slightly away from the center (site 6d) and refining the atom as 4-fold split at a site 24k. Such off-centering was noted in both Cu−Sn and Zn−Sn systems for the smaller K and Rb atoms in the oversized 24-atom cage, while the very large Cs atoms did not show propensity for being displaced from the geometric center of this polyhedron (Figure 2). The atomic coordinates and equivalent isotropic displacement parameters for the two refined structures compared in Figure 2 are given in Table 2; selected interatomic distances are summarized in Table 3. The anisotropic displacement parameters of both compounds as well as the atomic coordinates and equivalent isotropic and anisotropic displacement parameters of the other four compounds can be found in the Supporting Information. For additional details, we refer the reader to the CIFs, which can be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (49) 7247−808−666; e-mail: crysdata@fiz. karlsruhe.de) on quoting the depository numbers CSD-426395 for K8Cu2.77Sn43.23(6), CSD-426396 for Rb8Cu2.8Sn42.2(1), CSD-426397 for Cs8Cu2.69Sn43.31(5), CSD-426398 for K8Zn3.78Sn42.22(5), CSD-426399 for Rb8Zn3.52‑Sn42.48(7), and CSD-426400 for Cs8Zn3.44Sn42.56(5), respectively, or from the Supporting Information. EDX Analysis. Multiple crystals from each compound were subjected to microprobe elemental analysis by means of energy dispersive X-ray spectroscopy (EDX). The experiments were carried out on a ZEISS AURIGA 60 high resolution focused ion beam and scanning electron microscope equipped with an OXFORD Synergy XMAX80 detector. Multiple crystals were analyzed, and the results were then averaged. The determined compositions were consistent (within the error of the analysis) with the refined formulas. Thermal Analysis. Differential scanning calorimetry and thermogravimetric (DSC-TG) analyses were performed for all samples using a calorimeter supplied by TA Instruments (model SDT Q600). The samples were ground into powder and loaded in small alumina pans. In a typical run, the temperature was ramped at a rate of 5 °C/min under a constant flow (100 mL/min) of high purity argon in order to avoid oxidation in ambient air. The title compounds appear to melt incongruently, and the determined points of decomposition are

Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters (Ueqa) of K8Cu2.67Sn43.33 and Cs8Cu2.67Sn43.33 atom

site

K8Cu2.67Sn43.33 K1b 24k K2 2a Sn1 24k Sn2 16i M3c 6c Cs8Cu2.67Sn43.33 Cs1 6d Cs2 2a Sn1 24k Sn2 16i M3d 6c

x

y

z

Ueq (Å2)

0 0 0 0.1826(1) 1/4

0.250(4) 0 0.3162(1) 0.1826(1) 0

0.471(1) 0 0.1187(1) 0.1826(1) 1/2

0.053(5) 0.024(1) 0.0217(2) 0.0127(1) 0.0126(4)

1/4 0 0 0.1831(1) 1/4

1/2 0 0.3152(1) 0.1831(1) 0

0 0 0.1194(1) 0.1831(1) 1/2

0.0311(2) 0.0123(3) 0.0197(2) 0.0107(1) 0.0107(3)

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. bOff-centered to a neighboring site with 4 times greater multiplicity, site occupancy adjusted accordingly. cMixed occupied by Sn and Cu with a refined ratio 0.54(1)/0.46. dMixed occupied by Sn and Cu with the ratio 0.55(1)/0.45. Information). The other two framework sites 24k and 16i remain devoid of disorder and fully occupied by Sn atoms (Figure 2). The final refined compositions (Table 1) are in very good agreement with the EDX results (vide infra) and the stoichiometry expected from the application of the Zintl concept,5 A8Cu2.67Sn43.33 and A8Zn4Sn42, respectively.32 Therefore, for the sake of simplicity, the idealized Zintlbased formulas are used throughout this manuscript. The occupation factors of the guest alkali metal atoms were verified in a similar mannerthere was no evidence for partial filling of either type cage in all six refined structures. However, we noticed unusual 3739



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Table 3. Selected Interatomic Distances (in Å) in A8Cu2.67Sn43.33 and A8Zn4Sn42 (A = K, Rb, Cs)a Sn1−

Sn2− M3− a

Sn1 Sn2 (×2) M3 Sn1 (×3) Sn2 Sn1 (×4)

K8Cu2.67Sn43.33

Rb8Cu2.67Sn43.33

Cs8Cu2.67Sn43.33

K8Zn4Sn42

Rb8Zn4Sn42

Cs8Zn4Sn42

2.8680(9) 2.8393(3) 2.7278(5) 2.8393(3) 2.8186(9) 2.7278(5)

2.868(2) 2.8375(6) 2.739(1) 2.8375(6) 2.812(2) 2.739(1)

2.8959(5) 2.8439(3) 2.7437(5) 2.8439(3) 2.8087(8) 2.7437(5)

2.8577(7) 2.8333(3) 2.7346(4) 2.8333(3) 2.8168(8) 2.7346(4)

2.868(1) 2.8359(4) 2.7439(5) 2.8359(4) 2.814(1) 2.7469(5)

2.8837(5) 2.8394(2) 2.7560(3) 2.8394(2) 2.8100(5) 2.7560(3)

M denotes mixed occupied Sn/Cu and Sn/Zn.

gathered in Table 4. For some samples, an endothermal event around 230 °C can also be seen, which is likely due to small amounts of Sn (mp 232 °C).

hexagonal faces to three perpendicular but not interpenetrating columns running along [100], [010], and [001] of the unit cell, encapsulating the isolated 20-atom polyhedra. The alkali metals occupy the available eight cagesthe ideal positions are the centers of the pentagonal dodecahedra and the tetrakaidecahedra, the 2a and 6d sites, respectively (Figure 2). As seen from Table 4, the unit cells for both series increase with the size of incorporated alkali metals. Specifically, in the Cu-substituted clathrates, the cell lengthens from 12.0769(3) Å for K8Cu2.67Sn43.33 to 12.0868(8) Å for Rb8Cu2.67Sn43.33 and to 12.1234(6) Å for Cs8Cu2.67Sn43.33. For the Zn-substituted clathrates, the variation is from 12.0671(4) Å for the K version, to 12.0913(5) Å for the Rb one, and to 12.1239(4) Å for the Cs analog. Notice that due to the varying amounts of Cu and Zn, a good correlation cannot be found. The direct comparison of our unit cell data with the metrics from the literature is somewhat hindered because the measurements have been taken at different temperatures; nonetheless, it is apparent that the unit cell volumes of Rb8Zn4Sn42 (a = 12.112(1) Å)13 and that of Rb8Zn3.52Sn42.48(7) (a = 12.123(2) Å) are in good agreement after an extrapolation with a presumed linear expansion coefficient β ≈ 5 × 10−5 K−1. This is not the case, however, for the unit cells of the previously reported Cs8Zn4Sn42 (a = 12.123(2) Å)13 and our Cs8Zn3.44Sn42.56(5) (a = 12.1239(4) Å), which are virtually the same. Unfortunately, the earlier reports have not indicated that the Zn/Sn ratio were refined, and any conclusion here will be largely speculative. We could reason that the Zn content of the earlier reported Cs compound (obtained from a stoichiometric reaction) is closer to the nominal ratio, or that there are small amounts of vacancies in the framework.32 The latter hypothesis is less likely, based on our attempts to refine Cs8Zn3.44Sn42.56(5) as Cs8ZnxSn46−x−y□y (x ≈ 3.5; y ≈ 0.25)a summary of this work presented in the Supporting Information section, does not lend support for vacancies in the flux-grown samples. The Cu and Zn atoms substitute Sn with a strict preference for one of the three framework sites, namely 6c,3d thereby avoiding homoatomic Cu−Cu and Zn−Zn bonding throughout the framework. The same “coloring” of the framework is reported for A8Zn4Sn42 (A = Rb, Cs),13,14 Cs8Cd4Sn42,15 A8Hg4Sn42 (A = K, Rb, Cs),16 and Ba8Zn8Ge38,17c among others. We looked at multiple long-exposure zone images to find possible superstructure reflections (similar to A8Sn44□2,22−25 where ordering of the vacancies in the Rb and Cs compound is resulting in a 2 × 2 × 2 superstructure with the cubic space group Ia3̅d) but found no indications for doubling or tripling the unit cell axes. The refined interatomic distances involving the atoms at the site 6c are the “averaged” Sn−Sn and Cu/Zn−Sn contacts. For A8Cu2.67Sn43.33 (A = K, Rb, Cs) they are 2.7278(5) Å, 2.739(1) Å, and 2.7437(5) Å, respectively. These are slightly shorter than those in their Znanalogs2.7346(4) Å, 2.7469(5) Å, and 2.7560(3) Å for

Table 4. Lattice Parameters and Melting Points (Where Available) of Different Tin Based Type-I Clathrates (Space Group Pm3n̅ ) compound

a (Å)

T (K)a

mp (°C)

reference

K8Sn44□2 K8Cu2.67Sn43.33 K8Zn4Sn42 K8Hg4Sn42 Rb8Sn44□2 Rb8Sn44□2b Rb8Cu2.67Sn43.33 Rb8Zn4Sn42 Rb8Zn4Sn42 Rb8Hg4Sn42 Cs8Sn44□2 Cs8Sn44□2b Cs8Cu2.67Sn43.33 Cs8Zn4Sn42 Cs8Zn4Sn42 Cs8Cd4Sn42 Cs8Hg4Sn42

12.03(1) 12.0769(3) 12.0671(4) 12.1255(4) 12.054(1) 24.131(4) 12.0868(8) 12.0913(5) 12.112(1) 12.1838(4) 12.105(1) 24.2234(2) 12.1234(6) 12.1239(4) 12.123(2) 12.2357(1) 12.2130(4)

298 200 200 298 298 298 200 200 298 298 298 298 200 200 298 298 298

395 453 526 446

16,22 this work this work 16 16,23a 23b this work this work 13 16 16,25a 25b this work this work 14 15 16

a

511 527 607 548 577 591 634

627

Temperature at which the structures were elucidated. model with superstructure in space group Ia3d̅ .

b

Ordered

Measurements of the Seebeck Coefficient and Electrical Resistivity. Large single crystals of K8Zn4 Sn42, Rb 8Zn4Sn42, K8Cu2.67Sn43.33, and Cs8Cu2.67Sn43.33 were available, and they were picked out and their surfaces cleaned from possible residual Sn. After that, small bars with approximate dimensions 1 × 0.5 × 0.5 mm were cut out of them. The smaller size of the Cs8Zn4Sn42 single crystals was inadequate for resistivity measurements. At least two crystals of each batch were measured to ensure reproducibility. Measurements of the Seebeck coefficient were carried out in the temperature range 300− 500 K with a commercial MMR instrument equipped with a K-20 programmable temperature controller and a SB-100 Seebeck controller using a two-contact setup. The resistance R was measured in the range 100−400 K using the four-probe method with an excitation current of 1 mA. The electrical resistivity ρ was determined from the equation ρ = RA/l, for which A is the cross-section area of the crystal, and l is the distance between the outer contacts.

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RESULTS AND DISCUSSION Structure Determination and Crystal Chemistry. The title compounds adopt the type-I structure with the primitive cubic space group Pm3̅n. The unit cell of this structure contains 46 framework-building atoms, located on the three Wyckoff sites24k, 16i, and 6cforming a space-filling network of two pentagonal dodecahedra and six tetrakaidecahedra (Figure 1). The larger 24-atom tetrakaidecahedra connect via their two 3740



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In contrast, the larger Cs atoms in Cs8Cu2.67Sn43.33 and Cs8Zn4Sn42 have the “right” size to fill the 24-atom cages, thus, they remain at the center. Nevertheless, the Ueq values for Cs1 are still approximately twice as large as Cs2 (see Table 2 and Table S1). All of the above bolsters the hypothesis for “rattling” behavior, due to the mismatch between the guest atom and the host lattice.3 One more aspect of the structure of the title compounds, which we briefly touched upon above, but need to elaborate on, is the possible phase width. We already mentioned the likelihood for a different Zn content between Cs8Zn4Sn42 (Zn/Sn ratio not established) 14 and our sample of Cs8Zn3.44Sn42.56(5) (Table 1) based on their unit cell volumes. However, since we performed multiple structure determinations for single-crystals of various batches (both as-synthesized and annealed at 300 °C for 1 week), and since they all turned in virtually identical refined compositions, we could argue that the phases in question do not have wide homogeneity ranges. The exact limits remain unknown, but based on the fact we are producing the crystals from Sn-rich solution, one could expect that the formula Cs8Zn3.44Sn42.56(5) represents the Zn-leanest range, while Cs8Zn4Sn42 represents the upper limits of the Zn uptake. The basis for this argument is the Zintl concept,5 according to which, the ideal formula of A8Zn4Sn42 can be rationalized as [A + ] 8 [4b-Zn 2− ] 4 [4b-Sn 0 ] 42 . Similarly, A8Cu2.67Sn43.33 can be broken down to [A+]8[4b-Cu3−]2.67[4bSn0]43.33. However, one could notice that the structure refinements show deviations from the limiting compositions defined by the Zintl reasoning. For the Cu−Sn clathrates, the deviations between the refined and the idealized A8Cu2.67Sn43.33 formula are not statistically significant (Table 1). In the case of the Zn− Sn clathrates, however, the refined formulas have lower Zn content than the ideal formula of A8Zn4Sn42. Take for example Rb8Zn3.52Sn42.48(7) and Cs8Zn3.44Sn42.56(5), which show 12−14 atom % less Zn than what is expected. One could argue that the lower Zn content might be a hint for vacancies in the framework, as demonstrated by Rogl et al. for Ba8ZnxGe46−x−y□y.17b We attempted structure refinements following the latter model, but they did not support the notion of vacancies at the 6c site (Supporting Information). The slightly elongated anisotropic displacement parameter for Sn1 (the framework Sn atom that neighbors the Zn)30 should therefore be attributed to the substitution of the larger Sn with the smaller Zn at the 6c site, which causes the small positional disorder of the nearest framework-atoms at the 24k site (Figure 2). Since the Cu atom is even smaller than Zn, this hypothesis could be tested experimentally from the diffraction data for Rb8Cu2.8Sn43.2(1), where we successfully modeled the 24k site as split between two nearby sites with the same multiplicity and Wyckoff letterin an approximate ratio 3:1. Additional information pertaining to these structure refinements is provided as Supporting Information as well. The statistically significant improvements (Table 1 and Table S4) support our reasoning; however, the final answer as to what is the origin of the effect and whether or not the proposed splitting is the best model requires additional structural work including 119Sn Mössbauer spectroscopy. We will note that similar positional disorder is not without precedents and has been already noted for K7B7Si3933 and Ba8CdxGe43−5x/8□3−3x/8,34 where there also exist significant size differences between the framework elements B and Si and elements Cd and Ge, respectively. The inverse type-I clathrates Sn17Zn7P22I8 and Sn17Zn7P22Br8

K8Zn4Sn42, Rb8Zn4Sn42, and Cs8Zn4Sn42, respectively. These values are on the short side for being normal Sn−Sn covalent bonds but are “right” for Cu−Sn and Zn−Sn bonding (the corresponding Pauling radii are rSn = 1.42 Å, rZn = 1.21 Å and rCu = 1.18 Å, respectively31). The slight increase in the distances as a consequence of the incorporation of larger alkali metals (from K to Rb to Cs) mirrors the above-mentioned unit cell expansion. The rest of the clathrate framework is invariant of the Cu/Zn substitution at the site 6c, which is why the remaining Sn−Sn distances are longer, consistent with the larger size of the Sn atom.31 The distances fall in the range from 2.8186(9) Å to 2.8680(9) Å for K8Cu2.67Sn43.33, from 2.812(2) Å to 2.868(2) Å for Rb8Cu2.67Sn43.33, from 2.8087(8) Å to 2.8959(5) Å for Cs8Cu2.67Sn43.33, from 2.8168(8) Å to 2.8577(7) Å for K8Zn4Sn42, from 2.814(1) Å to 2.868(1) Å for Rb8Zn4Sn42, and from 2.8100(5) Å to 2.8837(5) Å for Cs8Zn4Sn42, as shown in Table 3. All distances are in good agreement with those previously reported for Cs8Zn4Sn42 (dSn−Sn/Zn = 2.757(2) Å, dSn−Sn = 2.810(4)−2.880(4) Å),14 Cs8Cd4Sn42 (dSn−Sn/Cd = 2.8217(5) Å, dSn−Sn = 2.827(1)−2.869(1) Å),15 K8Hg4Sn42 (dSn−Sn/Hg = 2.7922(3) Å, dSn−Sn = 2.8201(5)−2.8514(7) Å),16 Rb8Hg4Sn42 (dSn−Sn/Hg = 2.8072(6) Å, dSn−Sn = 2.8240(5)− 2.8695(7) Å),16 and Cs8Hg4Sn42 (dSn−Sn/Hg = 2.8139(4) Å, dSn−Sn = 2.8297(5)−2.8850(9) Å).16 All stated reference values are determined at room temperature. The nearly identical ranges of the Sn−Sn bonds are another indication for a rigid framework with a very small thermal expansion. The rigidity of the framework is also evident from the behavior of the alkali metal atoms encapsulated within the voids. As mentioned in the Experimental Section, the anisotropic displacement parameters (ADP) for the alkali metal guests in the large tetrakaidecahedral cages are significantly distorted and elongated compared to those residing in the smaller dodecahedral cages. The ADP values decrease from the K-containing compounds to the Cs compounds, which means that the Cs atoms provide for much “tighter” fit. Considering the corresponding radii of the alkali metals (rK = 2.03 Å, rRb = 2.16 Å, rCs = 2.35 Å31), it is understandable that the K atoms are relatively small for the cages, whereas the Cs atoms provide a nearly optimal fit in them, with Rb atoms somewhere in between in terms of size match. Therefore, the decrease of the ADP values from K to Rb to Cs is in accordance with the presumed “rattling motion” of the alkali metals in the framework cages, which must be sizedependent. In fact, the ADPs values in the tetrakaidecahedra are so large for K8Cu2.67Sn43.33, Rb8Cu2.67Sn43.33, K8Zn4Sn42, and Rb8Zn4Sn42 that an off-centering of the K and Rb atoms is needed to adjust the electron density (from the center of the cages, site 6d, to a 24k site as shown in Table S2 and Figure 2). The 4-times greater multiplicity of this off-center site requires a 1/4 partial occupation. Such disorder has not been noted previously for the other known d-metal substituted Snclathrates, although from the published Ueq values, we could argue that analogous disorder is present, but likely overlookedfor example the K and Rb atoms at the site 6d in the structures of K8Hg4Sn42 and Rb8Hg4Sn4216 have extremely large Ueq’s of 0.118(4) Å2 and 0.0730(9) Å2, respectively, which are orders of magnitude higher than the rest. Similarly large Ueq’s are seen if the A8Cu2.67Sn43.33 and A8Zn4Sn42 (A = K, Rb) structures are refined without off-centering of the atoms in the tetrakaidecahedral cages (Table S3). 3741



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are also shown to have analogous, albeit more complicated, disorder at the same site.35 Thermal Analysis. A list of the melting points for several type-I tin clathrates A8Sn44□2 and A8(M,Sn)46 (A = K, Rb, Cs; M = Cu, Zn, Cd, Hg; □ = vacancy)13−16,22−25 is provided in Table 4. In accordance with the literature, the term “melting points” is used; however, the given values reflect temperatures of peritectic decomposition. In addition to the small unit cell expansion that follows the increase of the size of the guest alkali metal atoms, the thermal stability of such compounds appears to increase too (i.e., Cs-containing clathrates have the highest melting points). The “inverse dependence” on the sizes of the substituted framework atoms can be seen toothe Zncontaining clathrates have higher melting points than their Cd or Hg counterparts. However, for almost all compounds, melting is followed by decomposition, which is confirmed by our study as well. Incorporation of Cu (highest melting element of all) does not appear to affect the overall thermal stability. Properties. The clathrate compounds with general formulas A8Cu2.67Sn43.33 and A8Zn4Sn42 (A = K, Rb, Cs) are expected to be semiconductors, since they can be rationalized as electronprecise Zintl phases.5 Correspondingly, because of their openframework structures with “vibrating” atoms within the voids all desirable characteristics of the thermoelectric materialswe set out to conduct preliminary property measurements for single crystals of K8Cu2.67Sn43.33, Cs8Cu2.67Sn43.33, K8Zn4Sn42, and Rb8Zn4Sn42. We gathered temperature dependent data of the Seebeck coefficient (S) and the electrical resistivity (ρ). The available crystals of Cs8Zn4Sn42 were inadequate in size, and resistivity measurements could not be carried out. Data for Rb8Cu2.67Sn43.33 are also not shown since reproducibility was poorthe issue is likely due to synthetic challenges already described in the Experimental Section. Temperature dependent measurements of the Seebeck coefficient between 300 and 500 K (shown in Figure 3) and electrical resistivity in the range of 100−400 K (displayed in Figure 4) were performed. As seen in Figure 3, all five samples show negative Seebeck coefficient values over the entire measured temperature range, indicating that the dominant carriers are electrons; thus all specimens should be n-type semiconducting materials. This agrees particularly well with the observed Zn deficiency of the refined structures relative to the ideal ones. The higher absolute value for K8Zn4Sn42 (−85 μV/ K at 300 K) compared to that for Cs8Zn4Sn42 (−71 μV/K at 300 K) might be due to the enhanced “rattling” of the K guest atoms vs the Cs atoms. The absolute value for Rb8Zn4Sn42 (−100 μV/K at 300 K) is the highest of all three, which could be interpreted as the synergistic effect of the favorable displacement of the guest Rb atoms and higher charge-carrier concentrations. However, the difference between the absolute value of the Seebeck coefficient of the Cu-containing clathrates appears to contradict this logic: −45 μV/K for K8Cu2.67Sn43.33 vs −124 μV/K for Cs8Cu2.67Sn43.33. In both Cu-containing specimens, increasing the temperature also leads to a small change in the Seebeck coefficient with relatively poor linearity up to ca. 370 K, and with very large scatter above 450 K. Whether this is an intrinsic behavior remains to be confirmed; yet, we can speculate that among the many factors which could be the root for this, a combination of charge-carrier concentrations (doping), the displacement of the guest atoms (i.e., rattling), and the degree of positional disorder in the framework (recall that split-model refinement of the Sn1 atoms in A8Cu2.67Sn43.33) ought to be considered. Clearly, there are

Figure 3. Temperature dependence of the Seebeck coefficient for (a) A8Cu2.67Sn43.33 (A = K, Cs) and (b) A8Zn4Sn42 (A = K, Rb, Cs).

Figure 4. Temperature dependence of the resistivity of singlecrystalline K 8 Cu 2.67 Sn 43.33 , Cs 8 Cu 2.67 Sn 43.33 , K 8 Zn 4 Sn 42 , and Rb8Zn4Sn42.

many clues from the discussed single-crystal work, which could aid the interpretation of more elaborate property measurements in the future. The plots of the resistivity vs temperature for K8Cu2.67Sn43.33, Cs8Cu2.67Sn43.33, K8Zn4Sn42, and Rb8Zn4Sn42 are displayed in Figure 4. All measurements suggest semiconducting-like behavior, as expected for Zintl phases. However, we recall that based on the refined compositions (Table 1), one would expect that metallic behavior is more likely. This discrepancy is puzzling and cannot be explained from the structural and property data available to date. At room temperature, Rb8Zn4Sn42 and Cs8Cu2.67Sn43.33 have ρ = 273 mΩ·cm and ρ = 341 mΩ·cm, respectively, while both potassium containing samples exhibit lower resistivity with ρ = 38 mΩ·cm for K8Zn4Sn42 and ρ = 18 mΩ·cm, for K8Cu2.67Sn43.33. 3742



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The results above need to be confirmed (and thermal conductivities need to be measured), but with their electrical resistivity being in favorable ranges, the K8Zn4Sn42 and K8Cu2.67Sn43.33 clathrates should be compared with some other Sn-based clathrates that are suggested as possible thermoelectric materials. Using the measured values and an estimated thermal conductivity of 1 Wm−1K−1, a thermoelectric figure of merit of ZT ≈ 0.06 at 300 K can be calculated for K8Zn4Sn42. In comparison, the reported room temperature data for Rb8Zn4Sn42 and Cs8Zn4Sn42 are the following: S = −250 μV/K and ρ = 100 mΩ·cm and with S = −200 μV/K and ρ = 50 mΩ·cm, respectively.13,14 Factoring the thermal conductivities, their room temperature ZTs are ca. 0.02. Similar values for the figure of merit are reported for K8Ga8−xSn38+x (ZT ≈ 0.07− 0.17)6,36 and Ba8Ga16−xSn30+x (ZT ≈ 0.05−0.09).7 Apparently both previously reported Rb8Zn4Sn42 and Cs8Zn4Sn42 samples exhibit higher Seebeck coefficients and lower resistivity values than any of the herein reported ones. We should mention again that the quoted numbers are obtained on hot-pressed polycrystalline samples and that their actual compositions have not been verified through singlecrystal work. We already speculated that since these samples have been synthesized by fusing stoichiometric mixtures of the respective elements, their compositions might be closer to the idealized composition. Such slight variations can cause large differences in the corresponding absolute values of the Seebeck coefficients and for the resistivity, as demonstrated already by Hayashi et al. for n-type K8Ga8−xSn38+x (0 ≤ x ≤ 2)6 and by Suekuni et al. in their study of type-I and type-VIII Ba8Ga16−xSn30+x single crystals.7 In the latter compounds, subtle variations in the compositions originating from small alterations of the preparatory method are shown to lead to changes from n- to p-type semiconducting behavior.7 All of the above underscores the importance of understanding the synthesis and how it affects the structure and properties in such convoluted systems.

AUTHOR INFORMATION

Corresponding Author

*Fax: (302) 831-6335. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Parts of this work were presented at the 245th ACS Meeting held in New Orleans, LA (April 7−11, 2013).

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ACKNOWLEDGMENTS S.B. acknowledges financial support from the U.S. Department of Energy through grants DE-SC0001360 and DE-SC0008885. REFERENCES

(1) Jeffrey, G. A. In Inclusion Compounds; Atwood, I. J. L., Davies, J. E. D., MacNichol, D. D., Eds.; Academic Press: London, 1983; pp 135−185. (2) Kasper, J. S.; Hagenmuller, P.; Pouchard, M.; Cros, C. Science 1965, 150, 1713. (3) (a) Slack, G. A. In CRC Handbook of Thermoelectrics; Rowe, D. M. , Ed.; CRC Press: Boca Raton, FL, 1995; pp 407−440. (b) Sales, B. C.; Chakoumakos, B. C.; Mandrus, D.; Sharp, J. W. J. Solid State Chem. 1999, 146, 528−532. (c) Christensen, M.; Lock, N.; Overgaard, J.; Iversen, B. B. J. Am. Chem. Soc. 2006, 128, 15657−15665. (d) Christensen, M.; Johnson, S.; Iversen, B. B. Dalton Trans. 2010, 39, 978−992. (e) Toberer, E. S.; May, A. F.; Snyder, G. J. Chem. Mater. 2010, 22, 624−634. (4) (a) Bobev, S.; Sevov, S. C. J. Solid State Chem. 2000, 153, 92− 105. (b) Beekman, M.; Nolas, G. S. J. Mater. Chem. 2008, 18, 842− 851. (c) Shevelkov, A. V.; Kovnir, K. In Structure Bonding; Springer: Berlin, 2011; Vol. 990, pp 97−142. (d) Kirsanova, M. A.; Olenev, A. V.; Abakumov, A. M.; Bykov, M. A.; Shevelkov, A. V. Angew. Chem., Int. Ed. Engl. 2011, 123, 2419−2422. (5) Kauzlarich, S. M. Chemistry, Structure, and Bonding of Zintl Phases and Ions; Wiley-VCH: New York, 1996; including references therein. (6) Hayashi, M.; Kishimoto, K.; Kishio, K.; Akai, K.; Asada, H.; Koyanagi, T. Dalton Trans. 2010, 39, 1113−1117. (7) Suekuni, K.; Avila, M. A.; Umeo, K.; Fukuoka, H.; Yamanaka, S.; Nakagawa, T.; Takabatake, T. J. Phys. (Paris) 2008, B 77, 235119. (8) Bentien, A.; Christensen, M.; Bryan, J. D.; Sanchez, A.; Paschen, S.; Steglich, F.; Stucky, G. D.; Iversen, B. B. Phys. Rev. 2004, B 69, 045107. (9) Bobev, S.; Meyers, J., Jr.; Fritsch, V.; Yamasaki, Y. Proc. Int. Conf. Thermoelectr. 2006, 25, 48−52. (10) Schäfer, M. C.; Fritsch, V.; Yamasaki, Y.; Bobev, S. Crystals 2011, 1, 145−162. (11) He, H.; Zevalkink, A.; Gibbs, Z. M.; Snyder, G. J.; Bobev, S. Chem. Mater. 2012, 24, 3596−3603. (12) (a) Schäfer, M. C.; Bobev, S. J. Am. Chem. Soc. 2013, 135, 1696−1699. (b) Schäfer, M. C.; Bobev, S. Acta Crystallogr. 2013, C 69, 319−323. (13) Nolas, G. S.; Cohn, J. L.; Dyck, J. S.; Uher, C.; Yang, J. Phys. Rev. 2002, B 65, 165201. (14) Nolas, G. S.; Weakley, T. J. R.; Cohn, J. L. Chem. Mater. 1999, 11, 2470−2473. (15) Wilkinson, A. P.; Lind, C.; Young, R. A.; Shastri, S. D.; Lee, P. L.; Nolas, G. S. Chem. Mater. 2002, 14, 1300−1305. (16) Kaltzoglou, A.; Ponou, S.; Fässler, T. F. Eur. J. Inorg. Chem. 2008, 538−542. (17) (a) Kuhl, B.; Czybulka, A.; Schuster, H. Z. Anorg. Allg. Chem. 1995, 621, 1−6. (b) Melnychenko-Koblyuk, N.; Grytsiv, A.; Fornasari, L.; Kaldarar, H.; Michor, H.; Rohrbacher, F.; Koza, M.; Royanian, E.; Bauer, E.; Rogl, P.; Rotter, M.; Schmid, H.; Marabelli, F.; Devishvili,

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CONCLUSIONS Reported are the synthesis and structural characterization of type-I clathrates A8Cu2.67Sn43.33 and A8Zn4Sn42 (A = K, Rb, Cs). All studied compounds are n-type materials and exhibit semiconducting-like behavior. The A8Zn4Sn42 series features the best thermal stability of all Sn-based clathrate phases with d-metal substitution, known up until now. The Cu-containing compounds expand the chemical range for exploring new intermetallic clathrates, which in turn may lead to the discovery of interesting thermoelectric and/or other physical properties. Currently, we are conducting more work aimed at precise control of the Zn (and Cu) content, which is a key element for increasing the absolute value of the Seebeck coefficient and for the optimization of the charge-carrier concentration.

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ASSOCIATED CONTENT

S Supporting Information *

Tables with the atomic coordinates and equivalent isotropic displacement parameters for Rb8Cu2.67Sn43.33 and A8Zn4Sn44 (A = K, Rb, Cs), tables of the anisotropic displacement parameters for all compounds, figures with anisotropic displacement parameters, EDS results, experimental and simulated powder X-ray diffraction pattern of Rb8Cu2.67Sn42.33, and plots from the DSC-TG analysis of Rb8Zn4Sn42 and Rb8Cu2.67Sn42.33. This material is available free of charge via the Internet at http:// pubs.acs.org. 3743



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A.; Dörr, M.; Giester, G. J. Phys.: Condens. Matter 2007, 19, 216223. (c) Christensen, M.; Iversen, B. B. J. Phys.: Condens. Matter 2008, 20, 104244. (18) Zhang, H.; Borrmann, H.; Ö schler, N.; Candolfi, C.; Schnelle, W.; Schmidt, M.; Burkhardt, U.; Baitinger, M.; Zhao, J.-T.; Grin, Yu. Inorg. Chem. 2011, 50, 1250−1257. (19) Cordier, G.; Woll, P. J. Less-Common Met. 1991, 169, 291−302. (20) (a) Hewaidy, I. F.; Busmann, E.; Klemm, W. Z. Anorg. Allg. Chem. 1964, 328, 283−293. (b) Baitinger, M.; von Schnering, H. G.; Grin, Yu.; Kniep, R. Z. Kristallogr.New Cryst. Struct. 1999, 214, 453− 454. (c) Baitinger, M.; Grin, Yu.; von Schnering, H. G.; Kniep, R. Z. Kristallogr.New Cryst. Struct. 1999, 214, 457−458. (21) Lidin, S.; Piao, S. Z. Anorg. Allg. Chem. 2009, 635, 611−613. (22) Gallmeier, J.; Schäfer, H.; Weiss, A. Z. Naturforsch. 1969, B24, 665−667. (23) (a) Zhao, J.-T.; Corbett, J. D. Inorg. Chem. 1994, 33, 5721− 5726. (b) Dubois, F.; Fässler, T. F. J. Am. Chem. Soc. 2005, 127, 3264− 3265. (24) Kaltzoglou, A.; Fässler, T. F.; Christensen, M.; Johnsen, S.; Iversen, B.; Presniakov, I.; Sobolev, A.; Shevelkov, A. J. Mater. Chem. 2008, 18, 5630−5637. (25) (a) von Schnering, H. G.; Kröner, R.; Baitinger, M.; Peters, K.; Nesper, R.; Grin, Yu. Z. Kristallogr.New Cryst. Struct. 2000, 215, 205. (b) Kaltzoglou, A.; Hoffmann, S. D.; Fässler, T. F. Eur. J. Inorg. Chem. 2007, 4162−4167. (26) SMART NT, version 5.63; Bruker Analytical X-ray Systems Inc., Madison, WI, 2003. (27) SAINT NT, version 6.45; Bruker Analytical X-ray Systems Inc., Madison, WI, 2003. (28) SADABS NT, version 2.10; Bruker Analytical X-ray Systems Inc., Madison, WI, 2001. (29) SHELXTL, version 6.12; Bruker Analytical X-ray Systems Inc., Madison, WI, 2001. (30) In our refinements of the three Cu-containing compounds in particular, the anisotropic displacement parameters (ADP) U22 and U33 for Sn1 on site 24k are unusually large (values are listed in Table S2 in the Supporting Information). Similar elongation is seen for the Zn-containing compounds as well, but the effect is less pronounced. The Sn1 atoms apparently “move” in the direction towards the 6c site (M3), on which the Sn atoms are mixed occupied with the smaller Cu/Zn atoms. The Sn1 can be split to account for this small positional disorder. A potential long-range ordering of the framework atoms needs to be examined with higher resolution X-ray diffraction or selected area electron diffraction (SAED) experiments. (31) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960. (32) The refined formula of the Zn-containing clathrates deviate more from the limiting composition, suggested on the basis of the Zintl concept, and are always Zn-deficient. Most likely, the reason for this is that Zn and Sn form a eutectic mixture with mp 198.5 °C; i.e., some amount of Zn is “lost” when the flux is removed at 400 °C. This is not the case with Cu and Sn, and in these cases, the refined formulae are very close to the idealized composition. Reactions with different Zn/Sn ratios (8−3: 38−50), however, did not yield any significant changes in the refined compositions, which may indicate that in addition to the Zn−Sn disorder, there are also vacancies present. For the phase Cs8Zn3.44Sn42.56(5), where the Zn deficiency is most apparent, this would equate to Cs8ZnxSn46−x−y□y (x ≈ 3.5; □ ≈ 0.25)such small variations in the composition could not be established reliably, neither from refinements nor from the EDS measurements. (33) Jung, W.; Lörincz, J.; Ramlau, R.; Borrmann, H.; Prots, Yu.; Haarmann, F.; Schnelle, W.; Burkhardt, U.; Baitinger, M.; Grin, Yu. Angew. Chem., Int. Ed. 2007, 46, 6725−6728. (34) Melnychenko-Koblyuk, N.; Grytsiv, A.; Berger, S.; Kaldarar, H.; Michor, H.; Rohrbacher, F.; Royanian, E.; Bauer, E.; Rogl, P.; Rotter, M.; Schmid, H.; Giester, G. J. Phys.: Condens. Matter 2007, 19, 046203. (35) Kovnir, K. A.; Shatruk, M. M.; Reshetova, L. N.; Presniakov, I. A.; Dikarev, E. V.; Baitinger, M.; Haarmann, F.; Schnelle, W.; Baenitz, M.; Grin, Y.; Shevelkov, A. V. Solid State Sci. 2005, 7, 957−968.

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Copper and Zinc Substitutions in Clathrates of Tin: Synthesis

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