Clathrates DOI:10.1002/anie.200701028 K 7 B 7 Si 39 , a Borosilicide with the Clathrate I Structure Walter Jung,* Josef Lörincz, Reiner Ramlau, Horst Borrmann, Yurii Prots, Frank Haarmann, Walter Schnelle, Ulrich Burkhardt, Michael Baitinger, and Yuri Grin* The clathrate I type silicides of the alkali metals were first prepared by thermal decomposition of the monosilicides in 1948. [1] However, their crystal structures and the chemical composition M 8 Si 46 (M = Na, K) have been known only since 1965. [2] The compounds exhibit metallic electrical conductiv- ity, because the valence electrons donated by the alkali-metal atoms are not required for the framework of four-bonded silicon atoms. [3] Partial substitution of silicon by trivalent atoms resulted in compounds such as K 8 Ga 8 Si 38 and Rb 8 Ga 8 Si 38 . With the electronic balances [K + ] 8 [Ga  ] 8 [Si 0 ] 38 and [Rb + ] 8 [Ga  ] 8 [Si 0 ] 38 , they follow the Zintl–Klemm concept and are semiconductors. [4] To date, the corresponding ternary clathrates with boron are not known. They are of special interest, however, because the substitution of silicon by boron results in a contraction of the clathrate framework. This contraction might facilitate the embedding of smaller cations into the cages. Such compounds are expected to be promising thermoelectric materials. [5] K 7 B 7 Si 39 is the first borosilicide with a clathrate I crystal structure, and it has the smallest lattice parameter among all known clathrate compounds based on Group 14 elements. The compound can be obtained in the form of well-shaped crystals, [6] which is unusual, since the silicon clathrates are normally obtained as fine polycrystalline powders by thermal decomposition [1] or oxidation [7] of reactive starting com- pounds. Single crystals were prepared in only a few cases. [2,4,8,9] The synthesis of K 7 B 7 Si 39 can be carried out by reaction of the elements in tantalum ampoules at approximately 900 8C; however, the conversion is not complete. The side products can be washed out, because K 7 B 7 Si 39 is stable towards sodium hydroxide solution as well as towards concentrated acids. Higher yields may be achieved if silicon and boron are allowed to pre-react in an arc melter. Energy-dispersive X-ray spectroscopy (EDXS) investiga- tions of K 7 B 7 Si 39 were performed in both scanning and transmission electron microscopes. The presence of boron was demonstrated qualitatively; potassium and silicon were found in a ratio of approximately 7:41, which is close to the expected ratio of 7:39. [10] Microcrystallites that clearly repre- sented the clathrate phase (as demonstrated by selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM)) were analyzed by electron energylossspectroscopy(EELS)intheregionsofthefracture edges (thickness less than 10 nm). The experimental EEL spectrum (Figure 1) agrees well with that simulated for the composition K 7 B 7 Si 39 and a sample thickness of 5–10 nm. The composition determined by means of the crystal structure analysis was thus confirmed within the estimated standard deviations. Metallographic investigation of polished single crystals confirmed the single-phase character of the material. How- ever, some larger crystals showed inclusions in their nuclei (volume share less than 10%) which, according to EDXS analysis, do not contain potassium and which do not appear in the diffraction pattern. The total content of these phases in the sample is very low. The Guinier powder diffraction pattern can be indexed with a cubic lattice and a lattice parameter of a = 9.952(1) . For samples with higher silicon content, the clathrate structure appears with larger lattice parameters up to a = 9.971(1) . On the other hand, an increase of the boron content does not reduce the lattice parameter. This suggests a small homogeneity range stretch- ing from K 7 B 7 Si 39 to higher Si:B ratios. For the binary phase K 7.0(1) Si 46 , the lattice parameter is clearly larger (10.278(1) ). [11] ThecompositionK 7 B 7 Si 39 was mainly established from the crystal structure determination with single-crystal X-ray diffraction data. According to this analysis, four-bonded Figure 1. EELspectrumofK 7 B 7 Si 39 .MicrocrystalliteofK 7 B 7 Si 39 inthe [111]orientation,boron K edgeat188eV. [*] Prof.Dr.W.Jung,J.Lörincz InstitutfürAnorganischeChemie UniversitätzuKöln Greinstrasse6,50939Köln(Germany) Fax:(+ 49)221-470-5083 E-mail:walter.jung@uni-koeln.de R.Ramlau,H.Borrmann,Yu.Prots,F.Haarmann,W.Schnelle, U.Burkhardt,M.Baitinger,Prof.Dr.Yu.Grin Max-Planck-InstitutfürChemischePhysikfesterStoffe NöthnitzerStraße40,01187Dresden(Germany) Fax:(+ 49)351-4646-4001 E-mail:grin@cpfs.mpg.de SupportinginformationforthisarticleisavailableontheWWW underhttp://www.angewandte.orgorfromtheauthor. Angewandte Chemie 6725 Angew. Chem. Int. Ed. 2007, 46, 6725 –6728 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim