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Al43.14C0i8.86Si11.86, monoclinic, C12/cl (no. 15), a = 20.010(2) Ä, b = 19.1498(7) Ä, c = 12.8227(8) Ä, β = 123.56(1)°, V= 4094.6 Ä, Ζ = 4, Rgi(F) = 0.035, wRItf(F) = 0.079, Τ = 293 Κ. Al42.51C019.49Si12.49, monoclinic, C12/cl (no. 15), a = 20.025(2) Ä, b = 19.173Γ2) A, c = 12.835(1) A, β = 123.63(1)°, V= 4103.2 A ,Ζ = 4,R$(F) = 0.044, wR,ef(F) = 0.108, Τ = 293 Κ. * Correspondence author (e-mail: klaus.richter@univie.ac.at) Source of material The title compound was observed during a systematic phase equilibria investigation in the Co-Al-Si system [1]. Samples were prepared from the pure elements by arc melting in inert gas atmosphere. Subsequent heat treatment (4 weeks at 800 °C) was carried out in alumina crucibles placed into evacuated silica ampules. Electron probe microanalysis shows that the homogeneity range of the γ phase is considerable: at 800 °C the Si content varies from 8.6 at.% to 17.0 at.% and the Co content varies between 25.6 at.% and 26.6 at.% [1], Single crystals were isolated from three individual samples with the nominal compositions C022AI62S116? C026AI62S112 and C028AI56S116. Hie data sets shown 116 Coi9-xAl43+ySii2-y (x = -0.14, y = 0.14; χ = 0.49, y = -0.49) here correspond to the single crystals isolated from the samples with the nominal compositions C022AI62S116 and C028AI56S116, i.e. from the Co-poor and Co-rich sides of the homogeneity range. At the annealing temperature, the γ phase in C022AI62S116 is in equilibrium with the liquid and in Co2sAls6Sii6 it is in equilibrium with χ and δ. Experimental details Samples were characterized by X-ray powder diffraction (Huber Image Plate Guinier camera G670, CoKai radiation, λ = 1.788965 A, 5° < 20 < 100°, LaBe as internal standard, a = 4.1569 Ä). Discussion Hie γ phase is one of five new complex ternary compounds that were found during the phase diagram investigation of the Co-AlSi system. At 800 °C it is in equilibrium with the adjacent binary compounds C0AI3 and C02AI9, with the ternary phases ό, φ and χ and with liquid phase [1]. It adopts an own structure type with no obvious structural relations to the adjacent phases in the binary system [2-6]. Important common structural motifs of these compounds, like the existence of condensed pentagonal prismatic channels [7] or the existence of a "cluster" formed by two trigonal prisms around cobalt and a distorted rectangular prism around aluminum [8] are not found in the title compound. One possibility to visualize the complex structure of the γ phase is based on the coordination of Co atoms. Six out of ten Co positions are coordinated only by Al and Si (Co3, Co4, C06, C08, Co9 and ColO) while the other four positions have also Co atoms in their first coordination sphere. Hie most regular coordination polyhedra (figure, bottom) include an icosahedron around C08 and capped pentagonal arrangements (coordination number 10) around Co3 and ColO which are arranged in slabs within the structure (figure, top). The γ phase exhibits a significant range of homogeneity connected with Al/Si substitution. It is possible to distinguish positions mainly occupied by Al atoms from those positions mainly occupied by Si atoms, as the smallest Co—Al distances are larger than 2.40 A while the smallest Co—Si distances are found below 2.40 Ä. The identification of Al and Si positions yields an ideal chemical formula C019AI43S112. The real extension of the γ phase at 800 °C (determined by ΕΡΜΑ) is Coi9±xAl43+ySii2->· with -0.5 < y < 3 [1]. This clearly indicates that a certain amount of Al/Si substitution is possible at these positions. The mechanism of nonstoichiometry connected with the small but significant variability of the Co content was investigated on three single crystals with different Co content. According to our refinements, the γ phase is stable between the compositions Coi8.86Al43+ySii2->> and Coi9 49Al43+ySii2-> which is in excellent agreement with the experimental determination of the phase boundaries of the γ phase by ΕΡΜΑ ( 0 < x < 0.7) [1]. The Co deficiency is due to a reduced occupation at the position Co7 (93 % occupation refined). This deficiency is connected with positional splitting observed at the adjacent Si7/A17 position: the occupation of the position by a Co atom is connected with a Si atom at the position Si7, while the position A17 is occupied in case of an empty position Co7. The structure refinement of a single crystal obtained from the Co-rich end of the composition range shows that the deviation to the Co-rich side of the ideal composition is due to the partial occupation of an additional Co position, Coll, with the atomic coordinates χ=0.0558, y=0.0951,2=0.3181. This partial occupation is limited to 28 % and it is connected with multiple splitting of adjacent Al and Si positions. It should be pointed out that the Al/Si ratio is in general not connected with the Co content. 1. Cobalt aluminum silicide, üiim(AI43.iAuk Table 1. Data collection and handling.

Crystal structures of cobalt aluminum silicide, Co 19+x Al 43+y (x = -0.14, y = 0.14; χ - 0.49, y = -0.49), the γ phase in the Co-Al-Si system