Ln₉Al₅Sn₁₀: aluminium stannides of the divalent rare earth elements Yb and Eu. Synthesis, crystal structure, and chemical bonding

The isotypic aluminium stannides Ln₉Al₅Sn₁₀ (Ln = Yb, Eu) are the first ternary compounds in the systems Yb-Al-Sn and Eu-Al-Sn. They were synthesized in the course of systematic experimental work on the phase stability along the sections LnAl₂ - LnSn₂ from the elements at maximum temperatures of 1170 K. Their crystal structures were determined using single-crystal X-ray diffraction [both monoclinic, space group C2/m, a = 2145.2(5)/2214.99(8), b = 1189.15(14)/1236.67(4), c = 1003.9(2)/1027.57(4) pm, β = 103.22(2)/103.300(3) , Z = 2, R₁ = 0.0492/0.0515 for Ln = Yb/Eu]. The two phases are isotypic to the respective calcium/strontium Al stannides. Their crystal structure can be best described as a complex three-dimensional network, consisting of three different building units (motifs I, II, and III), which are connected by two-bonded tin atoms (motif 0). The first motif is a planar zigzag chain of tin like in the CrB-type structure of e.g. EuSn. Ladders of four-bonded Al/Sn atoms (motif II) are also present in SrAl₂ (KHg₂ structure type). The trigonal-bipyramidal units [Al₃Sn₂] of motif III, which can alternatively be described as three AlSn₄ tetrahedra sharing one common edge, are also known from binary barium aluminides like Ba₇Al₁₀. In the Yb compound, this motif III is to a small extent of 17 % statistically substituted by an alternative but related motif IIIB, which consists of only two edge-sharing AlSn₄ tetrahedra. Despite the complex structure and some statistically occupied Al/Sn positions, the chemical bonding in the title compounds can be rationalized using the Zintl concept: Both the interpretation as aluminide/stannides and stannido-aluminates results in only a minuscule difference between the formal charge of the Al/Sn polyanion and the charge sum of the divalent rare earth cations. FP-LAPW bandstructure calculations of an ordered subgroup model of the europium compound confirm this interpretation in exhibiting a pronounced minimum of the tDOS slightly below the Fermi level. (Copyright copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)