Ln 9 Al 5 Sn 10 : Aluminium Stannides of the Divalent Rare Earth Elements Yb and Eu. Synthesis, Crystal Structure, and Chemical Bonding

The isotypic aluminium stannides Ln 9 Al 5 Sn 10 ( 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 Ln Al 2 – Ln Sn 2 from the elements at maximum temperatures of 1170 K. Their crystal structures were determined using single‐crystal X‐ray diffraction [both monoclinic, space group C 2/ 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, R 1 = 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 SrAl 2 (KHg 2 structure type). The trigonal‐bipyramidal units [Al 3 Sn 2 ] of motif III, which can alternatively be described as three AlSn 4 tetrahedra sharing one common edge, are also known from binary barium aluminides like Ba 7 Al 10 . 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 AlSn 4 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.