The binary tin-rich lanthanum stannides La2Sn5 and La3Sn7—A structural and bond theoretical study

The three tin-rich lanthanum stannides LaSn3, La2Sn5 and La3Sn7, which were known only from indexed powder diagrams, have been synthesized from melts of the elements by slow cooling of sealed sample tubes or arc melting and quenching. Their crystal structures were refined from X-ray powder diffractometer data using the Rietveld method. LaSn3 (cubic, space group Pm3¯m, a=477.77(1) pm, Z  =1, RF2=0.044)RF2=0.044) crystallizes with the common Cu3Au structure type, which can be described either as an ordered close packing or as a network of [Sn6] octahedra (closo clusters) connected via common vertices. The structures of La2Sn5 (Ce2Sn5 structure type, orthorhombic, space group Cmmm, a=463.85(3), b=3581.3(3), c=469.83(4) pm, Z  =4, RF2=0.075)RF2=0.075) and La3Sn7 (Gd3Sn7 structure type, orthorhombic, space group Cmmm, a=457.83(4), b=2628.40(2), c=467.99(4) pm, Z  =2, RF2=0.068)RF2=0.068), which retain some similarities to ordered close packings as well, show 2D blocks of corner-sharing [Sn6] octahedra terminated by [Sn5] square pyramids (nido clusters). In between these layers, isolated Sn zigzag chains similar to those observed in LaSn (CrB structure type) are interspersed. A simple v.e. count after Zintl and Wade/Jemmis for the polyanionic Sn building blocks resulting from these structure descriptions fits the observed v.e. numbers of the three compounds almost exactly. It is also possible to explain the minima of the tDOS close to EF, which have been calculated within the framework of the FP-LAPW DFT approach. However, the simple assumption of an exclusively Sn-p bonded polyanion would neglect the large Sn–La bonding contributions and the reduced amount of electron transfer from La to Sn when compared to related alkaline earth stannides.

Graphical AbstractLa2Sn5 and La3Sn7, stacking variants of LaSn3 (Cu3Au type, closo clusters) and LaSn (CrB type, zig-zag chains) Figure optionsDownload as PowerPoint slideHighlights
► Preparation of binary La stannides LaSn3, La2Sn5 and La3Sn7.
► La2Sn5 and La3Sn7 do not appear in published phase diagrams.
► Full pattern Rietveld refinements of the crystal structures of all compounds.
► Revised description of the structure types emphasizes polyanionic Sn framework.
► Structural motifs rationalized through DFT calculations and Zintl and Wade rules.