The binary tin-rich lanthanum stannides La_2Sn_5 and La_3Sn_7 - A structural and bond theoretical study

摘要

The three tin-rich lanthanum stannides LaSn_3, La _2Sn_5 and La_3Sn_7, 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. LaSn_3 (cubic, space group Pm3m, a=477.77(1) pm, Z=1, RF_2=0.044) crystallizes with the common Cu _3Au structure type, which can be described either as an ordered close packing or as a network of [Sn_6] octahedra (closo clusters) connected via common vertices. The structures of La_2Sn_5 (Ce_2Sn_5 structure type, orthorhombic, space group Cmmm, a=463.85(3), b=3581.3(3), c=469.83(4) pm, Z=4, RF_2=0.075) and La _3Sn_7 (Gd_3Sn_7 structure type, orthorhombic, space group Cmmm, a=457.83(4), b=2628.40(2), c=467.99(4) pm, Z=2, RF_2=0.068), which retain some similarities to ordered close packings as well, show 2D blocks of corner-sharing [Sn_6] octahedra terminated by [Sn_5] 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 E_F, 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.