Neue Lanthan-Stannide/Germanide: La3Sn4.4Ge0.6, La3Sn3.1Ge0.9 und La9Sn6.7Ge3.3 / New Lanthanum Stannide/Germanides: La3Sn4.4Ge0.6, La3Sn3.1Ge0.9 and La9Sn6.7Ge3.3

Abstract

Three new ternary mixed lanthanum stannide/germanides have been synthesized and characterized in the course of a systematic study of the phase formation at the 3 : 5, 3 : 4 and 1 : 1 pseudo-binary sections of the ternary system La - Ge - Sn, i. e. a study of the ’coloring’ in mixed tetrelides. The structures of the title compounds have been determined using single-crystal X-ray data, and the electronic structure was analyzed by means of simple electron counting rules and FP-LAPW band structure methods. Even a very minor substitution of Sn by Ge in the 3 : 5 stannide La3Sn5 already changes the structure from the Pu3Pd5 type of the binary stannide towards the Tl4PbTe3 type (La3Sn4.4Ge0.6: tetragonal, space group I4/mcm, a = 861.35(10), c = 1211.48(13) pm, Z = 4, R1 = 0.0420). According to the anionic building blocks present, isolated [Ge/Sn]4− anions and heavily puckered 4.82 nets of three-bonded tin atoms (i. e. [Sn4]4− Zintl ions), a small formal electron excess (+9/−8) occurs, which is also apparent from the position of the minimum in the calculated tDOS. In contrast, starting from the known binary stannide La3Sn4 (orthorhombic, space group Cmcm, Er3Ge4 structure type) a substitution of tin by germanium is possible without a change of the structure type up to the border composition La3Sn3.1Ge0.9 (a = 448.61(5), b = 1170.68(14), c = 1556.0(2) pm, Z = 4, R1 = 0.0380). Germanium preferentially occupies the sites of the [Ge/Sn]3 trimers, whereas the site of the square-planar coordinated Sn(3), which is stabilized by hypervalent bonding, is not populated by germanium. Taking the respective partial bonding into account, the Zintl electron count is also nearly exact (+9/−8). The new complex mixed tetrelide La9Sn6.7Ge3.3 (tetragonal, space group P42/ncm, a = 1602.04(3), c = 1724.42(5) pm, Z = 8, R1 = 0.0706) exhibits nine crystallographically different tetrel positions, which are occupied by isolated Sn and Ge atoms, [Sn/Ge]2 dumbbells, bent [Sn3] trimers, planar four-membered rings [Ge4], and planar six-membered rings [M6]. In this compound, the electron count following the Zintl concept also reveals only a very small formal excess of electrons (+54/−52).