La15Ge9Z. Interstitial Derivatives with an Ordered Superstructure of the Mn5Si3 Structure Type. Property Trends in a Series of Homologous Intermetallic Phases

The phases La 15 Ge 9 Z, Z = Mn, Fe, Co, Ni, Cu, Ru, C, O, P, have been obtained from reactions of either the elements or suitable binary compounds in sealed Ta containers at 1200−1350 °C. All exhibit an ordered √3 a × √3 a × c superstructure of the Mn 5 Si 3 type (La 5 Ge 3, La 5 Ge 3 Z, P 6 3 / mcm ) in which two-thirds of the slightly distorted confacial chains of trigonal antiprisms (La 6/2 Ge 6/ 2 ) have Z bound in alternate, contracted interstitial sites. The room-temperature structures for Z = Mn, Fe, Co, Ni, C, P were refined from single-crystal data (space group P 6 3 mc, Z = 2; for the protypical Fe, a = 15.4810(2) Å and c = 6.8768(3) Å). The unit cell lengths and volumes and d (Z−Z) values show notably smaller variations with Z than in La 5 Ge 3 Z or related systems. The buffering effect of empty cavities in the commensurate chains and the weak La−La bonding appear to be responsible. Magnetic susceptibility data and XPS core shifts are reported for the Fe and, in part, the Co and Ni phases La 15 Ge 9 Z. All three are metallic with decreasing moments of ∼1.83, ∼0.3, and ∼0 μ B, respectively. In contrast, the Fe-richer La 5 Ge 3 Fe is a very soft ferromagnet at room temperature. Members of the sequence Fe, La 5 Ge 3 Fe, La 15 Ge 9 Fe exhibit regularly decreasing moments as well. The Fe 2p 3/2 core levels show a 1.7 eV decrease over the same series, i.e., increasing reduction, while an evidently regular oxidation of La from the element through La 5 Ge 3 and La 15 Ge 9 Fe to La 5 Ge 3 Fe is indicated by a regular overall increase of 2.7 eV in La 3d 5/2 binding energies.