LA(15)GE(9)Z - 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 bee n obtained from reactions of either the elements or suitable binary co mpounds in sealed Ta containers at 1200-1350 degrees C. All exhibit an ordered root 3a x root 3a x c superstructure of the Mn5Si3 type (La(5 )Ge(3)Z, La(5)Ge(3)Z, P6(3)/mcm) in which two-thirds of the slightly d istorted confacial chains of trigonal antiprisms (La6/2Ge6/2) have Z b ound in alternate, contracted interstitial sites. The room-temperature structures for Z = Mn, Fe, Co, Ni, C, P were refined from single-crys tal data (space group P6(3)mc, Z = 2; for the protypical Fe, a = 15.48 10(2) Angstrom and c = 6.8768(3) Angstrom). 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 cavit ies in the commensurate chains and the weak La-La bonding appear to be responsible. Magnetic susceptibility data and XPS core shifts are rep orted for the Fe and, in part, the Co and Ni phases La(15)Ge(9)Z. All three are metallic with decreasing moments of similar to 1.83, similar to 0.3, and similar to 0 mu(3), respectively. In contrast, the Fe-ric her La5Ge3Fe is a very soft ferromagnet at room temperature. Members o f the sequence Fe, La5Ge3Fe, La15Ge9Fe exhibit regularly decreasing mo ments 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 regula r oxidation of La from the element through La5Ge3 and La15Ge9Fe to La( 5)e(3)Fe is indicated by a regular overall increase of 2.7 eV in La 3d (5/2) binding energies.