THE LANTHANUM-GERMANIUM SYSTEM - 19 ISOSTRUCTURAL INTERSTITIAL COMPOUNDS OF THE LA5GE3 HOST

Sintering reactions in sealed Ta containers afford single-phase interstitial derivatives La5Ge3Z of the line compound La5Ge3 (Mn5Si3 type) for Z = B(x), C(x), N, O, P, S, Cl, As, Se, Sb, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, and Cd. Guinier X-ray techniques were not sufficiently sensitive to discern the exact stoichiometries of the boron and carbon examples, but x probably falls in the ranges 0.7 +/- 0.1 and 0.7-1.0, respectively. The lattice dimensions and relative inertness previously reported for some La5Ge3 samples appear to be those of La5Ge3O, including for the germaniothermal synthesis from La2O3 and Ge. Single crystal X-my studies were carried out for La5Ge3 and La5Ge3Cr (P6(3)/mcm, Z = 2, a = 8.941 (1), 9.004 (1) angstrom, c = 6.878 (1), 7.105 (1) angstrom, R/R(w) = 3.4/3.8, 2.4/2.9%, respectively). The short La-Cr distance in the latter, 2.801 (1) angstrom, is distinctive. Cell volumes increase on Z insertion into La5Ge3 for all but N and O. The changes within the Zn-Se series of Z are markedly greater than in electron-richer zirconium host analogues. La5Ge3 is Pauli-paramagnetic and metallic, while LasGe3P is a diamagnetic semiconductor (Zintl phase) appropriate to the presence of just three excess (conduction) electrons in the La5Ge3 host. Extended-Huckel band calculation results for La5Ge3 and La5Ge3P are consistent with these properties. Valence and conduction band overlap that is present in La5Ge3 through strong La-Ge bonding and large band dispersions is removed on oxidation with P. Calculations for the more electropositive Fe interstitial place the unsplit Fe d levels near E(F), with Fe 4s providing much of the binding. La3Ge has a Ti3P-type structure.