The lanthanum-germanium system. Nineteen isostructural interstitial compounds of the La[sub 5]Ge[sub 3] host

Sintering reactions in sealed Ta containers afford single-phase interstitial derivatives La[sub 5]Ge[sub 3]Z of the line compound La[sub 5]Ge[sub 3] (Mn[sub 5]Si[sub 3] type) for Z = B[sub x], C[sub x], N, O, P, S, Cl, As, Se, Sb, Cr, Mn, Fe, Co, Ni, C, 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 La[sub 5]Ge[sub 3] samples appear to be those of La[sub 5]Ge[sub 3]O, including for the germaniothermal synthesis from La[sub 2]O[sub 3] and Ge. Single crystal X-ray studies were carried out for La[sub 5]Ge[sub 3] and La[sub 5]Ge[sub 3]Cr (P6[sub 3]/mcm, Z = 2, a = 8.941, 9.004 [angstrom], c = 6.878, 7.105 [angstrom], R/R[sub w] = 3.4/3.8, 2.4/2.9%, respectively). The short La-Cr distance in the latter, 2.801 [angstrom], is distinctive. Cell volumes increase on Z insertion into La[sub 5]Ge[sub 3] 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. La[sub 5]Ge[sub 3] is Pauli-paramagnetic and metallic, while La[sub 5]Ge[sub 3]P is a diamagnetic semiconductor (Zintl phase) appropriate to the presence of just three excess (conduction) electrons in the La[sub 5]Ge[sub 3] host. Extended-Hueckel band calculation results for La[sub 5]Ge[sub 3] and La[sub 5]Ge[sub 3]P are consistent with these properties. Valence and conduction band overlap that is present in La[sub 5]Ge[sub 3] 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[sub F], with Fe 4s providing much of the binding. La[sub 3]Ge has a Ti[sub 3]P-type structure.