Syntheses and structure of the La5Ge3Z phases (Z=Si, Sn, Pb, Ga, In): Structural relationships among the M5X4-type structures

The indicated reactions under arc-melting or high-temperature sintering conditions in Ta containers lead to (1) the apparent Zintl phase La5Ge3Si0.75, stuffed Mn5Si3-type, image from quenching; (2) α-La5Ge3Si (Sm5Ge4-type, image); (3) β-La5Ge3Si (Zr5Si4-type, image) at high temperatures; (4) La5Ge3Tt, Tt=Sn, Pb, phases isotypic with 2); (5) the isotypic La5Ge3Tr, Tr=Ga, In, (Gd5Si4-type, image). The structures of compounds 3 and 5 (for Tr=Ga) have been refined from single crystal X-ray diffraction data. A general description of the three electron-poorer M5X4 structure types 2,3,4 (and of Eu5As4-type (image)) is given in terms of their common building block, an La9Tt6 cubeoctahedra centered about the tightest bound La. Some electronic bonding effects are also generalized with regard to the dominance of extra free electrons beyond simple Zintl expectations.