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Ternary BaAl4-type derivative stluctures are frequently found with rare-earth-transition-metal silicides and germanides with the rare-earth atoms occupying the Ba sites and the two other kinds of atoms distributed in ordered fashion on the A1 sites. The possible ternary BaA14-type derivative structures with the same unit cell as BaA14 are investigated. If it is assumed that transition-metal-transition-metal contacts are to be excluded only seven hypothetical BaAl4-type derivative structures are possible. Three of these are already known as ThCr2Si2, CaBe2Ge 2 and BaNiSn 3 types. Segments of these three structures and of other BaAl4-type derivative structures are found in various intergrowth structures. The BaA14-type derivative structures can be classified according to the type of coordination polyhedron around the transition metal, which can be a tetrahedron or a square pyramid. The tendency of the transition metal to have one or the other coordination polyhedron determines which BaAl4-type derivative structure is formed and explains also the occurrence of the HfFe2Si 2 and U2C03Si 5 types -BaAla-type derivative structures with larger unit cells.

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