CRYSTAL STRUCTURES OF TERNARY RARE EARTH-3d TRANSITION METAL COMPOUNDS OF THE RT₆Al₆ TYPE.

The structural behavior and phase relationships of materials of composition RT//6Al//6 (R EQUVLNT rare earth; T EQUVLNT Cr, Mn, Fe, Cu) were investigated. Detailed X-ray analysis yields that for the heavy rare earths the compounds crystallize in the body-centered tetragonal ThMn//1//2 structure. The space group is I4/mmm and the unit cell contains two formula units. The rare earth occupies the 2(a) sites, the transition metal and aluminum atoms the 8(f) and 8(i) sites respectively, while the remaining transition metal and aluminum atoms are distributed at random over the 8(j) sites. The RCu//6Al//6 (R EQUVLNT La-Eu) compounds crystallize in the f. c. c. NaZn//1//3 structure, and the RMn//6Al//6 (R equivalent La-Sm) compounds crystallize in the hexagonal Th//2Zn//1//7 structure. The various effects of the atomic sizes on these structures are discussed.