Abstract
The new title compounds were prepared by reaction of the elemental components at high temperatures. They crystallize with a rhombohedral cell in space group R 3m. The structure was determined for Pr/sub 2/Mn/sub 17/C/sub 3-x/ (x = 1.23), which has the lattice constants a = 8.8714 (7) A and c = 12.783 (2) A with Z = 3 formula units in the hexagonal setting of the cell. The structure refinement from single-crystal X-ray data resulted in a residual of R = 0.023 for 25 variable parameters and 414 unique structure factors. The Pr and Mn positions correspond to those of the Th and Zn positions in Th/sub 2/Zn/sub 17/. The carbon atoms fill voids of nearly octahedral shape formed by a rectangle of Mn atoms (Mn-C distances 1.94 and 1.95 A) and two Pr atoms at 2.57 A. The idealized composition with all octahedral voids filled is Pr/sub 2/Mn/sub 17/C/sub 3/. The refinement of the occupancy parameter for this position resulted in a value of 59 +/- 3% for the crystal used for the structure determination. The discussion reviews the correspondence of binary intermetallic and ternary complex carbide phases. 28 references, 2 figures, 5 tables.
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