TETRAGONAL AND CUBIC-CRYSTAL STRUCTURES OF SOME BINARY AND TERNARY METAL DICARBIDES IN THE SERIES CE-ER, CE-LU, U-LA, AND U-CE

The binary dicarbides LaC2 and ErC2 (tetragonal, space group I4/mmm, Z = 2) and the ternary dicarbides Ce0.50Er0.50C2, Ce0.75Lu0.25C2, Ce0.50Lu0.50C2, Ce0.25Lu0.75C2, U0.75La0.25C2, U0.50La0.50C2, U0.25La0.75C2, U0.75Ce0.25C2, U0.50Ce0.50C2, and U0.25Ce0.75C2 (cubic, space group Fm3m, Z = 4) have been prepared by arc melting, and their metal compositions confirmed by point energy dispersion X-ray analysis by scanning electron microscopy. Their neutron powder diffraction patterns have been recorded at room temperature and, for the three Ce-Lu compounds, also at 4.2 K (at which temperature the cubic structure is retained). For LaC2 and ErC2, mild hydrolysis indicates the presence of acetylide units, and Rietveld profile analysis of the neutron data has enabled the unit-cell dimensions (a, c = 3.937(1), 6.579(2) Å and 3.622(1), 6.106(1) Å, respectively) and the C-C bond lengths (1.284(6) and 1.275(3)Å, respectively) to be refined. For the ternary compounds (some of which contain two face-centered cubic (f.c.c.) phases), the distinctive martensitic microstructure is absent from micrographs, and the limited neutron integrated intensity data appear to be consistent with f.c.c. structures having either static or rotating C-C groups. © 1991 Academic Press, Inc.