Pr62Fe21M16C32 Versus Pr21Fe8M7'C12 (M = Si, P; M' = Si, Ge, Sn): Competing Intermetallic Carbides Grown from a Pr/Ni Flux.

Reactions of silicon, carbon, and iron in a low-melting flux mixture of praseodymium and nickel produced two competing intermetallic compounds. Pr62Fe21Si16C32 has a new structure type in tetragonal space group P4/ mmm ( a = 15.584(2) Å, c = 11.330(1) Å, Z = 1) that features trigonal planar FeC3 units that share corners to form a framework of cylindrical channels encompassing a network of silicon-centered praseodymium clusters. Slight variation of reactant ratio and heating profile produced Pr21Fe8Si7C12 instead; this compound has the previously reported cubic La21Fe8Sn7C12 structure type. Identical Pr/Si clusters and FeC3 subunit motifs are found in both structure types. In addition to reactant ratio and heating profile, size effects play a role in determining which structure forms. Replacing silicon with smaller phosphorus atoms produces only the tetragonal structure; replacement with larger elements (M = Ge, Sn) yields only cubic Pr21Fe8M7C12. Magnetic susceptibility measurements on single crystals of Pr62Fe21Si16C32 indicate antiferromagnetic ordering of the Pr moments below 17 K and no magnetic moment on iron atoms. The behavior of Pr21Fe8Si7C12 is more complex, revealing magnetic contributions from both Pr and Fe atoms and possible spin frustration.