PrFeMC Versus PrFeMC (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. PrFeSiC 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 FeC 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 PrFeSiC instead; this compound has the previously reported cubic LaFeSnC structure type. Identical Pr/Si clusters and FeC 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 PrFeMC. Magnetic susceptibility measurements on single crystals of PrFeSiC indicate antiferromagnetic ordering of the Pr moments below 17 K and no magnetic moment on iron atoms. The behavior of PrFeSiC is more complex, revealing magnetic contributions from both Pr and Fe atoms and possible spin frustration.