PREPARATION AND CRYSTAL-STRUCTURE OF SUPERCONDUCTING Y2FEC4 AND ISOTYPIC LANTHANOID IRON CARBIDES

The new compounds R//2FeC//4 (R equals Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) were prepared by arc melting of the elemental components. They are formed by a peritectic reaction of the carbides RC//2 with iron. Their crystal structure was determined from X-ray powder diffraction data for Tm//2FeC//4 and from neutron diffraction data for Er//3FeC//4. It is orthorhombic, space group Ibam with Z equals 4 formula units per cell. The lattice constants for Er//2FeC//4 are a equals 750. 6(1) pm, b equals 942. 6(2) pm, c equals 500. 6(1) pm, V equals 0. 3542 nm**3. The residual for a Rietveld peak shape refinement of the Er//2FeC//4 neutron diffraction data is R equals 0. 024 (295 data and 14 variable parameters). The structure is of a new type with carbon pairs (bond distance 133 pm) in an environment of five rare earth atoms and two adjacent iron atoms.