Th7Fe3-Type Related Structures in Pd(Pt)-Cu-B Systems: Pd6CuB3A New Structure Type for Borides

A new member of the series of Th7Fe3-type derivative structures, h-(Pd0.86Cu0.14)(7)B-3 (Pd6.02Cu0.98B3, unique structure type Pd6CuB3, space group P6(3)cm, a=12.9426(9)angstrom, c=4.8697(4)angstrom, single-crystal X-ray diffraction (XRD) data) was obtained from as cast alloys and alloys annealed at 600-650 degrees C. Further substitution of Cu by Pd led to formation of a Mn7C3-type structure, o-(Pd0.93Cu0.07)(7)B-3 (Pd6.51Cu0.49B3, space group Pnma, a=4.8971(2)angstrom, b=7.5353(3)angstrom, c=12.9743(6)angstrom, single-crystal XRD). Isotypic LT h-(Pt0.70Cu0.30)(7)B-3 (Pt4.90Cu2.10B3) was observed in the Pt-Cu-B system as a low-temperature (LT) phase (T600 degrees C) (powder XRD), whereas the Th7Fe3-type (high-temperature (HT) h-(Pt0.73Cu0.27)(7)B3Pt5.11Cu1.89B3, space group P6(3)mc, a=7.4671(1)angstrom, c=4.9039(1)angstrom, powder XRD) proved to be stable at high temperature. The three structures are built of columns of face connected metal octahedra and columns of metal tetrahedra alternatingly fused by common faces and vertices. Boron atoms are found in trigonal prisms formed by metal atoms. The volumes of the three new Th7Fe3-type derivative borides relate as 1:2:3. Superconductivity was discovered for Pt4.9Cu2.1B3 (Pd6CuB3-type) and Pt5.1Cu1.9B3 (Th7Fe3-type) below 0.67 and 0.66K, respectively. Despite the close value of the transition temperature the values of the upper critical field at 0K differ as 0.37T and 0.27T for the two compounds.