Th Fe -Type Related Structures in Pd(Pt)-Cu-B Systems: Pd CuB -A New Structure Type for Borides.

A new member of the series of Th Fe -type derivative structures, h-(Pd Cu ) B (≡Pd Cu B , unique structure type Pd CuB , space group P6 cm, a=12.9426(9) Å, c=4.8697(4) Å, single-crystal X-ray diffraction (XRD) data) was obtained from as cast alloys and alloys annealed at 600-650 °C. Further substitution of Cu by Pd led to formation of a Mn C -type structure, o-(Pd Cu ) B (≡Pd Cu B , space group Pnma, a=4.8971(2) Å, b=7.5353(3) Å, c=12.9743(6) Å, single-crystal XRD). Isotypic LT h-(Pt Cu ) B (≡Pt Cu B ) was observed in the Pt-Cu-B system as a low-temperature (LT) phase (T≤600 °C) (powder XRD), whereas the Th Fe -type (high-temperature (HT) h-(Pt Cu ) B ≡Pt Cu B , space group P6 mc, a=7.4671(1) Å, c=4.9039(1) Å, 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 Th Fe -type derivative borides relate as 1:2:3. Superconductivity was discovered for Pt Cu B (Pd CuB -type) and Pt Cu B (Th Fe -type) below 0.67 and 0.66 K, respectively. Despite the close value of the transition temperature the values of the upper critical field at 0 K differ as 0.37 T and 0.27 T for the two compounds.