Boron induced structure modifications in Pd–Cu–B system: new Ti2Ni-type derivative borides Pd3Cu3B and Pd5Cu5B2

Abstract

The formation of two distinct derivative structures of Ti₂Ni-type, interstitial Pd₃Cu₃B and substitutive Pd₅Cu₅B₂, has been elucidated in Pd–Cu–B alloys from analysis of X-ray single crystal and powder diffraction data and supported by SEM. The metal atom arrangement in the new boride Pd₃Cu₃B (space group Fdm, W₃Fe₃C-type structure, a = 1.1136(3) nm) follows the pattern of atom distribution in the CdNi-type structure. Pd₅Cu₅B₂ (space group F3m, a = 1.05273(5) nm) exhibits a non-centrosymmetric substitutive derivative of the Ti₂Ni-type structure. The reduction of symmetry on passing from Ti₂Ni-type structure to Pd₅Cu₅B₂ corresponds to the loss of an inversion centre delivered by an ordered occupation of the Ni position (32e) by dissimilar atoms, Cu and B. In both structures, the boron atom centers Pd forming [BPd₆] octahedra in Pd₃Cu₃B and [BPd₆] trigonal prisms in Pd₅Cu₅B₂. Neither a perceptible homogeneity range nor mutual solid solubility was observed for two compounds at 600 °C, while in as cast conditions Pd₅Cu₅B₂ exhibits an extended homogeneity range formed by a partial substitution of Cu atoms (in 24f) by Pd (Pd_5+xCu_5−xB₂, 0 ≤ x ≤ 1). Electrical resistivity measurements performed on Pd₃Cu₃B as well as on Pd-poor and Pd-rich termini of Pd_5+xCu_5−xB₂ annealed at 600 °C and in as cast conditions respectively demonstrated the absence of any phase transitions for this compounds in the temperature region from 0.3 K to 300 K.