Electron microscopy on YPd₅B₃C_x, x=0 or 0.35

Superconducting and non-superconducting specimens of YPd₅B₃C_x with x=0 and 0.35 have been studied with EPMA, electron diffraction, EDX element analysis and HREM. The superconducting specimen YPd₅B₃C_0.35 (superconducting fraction 10-20%) contains six different phases, of which four are crystalline with the approximate compositions: (1) YPd₂BC, I-centred tetragonal, a=0.375, c=1.07 nm, (2) YPd_1.2B_3.3, cubic, a=0.42 nm, with an incommensurate superstructure (q=0.285 (a^*+b^*+c^*)), (3) YPd₈B₃, I-centred orthorhombic, a=0.84, b=0.90, c=1.67 nm, and (4) YB _4-xC_x tetragonal, a=0.72, c=0.41 nm. In the specimen YPd₅B₃ (superconducting fraction 0.2%) only the phases YPd_1.2B_3.3 and YB_4-xC_x are observed in equal and higher amounts, respectively. The non-superconducting specimen YPd₅B₃C_0.35, obtained by annealing the superconducting specimen at 800°C, contains about five phases. In this specimen the phases YPd₂BC and YPd_1.2B_3.3 were distinctly decomposed. Because of its presence and absence in the various specimens and its plate-like morphology YPd₂BC is suggested to be the superconducting phase with a layer sequence {B_0.5-Pd₂-B_0.5-YC}_n. Considerations on the formation and stability of YPd₂BC are given.