Electron microscopy on YPd 5B 3C x, x=0 or 0.35

Abstract

Superconducting and non-superconducting specimens of YPd 5B 3C x with x=0 and 0.35 have been studied with EPMA, electron diffraction, EDX element analysis and HREM. The superconducting specimen YPd 5B 3C 0.35 (superconducting fraction 10–20%) contains six different phases, of which four are crystalline with the approximate compositions: (1) YPd 2BC, 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) YPd 8B 3, I-centred orthorhombic, a=0.84, b=0.90, c=1.67 nm, and (4) YB 4− x C x tetragonal, a=0.72, c=0.41 nm. In the specimen YPd 5B 3 (superconducting fraction 0.2%) only the phases YPd 1.2B 3.3 and YB 4- x C x are observed in equal and higher amounts, respectively. The non-superconducting specimen YPd 5B 3C 0.35, obtained by annealing the superconducting specimen at 800°C, contains about five phases. In this specimen the phases YPd 2BC and YPd 1.2B 3.3 were distinctly decomposed. Because of its presence and absence in the various specimens and its plate-like morphology YPd 2BC is suggested to be the superconducting phase with a layer sequence {B 0.5-Pd 2-B 0.5-YC} n. Considerations on the formation and stability of YPd 2BC are given.