UC Irvine

Several specimens in the system Th-Pd-B-C with nominal compositions ThPd3B2C and ThPd3B3C (as melted and annealed), containing superconducting phases exhibiting Tc's of 14.5 K and 21 K, have been studied with EPMA, electron diffraction, EDX element analysis and HREM. A number of phases (approximate compositions are given) have been observed: (1) ThPd2B2C, I-centered tetragonal, a = 0.375, c = 1.07 nm, which can be related to the Tc of 14.5 K; (2) ThPd0.65B4.7, P-type cubic, a = 0.42 nm, with short-range ordered superstructure resulting in diffuse streaks in the electron-diffraction pattern; this phase is suggested to be the 21 K superconducting phase; (3) ThPd3 P-type hexagonal, a = 0.58, c = 0.98 nm; (4) ThPd8B3, I-centred orthorhombic a = 0.84, b = 0.90, c = 1.67 nm; (5) ThPd3B2C, with unknown structure; (6) ThB4, tetragonal a = 0.72, c = 0.41 nm; (7) graphitic C. Annealing of the specimen ThPd3B2C at 1050°C results in the disappearance of ThPd2B2C and the loss of the Tc at 14.5 K but a strong increase of the 21 K superconducting fraction with a corresponding strong increase of the phases ThPd0.65B4.7 and ThPd3. HREM indicates that ThPd0.65B4.7 adopts a modified CaB6 structure in which the cube face, 1 2, 1 2, 0, which is vacant in CaB6, is partially occupied by Pd with the removal of some B atoms. The superstructure indicates a short-range ordering (clustering) of the Pd atoms. © 1994.