Abstract
Several specimens in the system Th-Pd-B-C with nominal compositions ThPd 3B 2C and ThPd 3B 3C (as melted and annealed), containing superconducting phases exhibiting T c'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) ThPd 2B 2C, I-centered tetragonal, a = 0.375, c = 1.07 nm, which can be related to the T c of 14.5 K; (2) ThPd 0.65B 4.7, 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) ThPd 3 P-type hexagonal, a = 0.58, c = 0.98 nm; (4) ThPd 8B 3, I-centred orthorhombic a = 0.84, b = 0.90, c = 1.67 nm; (5) ThPd 3B 2C, with unknown structure; (6) ThB 4, tetragonal a = 0.72, c = 0.41 nm; (7) graphitic C. Annealing of the specimen ThPd 3B 2C at 1050°C results in the disappearance of ThPd 2B 2C and the loss of the T c at 14.5 K but a strong increase of the 21 K superconducting fraction with a corresponding strong increase of the phases ThPd 0.65B 4.7 and ThPd 3. HREM indicates that ThPd 0.65B 4.7 adopts a modified CaB 6 structure in which the cube face, 1 2 , 1 2 , 0, which is vacant in CaB 6, is partially occupied by Pd with the removal of some B atoms. The superstructure indicates a short-range ordering (clustering) of the Pd atoms.
Highlights
Quite a number ofintermetallic materials containing boron and carbon have been reported to be superconducting [ 1-3 ]
For the electron-diffraction and HREM experiments, thin specimens were obtained by crushing under ethanol and mounting the crystal fragments on a Cu grid covered with a carbon coated holey film
The C-rich phase is probably pure C, since with electron diffraction no other structures than those of ThPd3, ThPdo.65B4.7, and graphitic C are observed in specimen 1331a
Summary
Quite a number ofintermetallic materials containing boron and carbon have been reported to be superconducting [ 1-3 ]. H. W..Zandbergen et al / Physica C 232 (1994) 328-336 electron microscopy (HREM) of several superconducting ( Tc= 23 K) and non-superconducting specimens in the Y-Pd-B-C system. W..Zandbergen et al / Physica C 232 (1994) 328-336 electron microscopy (HREM) of several superconducting ( Tc= 23 K) and non-superconducting specimens in the Y-Pd-B-C system They concluded that the superconducting phase is YPd2B2C having a structure quite similar to that of LuNi2B2C. Sarrac et al [ 3 ] have reported the presence of two superconducting phases in the system Th-Pd-B-C, one with composition YPd2B2C having a Tc o f 14.5 K and the LuNi2B2C structure [ 3,8 ] and one with a Tc of 21 K whose composition and structure are not known. These investigations confirm that the 14.5 K phase is ThPd2BEC adopting the LuNi2BEC structure, and indicate that the 21 K phase is ThPdo.65B4.7
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