Structure and composition analysis of the phases in the system Th-Pd-B-C containing superconductors with Tc = 14.5 K and Tc = 21 K

Abstract

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-centred tetragonal, a = 0.375, c= 1.07 nm, which can be related to the Tc of 14.5 K; (2) ThPdo.65B4.7, P-type cubic, a = 0.42 nm, with a 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) ThPdsB3, 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 T¢ at 14.5 K but a strong increase of the 21 K superconducting fraction with a corresponding strong increase of the phases ThPdo.65B4. 7 and ThPd3. HREM indicates that ThPdo.65B4.7 adopts a modified CaB6 structure in which the cube face, ½, l 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.

Some Chemical and Physical Properties of Quaternary Borocarbides

“…TEM (Transmission Electron Microscope) and electron microprobe are more or less insensitive to light elements like B and C. From high resolution electron microscopy (HREM), it was proposed (43-45) that a centered tetragonal phase with a&37-3.8 A > and c&10.6-10.8 A s is the right superconducting phase, that phase could be some non stoichiometric ''YPd B \V C.'' However, we point out that B and C have a very small effect on image contrast and that Y and Pd have similar scattering factors and consequently are not really distinguishable. A ternary cubic phase, possibly YPd B also exists and disappears when ¹ &23 K disappears (46). According to (47) this ''YPd B C'' is metastable and cannot be stabilized by Ni substitution.…”

“…Unfortunately, the authors do not discuss eventual changes of the a parameter during this process, but they mention that this cubic phase does not exist in superconducting quenched YPd B C and as-cast or quenched YPd B C . Let us note that in Th-Pd-B-C also, two superconducting phases have been reported (¹ &14-15 K and ¹ &19-20 K) (18,19) and that both quaternary tetragonal (19,46,50) and ternary cubic phases (46) have been found from HREM and are supposed to be responsible for superconductivity. The cubic phase (primitive, a"4.2 A > ) could be ThPd B (probably CaB -type), and it may be responsible for ¹ &21 K. In order to improve the knowledge of the phase diagram and look for other compositions, close to those of previous quaternaries that could be responsible for superconductivity, we prepared and studied Table 1) only in Y Pd B K C with m57 in a sample containing a large number of phases.…”

Rare-earth metal transition metal borocarbide and nitridoborate superconductors

“…By now it appears certain that in all multiphase samples a fraction of YPd 2 [B 2 C] is responsible for superconductivity at T c % 23 K [8][9][10][11]. The isotypic compound ThPd 2 [B 2 C] displays a critical temperature of 14.5 K [100,155]. It is interesting to note that the unit cell c-axis decreases on irradiation in an electron microscope to a value expected for a ThCr 2 Si 2 type structure.…”

“…Due to the high neutron absorption of Sm and Gd the magnetic structures of SmNi 2 [B 2 C] and GdNi 2 [B 2 C] have not been studied by neutron diffraction, but some information steam from X-ray scattering experiments [182,269,281] and 155 Gd Mößbauer spectroscopy [185]. For the Sm compound a magnetic propagation vector of k ¼ ( with the moments aligned perpendicular to the c-axis, below about 14 K a spin-reorientation leads to a spiral-like magnetic structure with moments rotating in (100) [182,185,282].…”

Evidence for secondary-phase superconductivity in YNi4BC0.2

“…7 (To=21 K) [4] and (4) YNiaBCo.2 with Tc= 12 K [5]. The latter system is especially controversial regarding its superconducting properties.…”