Search for Superconductivity in Thorium- and Uranium-Based Boro-Carbides

Abstract

We have observed superconductivity in thorium-based quaternary borocarbides ThM,B,Cz (M = Ni, Pd). Our studies reveal that the system Th-Pd-B-C has at least two superconducting phases with T, -20 K and 14 K. In Th-Ni-B-C system superconductivity is observed below T, -6 K. Superconductivity was not observed, down to 4.2 K in ThMzBpC alloys with M = CO, Os and Ir and in several U-based alloys. Introduction. -Nagarajan et al. [l] reported superconductivity in Y-Ni-B-C at -12 K. This led to a new class of superconductors, namely quaternary borocarbide superconductors and, naturally, generated a great deal of excitement. Later, superconductivity was observed with T, > 20 K in the multiphase Y-Pd-B-C [2,3] system which is obtained by replacing Ni by Pd. Siegrist et al. [4] determined the structure of the superconducting phase LuNi2B2C which is a dilled-in)) version of the well-known ThCr2Si2 structure. Here we report superconductivity in thorium-based borocarbide Th-M-B-C system (M = Ni, Pd).While this work was in progress, we noted from the high-T, update, June lst, 1994 that Sarrao et al. [5] have submitted a paper to Physica C in which they report superconductivity in Th-M-B-C system with M = Ni, Pd, Pt.

Quaternary borocarbides: Relatively high T intermetallic superconductors and magnetic superconductors

“…Our sample of nominal composition ThPd 2 B 2 C was not single phase and showed two transitions: one at T c $ 20 K (resistance not going to zero) and the other at T c $ 14 K with resistance going to zero [68]. The other work in this system also showed two transitions (higher T c $ 21.5 K), but in a sample of nominal composition ThPd 3 B 3 C [69].…”

Some Chemical and Physical Properties of Quaternary Borocarbides

“…The composition and structure of the SC phase(s) is still a topic of controversy that we will briefly discuss, giving new results. SC has also been found in Sc-Ni (17) and Th-Ni-and Th-Pd-based (18,19), as well as in some Pt-based (20)(21)(22)(23)(24) quaternary borocarbides. New borocarbides RNi \V Cu V BC (R"Y, Lu) having ¹ up to 8.9 K (25), claim (not confirmed by other groups) for SC up to 16 K in RNi B C (26), and new boronitrides R Ni B N (27,28) with ¹ up to 12 K, in Ni-based materials, give a pretty unique character to this family of quaternary materials.…”

“…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.…”