High superconducting transition temperatures of new rare earth ternary borides

Abstract

A new group of ternary borides has been found that show either ferromagnetism or superconductivity. Their general formula is MRh4B4 where M is a transition or rare-earth element. Their superconducting transition temperatures range from approximately 2.5 K for the Sm compound to approximately 12 K for the Lu compound. In the past, the superconducting transition temperatures of borides have always been rather low. The highest values, in the vicinity of 8 K, were reached with pseudobinary molybdenum borides which were metastable and had to be splat cooled (1).We have now discovered an entirely new group of superconducting borides which are true ternary compounds, as opposed to the previously known pseudobinary phases. These new compounds have the general formula MRh4B4 (where M is a three or four valent transition element), and their superconducting transition temperatures, by far the highest yet observed for any borides, are given in Table 1. Transition temperatures were a few tenths of a degree higher when the samples contained an excess of RhB and the values given are for compositions MRh4B4 + 2 [RhB]. This is only the second true ternary system of high-temperature superconductors found thus far. The first was that of the double molybdenum sulphides (2) originally reported by Chevrel et al. (3).In contrast to the double sulphides, those compounds in this new superconducting system which have high transition temperatures are the very ones formed with elements which by themselves are not superconducting above 0.1 K. However, some striking analogies to the double sulphides have also been observed. When M is a magnetic rare-earth element such as erbium or thulium the transition temperature is fairly high, even higher than that of the corresponding molybdenum sulphides (4) or selenides (5). And yet, in this new system there is now a sudden switch to magnetism upon replacing erbium by holmium, a phenomenon never known before (see Table 2).The two crystallographic systems are quite different from each other. Whereas the sulphides are rhombohedral, the new system is tetragonal (6). For the sulphides, values of the metallic radii were not at all critical, but now only values between 1.70 A and 1.82 A for M in MRh4B4 are able to form this new phase. Again, in contrast to the sulphides, the electron/atom ratio is of decisive importance here, as shown by comparison of the yttrium and lutetium compounds to the thorium compound. Whereas the rhombohedral crystal structure of the molybdenum sulphides may be closely related to a more symmetrical

Ternary transition metal phosphides: High-temperature superconductors

“…In fact, it is also one of the few true ternary systems of high-temperature superconductors. Two other examples are the double molybdenum sulfides (14) and the rare earth rhodium borides (15).…”

History and Introduction

“…compound of the so-called Chevrel phases RMo 6 Se 8 (R. =rare earth) m 1972 [25], followed by the rare-earth rhodium borides R.Rb.iB 4 [26]. The rare-earth ions carry local magnetic moments (with certain well-understood exceptions) due to incompletely filled orbitals of 4f electrons.…”

Superconductivity and magnetic ordering in the mixed ternary ErxY1−xRh1.1Sn3.6 system

“…Since the discovery of ternary compounds like the Chevrel phases RExMO6S 8 [1] and RExMo6Se 8 [2] and the RERhaB 4 series [3], which show for some of the rare-earth (RE) metals superconductivity as well as magnetic ordering, there is a growing interest in the study of the interaction between these phenomena. Mixed ternary systems, formed by alloying two members of a family of compounds, offer a possibility to investigate the competition between superconductivity and magnetism as a function of the concentration of magnetic moments.…”