Superconductivity in the Yttrium Sesquicarbide System

Abstract

At the present time there are only three known crystal structures in which superconductivity with transition temperatures above 15° K have been observed. One of these structures is the A-15 structures which contains the Nb₃Ge system, the Nb₃Ga system, the NB₃Al and V₃Si systems.¹ The second structure, NaCl rock salt, occurs in the NB-N-C system having a T_c up to 18°K.² The third structure is the Pu₂C₃ type in which, primarily, nonstoichiomatic alloys of Y-Th-C have been observed with a T_c of 17°K.³ In the A-15 and the NaCl structure systems the high T_c compositions are regarded as normal high T_c systems in that they have an e/a ratio of 4.5 – 4.8 in accord with Matthias rules.⁴ The Pu₂C₃ systems having a high T_c, however, appear anomalous in that a simple application of the e/a rule would give an e/a ratio of 3.8 for the high T_c material.⁵ The high T_c material of the Pu₂C₃ structure type has only been synthesized under high pressures (15–25 Kilobars) and high temperatures (1200–1450°C). An apparently open question is whether these high T_c Pu₂C₃ structure phases are really so anomalous. A second question is whether different synthesis techniques could further raise the T_c of this system, possibly by producing phases in a composition range more in conformity with the e/a ratio rules of Matthias.

Supported in part by NSF DMR 78-03217