The superconductivity and magnetic susceptibility of some zirconium-transition-metal compounds; evidence for an anticorrelation

Abstract

The properties of the superconducting transition temperatureT cof the CuAl 2 -type compounds of Zr 2 Co, Zr 2 Ni, Zr 2 Rh, Zr 2 Fe, and Zr 2 Ir are discussed with respect to effects due to alloying and heat treatment. Dilute pseudobinary alloys of transition-metal elements with Zr 2 Rh (T c=11.3 K) produced lower transition temperatures suggesting that the valence-electron concentration of 5.67 electrons/atom gives a maximum inT c for this crystal type. Results for Zr 2 Co (T c=5.0 K) and Zr 2 Ir (T c=7.3 K) show that their transition temperatures are raised somewhat when the electron concentration is increased by alloying. A peak is seen near 5.72 electrons/atom. A peak in the room-temperature magnetic susceptibility in the Zr 2 Co-Zr 2 Ni system occurs near the same electron concentration as the peak inT c. Similar susceptibility behavior takes place in the Zr 2 Rh-Zr 2 Ni system, however, with no peak inT c. Results of susceptibility measurements on Zr 2 Ir and its isomorphic alloys indicate correlation betweenT c and susceptibility. An anticorrelation occurs for Zr 2 Co alloys suggesting the presence of Coulomb interactions. A sharp symmetric drop inT c near the stoichiometric composition as well as a marked decrease of the transition temperature with a low-temperature (600 C) anneal indicate that the coulomb interactions are sensitive to crystalline order. The lattice parameters of Zr 2 Ir are reported as a=6.508 A andc=5.721 A. The superconductivity of Zr 3 Co (T c=3.9 K) and Zr 3 Ir (T c=2.13 K) is reported.