The Superconductivity and Magnetic Susceptibility of Some Zirconium-Transition-Metal Compounds; Evidence for an Anticorrelation

The properties of the superconducting transition temperature T-c of the CuAl2-type compounds of Zr2Co, Zr2Ni, Zr2Rh, Zr2Fe, and Zr2Ir are discussed with respect to effects due to alloying and heat treatment. Dilute pseudobinary alloys of transition-metal elements with Zr2Rh (T-c = 11.3 K) produced lower transition temperatures suggesting that the valence-electron concentration of 5.67 electrons/atom gives a maximum in T-c for this crystal type. Results for Zr2Co (T-c = 5.0 K) and Zr2Ir (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 Zr2Co-Zr2Ni system occurs near the same electron concentration as the peak in T-c. Similar susceptibility behavior takes place in the Zr2Rh-Zr2Ni system, however, with no peak in T-c. Results of susceptibility measurements on Zr2Ir and its isomorphic alloys indicate correlation between T-c and susceptibility. An anticorrelation occurs for Zr2Co alloys suggesting the presence of Coulomb interactions. A sharp symmetric drop in T-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 Zr2Ir are reported as a = 6.508 angstrom and c = 5.721 angstrom. The superconductivity of Zr3Co (T-c = 3.9 K) and Zr3Ir (T-c = 2.13 K) is reported.