Sign change in c-axis thermal expansion constant and lattice collapse by Ni substitution in transition-metal zirconide superconductor Co(1−x)Ni(x)Zr(2)

Recently, c-axis negative thermal expansion (NTE) was observed in a CoZr(2) superconductor and related transition-metal zirconides. Here, we investigated the structural, electronic, and superconducting properties of Co(1−x)Ni(x)Zr(2) to achieve systematic control of c-axis NTE and switching from NTE to positive thermal expansion (PTE) by Ni substitution. At x ≤ 0.3, c-axis NTE was observed, and the thermal expansion constant α(c) approached zero with increasing x. At x = 0.4–0.6, c-axis thermal expansion close to zero thermal expansion (ZTE) was observed, and PTE appeared for x ≥ 0.7. On the superconducting properties, we observed bulk superconductivity for x ≤ 0.6, and bulk nature of superconductivity is suppressed by Ni heavy doping (x ≥ 0.7). For x ≤ 0.6, the evolution of the electronic density of states well explains the change in the superconducting transition temperature (T(c)), which suggests conventional phonon-mediated superconductivity in the system. By analyzing the c/a ratio, we observed a possible collapsed transition in the tetragonal lattice at around x = 0.6–0.8. The lattice collapse would be the cause of the suppression of superconductivity in Ni-rich Co(1−x)Ni(x)Zr(2) and the switching from NTE to PTE.