Sign change in c-axis thermal expansion constant and lattice collapse by Ni substitution in transition-metal zirconide superconductor Co1−xNixZr2

Abstract

Recently, c-axis negative thermal expansion (NTE) was observed in a CoZr2 superconductor and related transition-metal zirconides. Here, we investigated the structural, electronic, and superconducting properties of Co1−xNixZr2 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 αc 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 (Tc), 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 Co1−xNixZr2 and the switching from NTE to PTE.

Pressure‐Induced Volumetric Negative Thermal Expansion in CoZr2 Superconductor

“…NiZr 2 exhibits PTE both along the a-and c-axes, and thus, the coefficient of volumetric thermal expansion 𝛽 is positive (Figure 1a). [19,20] In the tetragonal crystal structure, 𝛽 can be calculated according to the following equation:…”

“…[16][17][18] In the TrZr 2 system, we revealed that the uniaxial NTE along the c-axis can be controlled by the lattice constant ratio c/a, through chemical element substitution. [19,20] The sign of the linear coefficient of thermal expansion along the c-axis changes from positive to negative upon the substitution of Ni with Co at ambient pressure. [20] The anomalous bonding states related to the uniaxial NTE along the c-axis in TrZr 2 have also been observed using X-ray absorption spectroscopy.…”

“…[19,20] The sign of the linear coefficient of thermal expansion along the c-axis changes from positive to negative upon the substitution of Ni with Co at ambient pressure. [20] The anomalous bonding states related to the uniaxial NTE along the c-axis in TrZr 2 have also been observed using X-ray absorption spectroscopy. [21] Moreover, Xu et al revealed that FeZr 2 exhibits a giant uniaxial NTE along the c-axis and proposed that the soft Fe-Fe bond and flexible structure caused by optical phonons play an important role in the origin of the uniaxial NTE along the caxis.…”