Pressure-Induced Volumetric Negative Thermal Expansion in CoZr₂ Superconductor

The study investigates the thermal expansion and superconducting properties of a CuAl2-type (tetragonal) superconductor CoZr2 under high pressures. High-pressure synchrotron X-ray diffraction is performed in a pressure range of 2.9 GPa < P < 10.4 GPa, and it is discovered that CoZr2 exhibits volumetric negative thermal expansion (NTE) under high pressures. Although uniaxial positive thermal expansion (PTE) along the a-axis is observed under ambient pressure, it is suppressed by pressure, whereas a large uniaxial NTE along the c-axis is maintained under the pressure regime. Because of the combination of the suppressed uniaxial PTE along the a-axis and uniaxial NTE along the c-axis, volumetric NTE is achieved under high pressure in CoZr2. The volumetric NTE mechanism is based on the flexible crystal structure caused by the soft Co-Co bond, as observed in the isostructural compound FeZr2, which exhibits a uniaxial NTE along the c-axis. High-pressure electrical resistance measurements of CoZr2 are performed and confirm superconductivity at 0.03 GPa < P < 41.9 GPa. Because of the coexistence of the two phenomena, volumetric NTE and superconductivity, in CoZr2 under high pressure, coexistence can be achieved under ambient pressure by tuning the chemical composition after the present observation.