${\mathrm{NiZr}}_2$ is a member of the transition metal (Tr) zirconide ${Tr\mathrm{Zr}}_2$ superconductor family having a ${\mathrm{CuAl}}_2$-type tetragonal structure. In this study, we report on two different pressure-induced properties of ${\mathrm{NiZr}}_2$. The first one is pressure-enhanced superconductivity; the transition temperature $\left(T_{\mathrm{c}}\right)$ reaches 4.8 K at $P=64.9$ GPa, while the $T_{\mathrm{c}}$ of ${\mathrm{NiZr}}_2$ at ambient pressure is lower than 2 K. From the calculated density of states (DOS) at the Fermi energy, we suggest that the enhancement of $T_{\mathrm{c}}$ cannot be explained by the change in DOS. The Debye temperature estimated from the resistance analysis increases with pressure, which suggests the possible correlation with the enhancement of superconductivity, while the $T_{\mathrm{c}}$ under pressure calculated based on a conventional model suggests that the conventional pairing is not sufficient to explain the pressure-enhanced $T_{\mathrm{c}}$. The second property is pressure-induced uniaxial negative thermal expansion (NTE). We performed synchrotron x-ray diffraction (XRD) under pressure at variable temperatures. From the temperature-dependent XRD patterns and the Rietveld refinement results, we confirmed the emergence of NTE along the $c$-axis in ${\mathrm{NiZr}}_2$. Since the NTE is one of the new functionalities in ${Tr\mathrm{Zr}}_2$, the pressure-induced NTE will be one of the important parts for developing new NTE materials and understanding the mechanism of NTE in the family and related materials.

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• Density of states
• High-pressure studies
• Superconductivity
• Thermal expansion