Superconducting Properties and Electronic Structure of CuAl₂-Type Transition-Metal Zirconide Fe_1-xNi_xZr₂

Transition-metal zirconides with a tetragonal CuAl₂-type crystal structure (<i>I</i>4/<i>mcm</i>, No. 140) are known as a superconductor family with a variable superconducting transition temperature (<i>T</i>_c) depending on the transition-metal (<i>Tr</i>) element. In this presentation, we report a new superconductor Fe_1-<i>x</i>Ni<i>_x</i>Zr₂ exhibiting a <i>T</i>_c dome. Polycrystalline samples of Fe_1-<i>x</i>Ni<i>_x</i>Zr₂ were synthesized by arc melting. From the synchrotron X-ray diffraction (SXRD), we confirmed that these compounds have a tetragonal CuAl₂-type crystal structure (<i>I</i>4/<i>mcm</i>, No. 140). Energy-dispersive X-ray spectrometry measurements confirmed that the actual Ni concentration was close to the nominal value, and X-ray photoelectron spectroscopy measurements showed that the trend of the estimated Ni concentration was also consistent with the nominal value. From magnetic susceptibility measurements, we observed bulk superconductivity for 0.4 ≤ <i>x</i> ≤ 0.8, and the highest <i>T</i>_c of 2.8 K was observed for <i>x</i> = 0.6. In contrast, the diamagnetic signals for <i>x</i> &gt; 0.8 are clearly small as a bulk superconductor, which indicates that the observed diamagnetic signals are caused by filamentary (trace) superconductivity states in those samples. No superconductivity was observed above 1.8 K for 0 ≤ <i>x</i> ≤ 0.3. Therefore, the <i>x</i> dependence of <i>T</i>_c shows a dome-shaped trend, which is similar to the case of unconventional superconductors where magnetic fluctuations are essential for superconductivity pairing. For <i>x</i> ≤ 0.3 or <i>x</i> ≥ 0.9, the bulk nature of superconductivity is suppressed. To explore a possible cause of the suppression of superconductivity, we estimated the <i>c</i>/<i>a</i> ratio of Fe_1-<i>x</i>Ni<i>_x</i>Zr₂ using the SXRD data. In a sister compound Co_1-<i>x</i>Ni<i>_x</i>Zr₂, a collapsed tetragonal transition was observed in a Ni-rich region, and the bulk nature of superconductivity is suppressed in the collapsed-tetragonal region. In the case of current system, although <i>c</i>/<i>a</i> linearly decreases with increasing <i>x</i>, the slope clearly changes at around <i>x</i> = 0.1–0.3 and <i>x</i> = 0.7–0.9. We consider that the change in the <i>c</i>/<i>a</i> ratio in the Ni-rich region is a kind of transition to collapsed tetragonal phases as revealed in Co_1-<i>x</i>Ni<i>_x</i>Zr₂. On the absence of bulk superconductivity in the Fe-rich region, we have no explanation at present, but we assume that the disappearance of bulk superconductivity would be related to strong spin fluctuations and/or the transition to collapsed tetragonal phase. To clarify that, further investigation on the physical and structural properties is needed.