[PCP2] Physics and Chemistry2

[PCP2-03] Superconducting properties and anomalous thermal expansion in a CuAl₂-type transition-metal zirconide Co_1-xFe_xZr₂

*Ryunosuke Shimada¹, Asato Seshita¹, Yuto Watanabe¹, Akira Miura², Yoshikazu Mizuguchi¹ (1. Tokyo Metropolitan Univ. (Japan), 2. Hokkaido Univ. (Japan))

Keywords:

Superconductivity,Anomalous thermal expansion,Transition-metal zirconide,Synchrotron XRD

Thermal expansion is a fundamental structural property of materials. In normal positive thermal expansion (PTE), lattice parameters and/or volume increase with temperature, whereas in negative thermal expansion (NTE), they decrease. Zero thermal expansion (ZTE) can be realized by combining PTE and NTE materials, and such composites are used in structural components and devices requiring ultrahigh positional stability. ZTE in a single-phase material is rare but offers advantages for practical applications. Achieving ZTE in a superconductor is particularly attractive, as it could enable superconducting devices, such as Josephson junctions, to maintain mechanical stability under repeated temperature cycling. Recently, we reported anomalous anisotropic thermal expansion in CuAl₂-type (tetragonal) transition-metal zirconide superconductors TrZr₂ (Tr: transition metal). For example, in CoZr₂ and FeZr₂, the c-axis exhibits NTE over a wide temperature range, whereas the a-axis shows PTE. We further revealed that the c/a ratio is a key factor controlling the c-axis expansion behavior. In this study, we focus on the solid-solution system Co_1-xFe_xZr₂ ( x = 0–1), in which CoZr₂ and FeZr₂ have T_c values of ~6 K and 0.17 K, respectively, and similar c/a ratios. Polycrystalline samples of Co_1-xFe_xZr₂ were synthesized by arc melting. Their crystal structures and phase purities were examined by synchrotron X-ray diffraction (SXRD) at BL13XU, SPring-8, confirming the tetragonal CuAl₂-type structure (I4/mcm) for all compositions. Temperature-dependent SXRD revealed PTE along the a-axis and NTE along the c-axis in all samples. With increasing Fe concentration x, the a-axis thermal expansion coefficient (α_a) increases, whereas the c-axis coefficient (α_c) decreases. A correlation was observed between c/a and α_c. From magnetic susceptibility measurements, bulk superconductivity was observed for x = 0–0.5. The highest T_c of 5.9 K was observed at x = 0, and T_c was found to be suppressed as the Fe concentration x increases. Specific heat measurements further confirmed the bulk nature of superconductivity. In this presentation, we will discuss how Fe substitution systematically modifies both the superconducting properties and the anomalous thermal expansion in Co_1-xFe_xZr₂.