Pair distribution function analysis of CoZr₂-structure alloys

The intermetallic compounds AZr₂ are a diverse array of materials with interesting properties. CoZr₂, for example, has a negative c-axis thermal expansion, even though the c-axis thermal expansion of the isostructural NiZr₂ is positive. Furthermore, CoZr₂ has an anomalous decrease in its unit cell volume on warming above ~400 K. The AZr₂ materials are also platforms for high-entropy alloys such as Co_0.2Ni_0.1Cu_0.1Rh_0.3Ir_0.3Zr₂, which can provide an opportunity to study superconductivity in the high-disorder regime. Characterization of the local structure would be helpful in understanding the properties of the AZr₂ compounds. To that end, we have performed pair distribution function (PDF) analysis on neutron diffraction data taken on NOMAD at the Spallation Neutron Source. We measured four samples, including CoZr₂ and variants with Co partially substituted by Cu, Cu/Rh, and Ni/Cu/Rh/Ir. Across all samples, our data show a consistent discrepancy in the local structure relative to the average structure of AZr₂. We discuss possible causes for this discrepancy, as well as the role that the additional elements have in modifying the thermal expansion behavior.