Anomalous Thermal Expansion in a CuAl2-type Superconductor CoZr2

We investigated the anomalous thermal expansion in the CuAl2-type superconductor CoZr(2 )and the alloyed compounds of TrZr2 (Tr: transition metal) over a wide temperature range. We performed neutron powder diffraction and X-ray powder diffraction on CoZr2 and observed remarkably anisotropic thermal expansion with a c-axis negative thermal expansion constant of alpha(c) < -15 mu K-1 over a wide temperature range of T = 50-573 K. With decreasing temperature, lattice constant a decreased, whereas lattice constant c continuously increased in CoZr2. The cause of the anisotropic shrinkage/expansion of the a-axis/c-axis during cooling was explained by the small change in the Co-Zr bond and the systematic decrease in the Zr-Co-Zr angle. Similar thermal expansion was observed in the alloyed systems, (Fe,Co,Ni)Zr-2 and (Fe,Co,Ni,Rh,Ir)Zr-2, which suggests that this phenomenon is a common feature in the TrZr2 system. We propose that zero-thermal expansion metals can be achieved by optimizing the contrasting thermal expansion of the a- and c-axes in TrZr2.