Temperature-dependent X-ray absorption study of the anisotropic negative thermal expansion of Co and Fe-based TrZr3 compounds

The local structure and unoccupied electronic states of binary TrZr3 (Tr = Co, Fe) superconductors have been investigated by x-ray absorption fine structure spectroscopy (XAFS). Extended x-ray absorption fine structure (EXAFS) measurements show that bonds aligned along the c-axis exhibit negative thermal expansion (NTE) while those along the a- or b-axes show positive expansion with increasing temperature. A specific Zr-Tr distance, appears to be a key factor in the NTE. The mean square relative displacement (MSRD) parameter reveals significant changes in the bond rigidity associated with NTE. X-ray absorption near-edge structure (XANES) analysis indicates superconducting properties primarily correlate with the density of states at the Fermi level. These results provide insights into atomic correlations governing anisotropic NTE in binary Zr-based systems, which could facilitate manipulation of their functional properties.