Observation of lattice softening at Tc in the FeSe0.5Te0.5 superconductor

►57Fe Mössbauer data of FeSe0.5Te0.5 reveal a change in hyperfine parameters at Tc. ► Both the second-order Doppler shift and absorption drop at Tc. ► The drop indicates a lattice softening and a ∼60 K dip in Debye temperature.