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

Polycrystalline samples of FeSe0.5Te0.5 were synthesized using a conventional solid-state reaction method. The onset of bulk superconductivity transition was confirmed by SQUID magnetometry at 12.5 K. 57Fe Mössbauer spectra in transmission geometry were recorded at temperatures between 6.0 and 320 K. Both the isomer shift and the total absorption started to drop about T c , indicating a softening of the lattice. The drop is estimated to correspond to at least 60 K from the original Debye temperature θD≈460 K. Seebeck measurements indicate that the samples are n-type conductors at low temperatures with a cross-over to p-type conductivity around 135 K. The zero Seebeck coefficient is seen below 10.6 K.