Observation of lattice softening at $T_{\mathrm{c}}$ in the FeSe${}_{0.5}$Te${}_{0.5}$ superconductor

Polycrystalline samples of FeSe${}_{0.5}$Te${}_{0.5}$ were synthesized using a conventional solid-state reaction method. The onset of bulk superconductivity transition was confirmed by SQUID magnetometry at 12.5~K. ${}^{57}$Fe M"ossbauer 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 $\theta_{\rm D}\approx 460$K\mathrm{.}Seebeckmeasurementsindicatethatthesamplesare$n$-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.