A ⁵⁷Fe Mössbauer study on the FeSe and Fe(Se,Te) superconductors: Discontinuities in the hyperfine parameters at T_c

Polycrystalline samples of FeSe _0.82 and FeSe _0.5Te _0.5 were synthesized using a solid-state reaction route. Bulk superconductivity was confirmed using SQUID magnetometry. The onset of T _c was at 8.0 K for FeSe _0.82 and 12.5 K for FeSe _0.5Te _0.5. Paramagnetic ⁵⁷Fe Mössbauer spectra were recorded at temperatures between 5.4 and 320 K in transmission geometry. All spectra exhibited simple quadrupole splitting. For FeSe _0.5Te _0.5 a small drop in the quadrupole splitting was observed about T _c upon cooling. Additionally, for both samples the isomer shift and the total absorption started to drop around T _c, indicating a softening of the lattice. The drop is estimated to correspond to at least 60 K from the original Debye temperatures.