Fe-excess Ions as Electronic Charge Suppliers for Zero Thermal Expansion in the Normal State of Fe1.16Te0.6Se0.4

We report on the observation of a zero thermal expansion of the crystalline lattice of Fe1.16Te0.6Se0.4 in the normal state, using neutron and x-ray diffraction, ac magnetic susceptibility, magnetization and resistivity measurements. Superconductivity develops below 15 K. Magnetic hysteresis loops are revealed at all temperatures studied, with the loop opening at 5 K being noticeably larger than that at 300 K. An extremely large thermal expansion of the lattice is observed in the superconducting state. Thermal expansion coefficients of the lattice are quenched upon loss of superconductivity. Zero thermal expansion is retained over a very broad temperature range from 20 to 200 K. These behaviors are understood as being due to the electronic charge redistribution, in which the excess Fe ions on the interstitial sites act as electronic charge suppliers that strengthen the electronic connections between the Te/Se and Fe ions on the lattice sites once the temperature is raised.