Figure 2 : Temperature dependence of resistivity ρ and its derivative dρ/dT for the SmFeAsO1 - xFx samples with nO and 18O.

From: A large iron isotope effect in SmFeAsO1 - xFx and Ba1 - xKxFe2As2

Figure 2

The peak in the derivative is considered to be the transition temperature. The peak temperature in dρ/dT corresponds to mid-transition temperature for the superconducting transition. An anomalous peak at about 147 K in resistivity is associated with the structural transition for the x = 0 sample, and the peak temperature in dρ/dT is very close to the SDW ordering temperature observed by neutron scattering23.