A Mössbauer study of the superconducting NdFeAsO0.82F0.18 oxypnictide compound

Using Mossbauer spectroscopy, we have studied the oxypnictide superconductor REFeAsO0.82F0.18 with RE = Nd for the first time. The Mossbauer spectra at 295 K predominantly consist of a singlet (94% of the spectral area) with an isomer shift δ = 0.43(1) mm s−1 with respect to α-iron at 295 K, and a minor doublet with δ = 0.29(1) mm s−1 and ΔEQ = 1.756(3) mm s−1, which is attributed to an FeAs2 impurity. The spectrum remains fundamentally unchanged down to 4.2 K. The isomer shift of the singlet with zero quadrupole splitting indicates that the iron in the Nd oxypnictide is probably in a low spin Fe2+ (S = 0) state. The temperature variation of the isomer shift can be reproduced assuming a Debye-type phonon spectrum with ΘD = 430 K.