A Mössbauer study of the superconductingNdFeAsO0.82F0.18 oxypnictide compound

Abstract

Using Mössbauer spectroscopy, we have studied the oxypnictide superconductorREFeAsO0.82F0.18 with RE = Nd for the first time. The Mössbauer spectra at 295 K predominantlyconsist of a singlet (94% of the spectral area) with an isomer shiftδ = 0.43(1) mm s−1 with respect toα-iron at 295 K, and aminor doublet with δ = 0.29(1) mm s−1 and ΔEQ = 1.756(3) mm s−1, which isattributed to an FeAs2 impurity. The spectrum remains fundamentally unchanged down to4.2 K. The isomer shift of the singlet with zero quadrupole splittingindicates that the iron in the Nd oxypnictide is probably in a low spinFe2+ (S = 0) state.The temperature variation of the isomer shift can be reproduced assuming a Debye-type phonon spectrumwith ΘD = 430 K.