Search for Fe magnetic ordering in the 40 K superconductor (Li0.8 Fe0.2)OHFeSe

Abstract We report the results of x-ray diffraction and 57 Fe Mossbauer spectroscopy study, complemented by ab-initio electronic structure and the hyperfine-interaction parameters calculations, of the 40 K superconductor (Li0.8 Fe0.2)OHFeSe. The superconductor crystallizes in the tetragonal space group P4/nmm with the lattice constants a = 3.7865(2) A and c = 9.2802(6) A. Evidence is provided for the presence of an anisotropic mixture of strong covalent and weak ionic chemical bonding and of metallic characteristics. The Mossbauer spectra consist of two quadrupole-doublet patterns corresponding to Fe2+ at the 2a and 2b sites. Contrary to the claims in the literature, we demonstrate unequivocally that there is no magnetic ordering of the 2a-site Fe magnetic moments down to 2.0 K. The calculated hyperfine-interaction parameters at the two Fe sites show general agreement with the experimental ones. The Debye temperatures for the 2a and 2b sites are found to be 186(21) and 397(4) K, respectively.