Magnetic Hyperfine Interactions In The Mixed-Valence Compound Fe-7(Po4)(6) From Mossbauer Experiments

Hyperfine parameters of Fe-7(PO4)(6) phosphate were examined using Fe-57 Mossbauer spectroscopy in a wide temperature range 4.2 <= T <= 300 K. Four different iron sites were successfully detected: two for the high-spin Fe2+ and two for Fe(3+ )ions. Mossbauer spectra below T-N1 approximate to 47 K were analyzed by diagonalization of the full nuclear-interaction Hamiltonian. The noticeable difference between the saturated hyperfine fields (B-hf) for the 5-fold-coordinated site (similar to 5.0 T) and the octahedral site (similar to 10.1 T) was related to the symmetry of the local crystal field. Using crystal field calculations of the energy levels based on the point symmetry of the (Fe2+O5) and (Fe2+O6) sites, we estimated the intra-atomic magnetic dipolar and electron orbital current contributions to the B-hf field. The observed line broadening below the second magnetic phase transition at T-N2 approximate to 16 K is approximated by the bimodal distribution of the hyperfine fields p(B-hf) that is characteristic of spin-modulated magnetic systems.