The 57Fe Mössbauer spectra of oriented single‐crystal and powder PbFe12O19 (abstract)

Abstract

The recent determination of the crystal structure of PbFe12O19 by means of single‐crystal x‐ray diffraction has made it possible to carry out a definitive and comparative analysis of the structure‐property relationship of PbFe12O19, SrFe12O19, and BaFe12O19. We have therefore performed 57Fe Mössbauer measurements on oriented, single‐crystal, and powder samples of PbFe12O19 at 298 K. For a single‐crystal oriented parallel to the c axis, five distinct patterns are observed. The hyperfine fields, isomer shifts, and quadrupole interactions are as follows: 12k: Heff=413 kOe; δ=0.33 mm s−1; Δ=0.35 mm s−1; 4f1: 487 kOe; 0.24 mm s−1; 0.13 mm s−1; 4f2: 514 kOe; 0.39 mm s−1; 0.29 mm s−1; 2a: 496 kOe; 0.30 mm s−1; 0.11 mm s−1; 2b: 399 kOe; 0.30 mm s−1; 2.00 mm s−1. The relative magnitude of each of these parameters is in good agreement with and exhibits the same trend observed for high‐purity SrFe12O19. The relative intensity of the 2b pattern for the crystal oriented parallel to the c axis is 8.2%, which approaches its theoretical value; of the total number of iron ions in the structure, 8.33% of them occupy the 2b site. The value for the polycrystalline sample is only 5.4% of the total intensity. However, for a single crystal oriented perpendicular to the c axis, the 2b pattern exhibits a negligible intensity within the experimental error. This result confirms the dynamic disorder in the position of the 2b iron ions and suggests a stronger anisotropy for the case of PbFe12O19 than for SrFe12O19 and BaFe12O19. Even though the trend in hyperfine parameters at 298 K is similar to Sr, Ba, and Pb hexaferrites, PbFe12O19 exhibits hyperfine fields that are consistently lower than those of the Ba and Sr analogues. This result is discussed in terms of the Curie temperatures, magnetic exchange interactions, and structural parameters.