Terahertz-infrared electrodynamics of single-crystalline Ba0.2Pb0.8Al1.2Fe10.8O19 M-type hexaferrite

Spectral response of single-crystalline Ba 0.2 Pb 0.8 Al 1.2 Fe 10.8 O 19 synthesized by a modified Czochralski method is investigated using terahertz-infrared spectroscopy. Reflectivity, transmissivity, and complex dielectric permittivity spectra of the compound are studied in the temperature range from 6 to 300 K and in the frequency interval 8-8000 cm −1 for two principle polarizations of the radiation electric field relative to the crystallographic c -axis, namely E || c and E ⊥ c . The resonance absorption lines observed above 80 cm −1 are assigned to polar lattice vibrations basing on a factor group analysis and a comparison with a dielectric response of isostructural compounds. A set of absorption bands is observed in a range of 8–80 cm −1 . To clarify their nature, a model is developed that considers electronic transitions within the fine-structured ground state of four-fold coordinated Fe 2+ . It is shown that the trigonal distortions of the crystal field lead to lowering of the symmetry of 4f 1 and 4e tetrahedral site-positions of Fe 2+ and, as a result, to further splitting of the ground state spin-orbital sub-levels. Electro-dipole transitions between the corresponding sub-levels are associated with the absorption lines observed in a low-energy response (at 8-80 cm −1 ) of the Ba 0.2 Pb 0.8 Al 1.2 Fe 10.8 O 19 compound. The study paves the way for the development of low cost materials with high dielectric permittivity (about 30) at terahertz frequencies that are promising for the manufacture of electronic devices with enhanced characteristics. • Terahertz-infrared electrodynamics of Ba 0.2 Pb 0.8 Al 1.2 Fe 10.8 O 19 crystal is studied in detail. • Electronic transitions within fine-structured Fe 2+ states are observed. • Crystal field trigonal distortions lead to symmetry lowering of Fe site-positions. • A model to interpret crystal field distortions is proposed. • The model fully describes observed absorption resonances between Fe 2+ levels.