Terahertz-infrared dielectric properties of lead-aluminum double-cation substituted single-crystalline barium hexaferrite

Highlights: • Terahertz-infrared electrodynamic response for Ba_0.2Pb_0.8Al_1-xFe_12-xO₁₉ crystals (x=0.0, 1.2, 3.0, 3.3) is studied in detail. • A set of THz excitations are observed and assigned to the transitions from A1-term of the fine structured ground state of Fe²⁺. • For all Al³⁺ concentrations, Al³⁺ ions mainly occupy octahedral site positions. • Concentration dependence of the Fe-O bond length is established. • The lowest A2u mode not soften indicating suppression of possible displacive phase transition in pure and lightly Pb-doped BaM. -- Abstract: Hexaferrite materials are highly demanded to develop and manufacture electronic devices operating at radio- and microwave frequencies. In the light of the prospects for their use in the forthcoming terahertz electronics, here, we present our results on the terahertz and infrared dielectric response of a typical representative of hexaferrites family, lead-substituted M-type barium hexaferrite doped with aluminum, Ba_0.2Pb_0.8Al_xFe_12-xO₁₉, x(Al) = 0.0, 3.0, and 3.3. We studied uniquely large and high-quality single crystals of the compounds prepared by spontaneous crystallization growth technique. Our aim was to explore the effect of aluminum substitution on the dielectric response of the compounds. Systematic and detailed investigations of the dependences of terahertz-infrared (frequencies 8–8000 cm⁻¹) spectra of complex dielectric permittivity on the temperature, 4–300 K, and on the chemical composition, x(Al) = 0.0, 1.2, 3.0, and 3.3, were performed for two principal polarizations of the electric field E-vector of the probing radiation relative to the crystallographic c-axis, namely E c and E⊥c. Furthermore, infrared phonon resonances are recorded and discussed. In contrast to undoped BaFe₁₂O₁₉, no softening of the lowest frequency A_2u phonon is observed, indicating suppression of a displacive phase transition in substituted compounds. A number of resonance absorption bands are discovered at terahertz frequencies and assigned to transitions between energy levels of the fine-structured ground state of Fe²⁺ (⁵E) ions. The temperature and aluminum-doping dependences of the resonances are analyzed with an account taken of disorder introduced by aluminum. Basing on dielectric data and detailed X-ray experiments, we find that for all concentrations of Al³⁺ ions, x(Al) = 0.0, 1.2, 3.0, and 3.3, they mainly occupy the 2a and 12k octahedral site positions and that the degree of substitution of iron in tetrahedral positions is not substantial. Along with fundamental findings, the obtained data on broad-band dielectric properties of Ba_0.2Pb_0.8Al_xFe_12-xO₁₉ crystals provides the information that can be used for development and manufacture of electronic devices with operating frequencies lying in the terahertz spectral band.