Crystal growth, structural characteristics and electronic structure of Ba1-xPbxFe12O19 (x = 0.23–0.80) hexaferrites

Abstract

The formation of (Ba,Pb)Fe12O19 crystals in the ternary system BaO–PbO–Fe2O3 and electronic structure of the crystals at high lead content are considered in the present study. The phase equilibria in the BaO–PbO–Fe2O3 system and related thermophysical parameters were defined. The analysis suggests that the use of lead oxide as a solvent reduces the minimum temperature for the hexaferrite crystal growth in the PbO–BaFe12O19 section to about 960 °C and, at the same time, gives an appropriate temperature range of about Δ500 °C (from 960 to 1465 °C). High-quality single crystals Ba1–xPbxFe12O19 with x = 0.23(2), 0.44(2) and 0.80(3) were grown by the flux technique and the structural properties were studied. The electronic structure of Ba0·2Pb0·8Fe12O19 hexaferrite was evaluated in parallel by X-ray photoelectron spectroscopy and density functional theory calculations to assess the effects of Pb2+ incorporation into the BaFe12O19 lattice. It is obtained by X-ray photoelectron spectroscopy measurements that the electronic parameters of Ba2+, Fe3+ and O2− ions are nearly the same in Ba0·2Pb0·8Fe12O19 and BaFe12O19 hexaferrites, and it indicates that the ionicities of Ba–O and Fe–O bonds are not affected by the presence of Pb2+ ions in the crystal lattice. On the base of density functional theory calculations, it is found that both the top of valence bands (from −10 eV to 0 eV) and bottom of the conduction bands (3 eV–8 eV) are dominantly occupied by Pb 6s 6p, Fe 3d and O 2p orbitals, indicating that the physicochemical properties of Ba0·2Pb0·8Fe12O19 related to the electron transitions across the forbidden bands are determined by Pb2+ cations, FeO4 and FeO6 groups. In particular, the incorporated Pb2+ cations have a more prominent contribution to the electronic states near the Fermi level than Ba2+ ions.