The effect of Bi$^{3+}$ ion substitution on the structural-phase state and features of the magnetic structure of the BaFe$_{12-x}$Bi$_x$O$_{19}$ solid solution

The results of studies of the features of the phase composition, crystal and magnetic structure of Bi-substituted barium hexaferrite BaFe$_{12-x}$Bi$_x$O$_{19}$ (0.1 $\le x\le$ 1.2) by methods of Mossbauer spectroscopy, X-ray phase analysis, as well as image analysis by scanning electron microscopy are presented. Samples of Bi-substituted hexaferrites – BaFe$_{12-x}$Bi$_x$O$_{19}$ (where $x$ = 0.1; 0.3; 0.6; 0.9 and 1.2) were synthesized by the method of solid-phase reactions with double annealing (at $T$ = 1100$^\circ$C for 6 h) and intermediate grinding (for 0.5 h). The X-ray phase analysis allowed us to establish the limit of substitution of Fe$^{3+}$ ions by Bi$^{3+}$ ions. It has been shown that at a low level of substitution ($x\le$ 0.3) there are no impurity phases detected and the samples are characterized by a single-phase state with the spatial group $P6_3/mmc$. When the degree of substitution increases ($x>$ 0.3) the formation of impurity phases is noted, which can be explained by the difficulties of ion diffusion in the process of solid-phase synthesis, as well as the formation of defects in the structure of magentoplumbite due to the large ionic radius of Bi$^{3+}$. As impurity phases in the studied compositions ($x>$ 0.3) Marked: BiFeO$_3$ (Pr. Gr. $Pnma$); BiO$_2$ (Pr. Gr. $Fm-3m$); BaBi$_2$O$_6$ (Pr. Gr. $R-3$) and BaO$_{0,5}$Bi$_{1,5}$O$_{2,16}$ (Pr. Gr. $Im-3m$). The content of the main phase (Pr. Gr. $P6_3mmc$) at the same time decreases from 95.11 to 88.27 vol.% when increasing x from 0.6 to 1.2, respectively. The analysis performed by the method of Mossbauer spectroscopy showed that all Fe ions have a charge of 3+. And all parameters lie within the values characteristic of Fe$^{3+}$ ions corresponding to the coordination of polyhedra: 12k. 4f2, 2a – octahedra, 4f1 – tetrahedron, and 2b – bipyramide. It is possible to single out a small monotonous decrease only for the 12k position. The analysis of SEM images showed an increase in the average particle size up to 10 mkm, depending on the concentration of bismuth oxide during the synthesis of hexaferrite.