Magnetic susceptibility and EPR study of Bi$_5$Nb$_{3-3x}$Co$_{3x}$O$_{15-\delta}$

Magnetic susceptibility, microstructure and EPR of cobalt-containing solid solutions with layered perovskite-like structure Bi$_5$Nb$_{3-3x}$Co$_{3x}$O$_{15-\delta}$ have been studied. Solid solutions of Bi$_5$Nb$_{3-3x}$Co$_{3x}$O$_{15-\delta}$ ($x \leqslant 0.005$) can be crystallized in tetragonal syngony (sp. gr. P4/mmm), as cobalt content increases, monoclinic distortion of the unit cell emerges at $0.005<x\leqslant 0.04$ (sp. gr. P2/m). The formation of exchange-bound aggregates of Сo(III) and Co(II) atoms predominantly with antiferromagnetic exchange types has been found in the solid solutions. EPR indirectly confirms that cobalt ions are in octahedral positions of substitution of Nb(V) ions.