Evolution of the Mössbauer spectra of ludwigite Co$_{3-x}$Fe$_x$О$_2$BO$_3$ with substitution of iron for cobalt

A concentration series of single crystals of iron-cobalt ludwigites Co$_{3-x}$Fe$_x$О$_2$BO$_3$ ($x$ = 0.0125, 0.025, 0.050, 0.10, 1.0) has been synthesized. The structure has been studied using X-ray diffraction and Mössbauer effect. A preferred occupation of nonequivalent crystallographic positions by iron in the ludwigite structure has been revealed. It has been found that the valence of substituting iron ions is three. It has been revealed that the structure of the $\gamma$-resonance spectrum of Co$_2$FeO$_2$BO$_3$ is complicated due to a composition disorder in the system.