The crystal structure, cation distribution and exchange interactions in the Co2.25Fe0.75O2BO3ludwigite have been explored through X-ray diffraction, electrical resistivity, ac-susceptibility and Mossbauer effect measurements. The crystal data have shown that iron atoms occupy the most symmetric crystallographic sites Fe4 and Fe2. The complex magnetic behavior with two magnetic transitions near 70 and 115 K at low temperatures was found. The Mossbauer data have displayed the trivalent iron states only. The values of superexchange energies have been estimated for Co3O2BO3 and Co2.25Fe0.75O2BO3 yielding a significant role of frustrations in the ludwigite magnetic system. Variable range Mott hopping conductivity law was proved to be valid in the wide temperature region, pointing out a localized character of charge carriers rather than collective.

This study was supported by the Russian Foundation for Basic Research (Project no.09-02-00171-a), the Federal Agency for Science and Innovation(Rosnauka)(Project no.MK-5632. 2010.2), Physical Division of the Russian Academy of Science,the program ‘‘Strongly Correlated Electrons’’,project 2.3.1. The financial support of Spanish MINCYT, MAT08/1077 and Aragonese E-34 project are also acknowledged.

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