In the Co3O2BO3 Ludwigite, there are four crystallographic sites for the metal ion Co. Three sites are occupied by Co2+ in a high-spin state, and one
site by Co3+ in low-spin state, resulting in a compound where the number of Co2+ ions is twice that of Co3+ ions. Given the particular distribution of magnetic and non-magnetic ions, the compound can be visualized as consisting of magnetic planes containing Co2+ ions, separated by non-magnetic planesformed by Co3+ ions. In this work, were studied the structural changes, especially magnetic ones, caused by the gradual substitution of low-spin Co3+ (S=0) by Cr3+ ions (S = 3/2). We synthesized the Co3−xCrxO2BO3 series, and X-ray experiments showed a gradual increase in unit cell volume with increasing Cr concentration. Magnetization measurements show that this type of substitution drastically increases the magnetic transition temperature, from 42 K for the un-doped compound to 115 K for the compound with the highest Cr concentration. The latter is the highest long-range magnetic ordering temperature recorded so far for cobalt Ludwigites. The results from specific heat and magnetization measurements show that this substitution changes the ground-state magnetic structure of the compound, transitioning from a ferrimagnetic state (at low Cr concentrations) to an antiferromagnetic state (at high Cr concentrations), in agreement with the results of exchange integral calculations.