Disorder- and correlation-induced charge carriers localization in oxyborate MgFeBO4, Mg0.5Co0.5FeBO4, CoFeBO4 single crystals

• The electrical resistance of the single crystalline warwickites has been measured.
• The temperature-induced changes in the charge transfer mechanisms have been found.
• The microscopic parameters of the electronic structure have been determined.
• The studied warwickites have been classified as disordered correlated systems.
• The Co substitution was found to lead to the weakening of the localization.

The temperature dependence of the resistivity of single crystalline Mg1−xCoxFeBO4 samples with x = 0.0, 0.5, 1.0 is investigated for the temperature range (210–400 K). The conduction was found to be governed by Mott variable-range hopping (VRH) in the low-temperature range (T = 210–270 K) and by thermo-activation mechanism in the high-temperature range (T = 280–400 K). Microscopic electronic parameters, such as the density of the localized states near the Fermi level, localization length, the hopping length, and the activation energy have been obtained. The change of the activation energy observed at high-temperature range was attributed to local structure distortions around Fe and Co atoms. The complicated behavior of charge transfer mechanisms is discussed based on two approaches: atomic disorder and electron correlations.