Anisotropic thermal expansion and electronic transitions in the Co$_3$BO$_5$ ludwigite

Abstract

The investigations of the crystal structure, magnetic and electronic properties of the Co$_3$BO$_5$ at high temperatures were carried out using powder x-ray diffraction, magnetic susceptibility, electrical resistivity, and thermopower measurements. The orthorhombic symmetry (Sp.gr. Pbam) was established at 300 K and no evidence of structural phase transitions was found up to 1000 K. The thermal expansion of the crystal lattice is strongly anisotropic. At $T<T_c=550$ K, a large thermal expansion along the c-axis is observed with simultaneous contraction along a-axis. The activation energy of the conductivity decreases significantly at high temperatures and follows the thermal expansion variation, that exhibits two electronic transitions at ~500 and ~700 K, in coincidence with the anomalies of the heat capacity. Electronic transport was found to be a dominant conduction mechanism in the entire temperature range. The temperature dependence of the effective magnetic moment reflects the evolution of the spin state of Co$^{3+}$ ions towards the spin crossover to a high spin state. The interrelation between the crystal structure and electronic properties is discussed.