Anisotropic thermal expansion and electronic transitions in the Co 3 BO 5 ludwigite.

The investigations of the crystal structure, magnetic and electronic properties of 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 observed at 300 K and no evidence of structural phase transitions was found up to 1000 K. The compound shows a strong anisotropy of the thermal expansion. A large negative thermal expansion along the a -axis is observed over a wide temperature range ( T = 300-600 K) with α a = -35 M K -1 at T = 500 K with simultaneous expansion along the b - and c -axes with α b = 70 M K -1 and α c = 110 M K -1 , respectively. The mechanisms of thermal expansion are explored by structural analysis. The activation energy of the conductivity decreases significantly above 700 K. Electronic transport was found to be a dominant conduction mechanism in the entire temperature range. The correlations between the thermal expansion, electrical resistivity, and effective magnetic moment were revealed and attributed to the evolution of the spin state of Co 3+ ions towards the spin crossover and gradual charge-ordering transition.