Correlation between Magnetic Properties and Mn/Co Atomic Order in LaMn_0.5Co_0.5O_3+δ: I. Second-Order Nature in Mn/Co Atomic Ordering and Valence State

The relaxation phenomenon around 1300 K in LaMn 0.5 Co 0.5 O 3+ δ and their valence states of Mn and Co atoms were investigated by TG-DTA, powder X-ray diffraction, magnetic susceptibility, and XANES of two samples cooled at 10 Kh - 1 in the range 1673−973 K and quenched from 1673 K. DTA clarified that the relaxation phenomenon is not due to a first-order phase transition and that the temperature dependence of relaxation time is a thermal activation type, which suggests that the order−disorder phase transition of Mn/Co atomic ordering is second-order type. The results of lattice parameters, XANES, and magnetic susceptibility measurements suggest that the ground valence state of the ordered phase is Mn 4+ /Co 2+ and that a fraction of Mn 3+ /Co 3+ is increased either by disordering the Mn/Co atomic configuration or by increasing temperature.