PHASE-EQUILIBRIA IN THE LN-MN-O SYSTEM (LN = PR, ND) AND GENERAL-ASPECTS OF THE STABILITY OF THE PEROVSKITE PHASE LNMEO3

Phase equilibria in the Pr-Mn-O and Nd-Mn-O systems have been studied between 1100 and 1500 K and in atmospheres with an oxygen partial pressure ranging between 10(-15) and 1 atm. LnMnO3 and LnMn2O5 have been found to form in both systems. PrMn2O5 was synthesized for the first time and has the orthorhombic DyMn2O5-type structure with cell parameters: a = 7.51, b = 8.63 and c = 5.69 angstrom. The thermodynamic stability of complex oxides was studied using TG and emf measurements in a galvanic cell with solid electrolytes. The equilibrium oxygen pressure, the standard Gibbs potential of the decomposition reactions 3LnMn2O5 = 3LnMnO3 + Mn3O4 + O2 and LnMnO3 = 1/2Ln2O3 + MnO + 1/4O2, and the standard Gibbs potential of the formation of the ternary oxides have been calculated. The variation of LnMeO3 stability for the rare earth manganates, and for the 3d-transition metal (Cr, Mn, Co, Ni, Cu) compounds for a given rare earth element is discussed based on geometrical crystallographic criteria and the outer 3d(n) electron configurations.