HIGH OXYGEN-PRESSURE SYNTHESIS OF LAMNO3-LESS-THAN-OR-EQUAL-TO-0.31)(DELTA WITH HIGH DELTA VALUES (DELTA)

Oxygen excess LaMnO3+delta samples with 0.11 less than or equal to del ta less than or equal to 0.31 have been prepared by thermal decomposit ion of metal citrates followed by annealings either in air or under hi gh oxygen pressure (up to 200 bar). Samples with 0.13 less than or equ al to delta less than or equal to 0.31 are single-phased rhombohedral perovskites, the distortion of which decreases as the oxygen content ( and the Mn4+ content) increases. LaMnO3.11 has orthorhombic symmetry. The observed contraction of the unit cell regularly increases with del ta in the samples obtained in air; moreover, there is an additional re duction in volume for the samples prepared under high oxygen pressure, which suggests the presence of interstitial oxygens in the structure. The thermal analysis under reducing conditions of LaMnO3+delta show t wo consecutive reduction processes. After the first one the orthorhomb ic LaMnO3.00 perovskite can be identified, the second one leading to m ixtures of MnO and La2O3.