Oxygen Stoichiometry in LaMn 1‐x Cu x O 3+y by Thermogravimetry

Thermogravimetry in reducing atmospheres was used to study the oxygen Stoichiometry of several Cu‐substituted lanthanum manganates. The oxygen content of the samples at various levels of reduction (to La 2 O 3 , MnO, and Cu) was calculated from the gravimetric data. Freeze‐dried samples prepared by firing at 1100°C in O 2 and cooling in O 2 had the following composition: LaMnO 3 . 13 , LaMn 0 . 7 Cuo. 3 O 2 . 96 , LaMn 0 . 5 Cu 0 . 5 O 2 . 93 , and LaMn 0.3 Cu 0.7 6 2.71 . Single‐phase perovskite structure was obtained in all but the last. The lanthanum manganate LaMnO 3.13 was reduced to LaMnO 3.00 , which shows considerable stability. A 50% substitution of Cu for Mn gave LaMn 0.5 Cu 0.5 O 2 . 93 , indicating that the ions involved were mostly Mn 4+ and Cu 2+ . This compound could be reduced to LaMng 0.5 Cu 0 . 5 O 2 . 26 (largely Mn 2+ and Cu 1+ ) without destroying the perovskite structure. This reduced phase could be reoxidized to the original oxygen level but subsequent reduction required considerably higher temperatures and was accompanied by the precipitation of Cu‐rich phases. Samples with a Cu:Mn ratio 30:70 and 70:30 were O‐deficient. The compound LaMn 0.7 Cuo. 3 O 2 . 96 reduced to LaMn 0 . 7 Cu 0 . 3 O 2 . 54 , a relatively stable composition. The highly Cu‐rich sample LaMn 0.3 Cu 0.7 O 2.71 was easily reduced.