Unprecedented High Solubility of Oxygen Interstitial Defects in La1.2Sr0.8MnO4+δ up to δ ∼ 0.42 Revealed by In Situ High Temperature Neutron Powder Diffraction in Flowing O2

The structural behavior of the n = 1 Ruddlesden-Popper (R-P) La1.2Sr0.8MnO4+delta phases has been monitored in situ in flowing O-2 on heating over the temperature range 65 < T/degrees C < 550 by means of neutron powder diffraction (instrument D20, ILL/Grenoble). Sequential Rietveld refinement showed that the I4/mmm nearly stoichiometric phase undergoes a first oxygen uptake in the temperature range 300-400 degrees C with phase separation into an oxygen-rich orthorhombic (Bmab) La1.2Sr0.8MnO4.30(1) phase and a stoichiometric tetragonal (I4/mmm) La1.2Sr0.8MnO4.00(2) phase. At T similar to 410 degrees C, the first oxidation step is complete and only the Bmab oxidized phase is present. The orthorhombicity decreases progressively on further heating, up to 510 degrees C with minor variation in the oxygen content. Above similar to 510 degrees C, the system undergoes a second abrupt oxidation step involving a single phase process to reach after prolonged isothermal heating at 550 degrees C, delta similar to 0.42(2). Such a high solubility for the excess-oxygen defects has never been reported so far for a K2NiF4-type structure. The La1.2Sr0.8MnO4+delta system is thus interesting for the fundamental studies of structural distortions induced by the intercalation of a large amount of oxygen defects owing to the flexibility of the R-P structure and high stability of the Mn4+ oxidation state in an oxygen octahedral environment.