The Phase Transition of La2CuO4+ Synthesized in High Oxygen Pressure

In this paper, La2CuO4+ compound with superconducting phase content over 30%, has been obtained by using high oxygen pressure 2.3 PGa techniques, which is the highest oxygen pressure exerted so far on the La2CuO4. It is noteworthy that the excess oxygen content determined by iodimetric titration (=0.050.01) is not consistent with that determined by TGA (in N2, =0.143). The product obtained by high oxygen pressure is heated in air at different temperatures, then quenched to room temperature. The superconductor shows a first order phase transition at temperatures between 200~300 ℃, and after this transition, the sample transformed to antiferromagnetic semiconductor. TGA and DSC also confirm the presence of this transition. The variation in the oxygen content before and after the phase transition just matches the value obtained by titration. These excess oxygen, which is responsible for the superconductivity, leads to a transformation from Cu2+ to Cu(2+2)+, and this rules out the possibility that the superconductivity is caused by superoxide.