SUPERCONDUCTING PROPERTIES IN LA2CUO4+DELTA WITH EXCESS OXYGEN

We have studied the thermal history dependence of the superconducting properties below and above room temperature (RT) in the ceramic La2CuO 4+delta with excess oxygen, which were prepared by annealing in high-p ressure oxygen gas or electrochemical oxidation. The diffusion of the excess oxygen is concluded not to occur below RT. In the sample with s mall delta, the low T-c (32 or 36 K) superconducting phase appears, of which T-c is changed by the annealing at about 200 K. This change is presumably due to some phase transition without the diffusion of the e xcess oxygen. On the other hand, when delta reaches some threshold, an other single super-conducting phase with high T-c (45 K), which is sta ble below 373 K at least, appears. This sample was prepared by the ele ctrochemical oxidation at 333 K for more than 48 h, where the excess o xygen can diffuse. In the sample electrochemically oxidized at RT, bot h the low- and high-T-c superconducting phases were formed only near t he surface area of the sample. These phases were homogenized by anneal ing at 373 K.