Hall effect in electrochemically oxidized La2CuO4+δ and (La1−xSrx)2CuO4+δ

We have prepared the superconducting La 2 CuO 4+ δ electrochemically oxidized at 333 K. Each sample is a single-phase superconductor; the low- T c phase ( T c =32 or 36 K, oxidation time=24 h, δ =0.068) or the high- T c phase (45 K, 72 h, 0.093). We have studied the relation between the hole concentration n H , determined by the Hall effect measurement as n H ≡1/ eR H and the valence of Cu 2+ p , determined by the iodometric titration in La 2 CuO 4+ δ . In highly oxidized La 2 CuO 4+ δ , the p /2-dependence of n H has been found to be different from that of the ordinary (La 1− x Sr x ) 2 CuO 4 . Similar phenomena were observed in (La 1− x Sr x ) 2 CuO 4+ δ ( x =0.050, 0.075, 0.100) electrochemically oxidized at 333 K. These results suggest that in La 2 CuO 4+ δ or (La 1− x Sr x ) 2 CuO 4+ δ , the intercalated excess oxygen plays a different role in conduction from Sr in the ordinary (La 1− x Sr x ) 2 CuO 4 .