Evidence for Phase Separating in a Superoxygenated La sub 2 CuO sub 4+delta Crystal

We present magnetic susceptibility, resistivity, and thermoelectric power measurements performed on a 3Kbar oxygen annealed La sub 2 CuO sub 4+delta single crystal. All three quantities show distinct anomalies between 200 and 280 K that result from a phase separation involving oxygen diffusion. Hysteresis in these anomalies indicates that the phase separation is weakly first order. We suggest that the transition may be partially driven by interactions between excess oxygen ions and copper spin magnetism.