We report a study of single-crystal ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$, a 38-K superconductor produced by high-pressure oxygenation. We employ ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ NMR to probe the behavior of copper spins in the ${\mathrm{CuO}}_2$ planes. At low temperatures we observe two signals, one originating from regions of the crystal rich in oxygen and a second having no excess oxygen (δ∼0). Upon warming through 265±5 K, the volume fraction of the crystal poor in oxygen goes to zero. The magnetic shift of the peak intensity of the line originating in the oxygen-rich portion of the crystal does not change in the vicinity of 265 K. These observations provide direct microscopic evidence for phase separation in the crystal.

• Received 29 May 1990

DOI:https://doi.org/10.1103/PhysRevB.42.6781