Superconducting superoxygenated cuprates: Search for phase separation inLa1.84Sr0.16CuO4+δ

We have studied high-pressure (2.3 kbar), high-temperature (575 °C), oxygen-annealed ceramic ${\mathrm{La}}_{1.84}$ ${\mathrm{Sr}}_{0.16}$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ by high-resolution neutron powder diffraction to determine its low-temperature structural properties in comparison to similarly oxygenated ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$. Unlike ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$, the Sr-containing superconductor shows no low-temperature phase separation. Further, the interstitial oxygen appears to be located at (1/4,1/4,z) with z=0.163(5) and to have a value of δ consistent with ${\mathrm{La}}_{1.84}$ ${\mathrm{Sr}}_{0.16}$ ${\mathrm{CuO}}_{4.019}$. Such a site is significantly displaced away from the interstitial location, (1/4,1/4,z), with z=0.25, observed in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ and ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$. The environment of such an interstitial site is suggestive of a smaller, neutral oxygen rather than a charged species.