Thermogravimetric analysis has revealed that reduction of ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{F}}_{2+\mathrm{\delta }}$ in ${10\%\mathrm{}\mathrm{H}}_2{/90\%\mathrm{}\mathrm{N}}_2,$ occurs in two distinct steps on heating up to 930 °C. Whereas reduction to give ${\mathrm{SrF}}_2,$ SrO, and Cu is complete above 800 °C, a stable intermediate forms in the region between 250 and 450 °C. This has been identified as a new tetragonal $\mathrm{}(I4/mmm)\mathrm{}$ phase, ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{F}}_2,$ with unit-cell dimensions $a=3.967\mathrm{}\left(1\right)\mathrm{}\AA$ and $c=12.816\mathrm{}\left(2\right)\mathrm{}\AA .$ The structure has been determined from powder x-ray-diffraction data and is related to ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ ($T^{\prime }$-type). Madelung energy, bond valence sum calculations, and F $1s$-XPS data clearly indicate that the ${\mathrm{F}}^{\mathrm{-}}$ ions occupy sites within the fluorite block $\left({\mathrm{Sr}}_2{\mathrm{F}}_2\right)$ insulating layers. This contrasts with the ${\mathrm{La}}_2{\mathrm{CuO}}_4$ ($T$-type) structure which is adopted by ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{F}}_{2+\mathrm{\delta }}.$

• Received 24 February 1997

DOI:https://doi.org/10.1103/PhysRevB.56.2831