Synthesis, structure, and XPS characterization of the stoichiometric phaseSr2CuO2F2

Thermogravimetric analysis has revealed that reduction of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}$ in ${10%\mathrm{}\mathrm{H}}_{2}{/90%\mathrm{}\mathrm{N}}_{2},$ occurs in two distinct steps on heating up to 930 \ifmmode^\circ\else\textdegree\fi{}C. Whereas reduction to give ${\mathrm{SrF}}_{2},$ SrO, and Cu is complete above 800 \ifmmode^\circ\else\textdegree\fi{}C, a stable intermediate forms in the region between 250 and 450 \ifmmode^\circ\else\textdegree\fi{}C. This has been identified as a new tetragonal $(I4/mmm)$ phase, ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2},$ with unit-cell dimensions $a=3.967(1)\AA{}$ and $c=12.816(2)\AA{}.$ The structure has been determined from powder x-ray-diffraction data and is related to ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ (${T}^{\ensuremath{'}}$-type). Madelung energy, bond valence sum calculations, and F $1s$-XPS data clearly indicate that the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions occupy sites within the fluorite block $({\mathrm{Sr}}_{2}{\mathrm{F}}_{2})$ insulating layers. This contrasts with the ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ ($T$-type) structure which is adopted by ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}.$