| Popis: |
Thermogravimetric analysis has revealed that reduction of Sr2CuO2F2+δ in 10% H2/90% N2, occurs in two distinct steps on heating up to 930 °C. Whereas reduction to give SrF2, 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 (I4/mmm) phase, Sr2CuO2F2, with unit-cell dimensions a = 3.967(1) Å and c= 12.816(2) Å. The structure has been determined from powder x-ray-diffraction data and is related to Nd2CuO4 (T′'-type). Madelung energy, bond valence sum calculations, and F 1 s-XPS data clearly indicate that the F- ions occupy sites within the fluorite block (Sr2F2) insulating layers. This contrasts with the La2CuO4 (T-type) structure which is adopted by Sr2CuO2F2+δ-. |
