Abstract
The partial substitution of small rare-earth RE3+ cations (RE=Y, Ho, Er, Yb) for La3+ in the cuprates La1.9Sr(Ca)1.1Cu2O6 results in ordering phenomena of the A cations in the oxygen-deficient double perovskite–rocksalt P2/RS intergrowth structure. The preference of these RE3+ cations for the cubic-like 8-fold coordination in the middle of the P2 blocks (A1 sites) is the driving mechanism of formation of the solid solutions La1.9−xRExSr(Ca)1.1Cu2O6−δ. The homogeneity range is rather wide in the case of Sr: xmax=0.5 (Yb)→0.8 (Ho), as its extent is much more limited in the case of Ca: xmax=0.15 (Yb)→0.3 (Ho), owing to a competition between RE3+ and Ca2+ for the occupancy of the A1 sites. In the Sr–Ho solid solution, Ho3+ is found both in the A1 and A2 sites (9-fold coordination): this can be understood in terms of a not so large destabilization of the 4f10 electronic structure of Ho3+ in the crystal field of the A2 sites. The thermal decomposition of La1.2Sr1.1Er0.7Cu2O5.95 at 1050°C gives rise to the formation of a T*-type cuprate whose likely composition is La1Sr0.3Er0.7CuO3.85, accompanied by a T-type cuprate and CuO. Such a result, never obtained up to now, exemplifies the strong geometrical similarities between the A–O polyhedral networks of the P2/RS and T* structures, namely: A[P2/RS]≡A[T*]+12A[T].
Published Version
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