A study of the upper limit of borate substitution in LnSr₂Cu_3-xB_xO₇

Compositions LnSr(2)Cu(3-x)B(x)O(7) (Ln=Y, La, Nd, Gd, Yb) have been investigated at 1000 degrees C. Almost single phase samples are obtained for Ln=Gd and Y up to x approximate to 0.8. Powder X-ray structure refinement for a nominal YSr2Cu2.25B0.75O7 sample shows the basic cell to be orthorhombic Pmmm; a=3.8176(2) Angstrom, b=3.8345(2) Angstrom, c=10.9708(5) Angstrom, but electron microscopy shows a well-defined 4Cu:1B ordering in the chain sites producing an ax5bx2c supercell. This confirms that the upper limit of borate substitution is n=0.8. The oxygen contents of the x=0.8 materials can be increased slightly by annealing, but they are not superconducting due to low hole concentrations within the copper oxide sheets. (C) 1997 Elsevier Science S.A.