A STUDY OF THE UPPER LIMIT OF BORATE SUBSTITUTION IN LNSR(2)CU(3-X)B(X)O(7) MATERIALS

Compositions LnSr(2)Cu(3-x)B(x)O(7) (Ln=Y, La, Nd, Gd, Yb) have been i nvestigated at 1000 degrees C. Almost single phase samples are obtaine d for Ln=Gd and Y up to x approximate to 0.8. Powder X-ray structure r efinement 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 c onfirms that the upper limit of berate substitution is n=0.8. The oxyg en contents of the x=0.8 materials can be increased slightly by anneal ing, but they are not superconducting due to low hole concentrations w ithin the copper oxide sheets. (C) 1997 Elsevier Science S.A.