ELECTRONIC-STRUCTURE AND PROPERTIES OF ISOELECTRONICALLY SUBSTITUTED COMPOUNDS Y1BA2-MMMCU3O7 AND Y1BA2-MMMCU4O8 (M=BE, MG, CA, SR, BA, RA)

According to cluster calculations, the electronic structures of compou nds based on Y1Ba2Cu3O7 and Y1Ba2Cu4O8 with isoelectronically substitu ted barium have some qualitative distinctions. These compounds behave differently upon barium substitution by other elements due to differen ces in the character of their highest occupied and lowest unoccupied m olecular orbitals. Substitution of barium by radium is expected to lea d to an increase in oxygen stability without a significant decrease in the critical temperature of superconduction transition T(s). In order to raise T(s), it is of interest to study the systems YBa2-m(Be or Mg )mCu3Ox and YBa2-m(Ca, Sr)mCu3Ox. On partial substitution of barium by calcium in YBa2Cu4O8, the mechanism of T(s) elevation may involve con traction of the forbidden band due to oxygen sublattice distortions in the vicinity of Ba centers.