A SEARCH FOR THE HERRINGBONE SUPERSTRUCTURE IN YBA2CU3O6+X

A careful search for the ordering of Cu(1)-O-Cu(1) monomers in a ''her ringbone'' (HB) superstructure with the unit cell 2 root 2a0 x root 2b (0) x c(0) at x=0.32(2) and x=0.37(2) yielded negative results. The up per limit for the partial volume of this phase in a slowly cooled crys tal with x=0.37(2) is estimated as 0.01%. A short-range ordering of ch ain fragments like in Ortho-II phase with an average chain length of t hree oxygen atoms is found instead while keeping the tetragonal symmet ry. The HE superstructure is found in as-grown crystals with x similar to 0.20, but it has not been observed in slowly cooled crystals with x=0.23(3). Possible mechanisms of the HE phase formation are discussed .