Recent Results

The well-known high-T_c superconductor, YBa₂Cu₃O₇ (Cu-1212 or so-called "123") accepts various elemental substitutions. Among the M-1212 phases, where the CuO layer of the Cu-1212 phase is replaced by an MO layer, those with M = Ga, Al, or Co have attracted significant interest as in their crystal structures oxygen atoms tetrahedrally coordinate the M atoms to form chains of corner-sharing MO₄ tetrahedra. In this report, we present results of high-resolution transmission electron microscopy (HRTEM) investigation for a whole homologous series of M = Co compounds, i.e. Co-12s2 phases (s=1-3) with compositions of CoSr₂YCu₂O₇, CoSr₂(Y_3/4Ce_1/4)₂Cu₂O₉ and CoSr₂(Y_1/3Ce_2/3)₃Cu₂O₁₁.

The electron diffraction (ED) patterns and HRTEM images were observed using a ultra-high-voltage transmission electron microscope (Hitachi H-1500) operated at an accelerating voltage of 820 kV. The TEM specimens were prepared by a crashing method.

We display in Figs 1 HRTEM images taken along [1-10] for the Co-12 s 2 phases. The fundamental 12s2-type stacking of layers along the c axis is clearly observed in each figure. Figures 2(a) and 2(b) show ED patterns taken along (a) [001] and (b) [1-20] for the Co-1212 phase. The superlattice reflections in Fig. 2(a) showing double periodicity along the b direction are indicative of a regular alternation of two different types of symmetry (L and R) of the CoO₄-tetrahedra chains along the direction. The superspots, (1, 1/2, l) and (3, 3/2, l), in Fig. 2(b) indicate that the two types of chains are nearly ordered along the c direction. We proposed the superstructure model with P2cm (No.28) orthorhombic symmetry and lattice parameters, a_s=a, b_s=2b and c_s=c, which is shown in Fig. 2(c). The superlattice reflections showing b_s=2b were also observed for the Co-1222 and Co-1232 phases. The intralayer ordering of the two types of chains is a general phenomenon for all the three phases. However, for the latter two phases in which an additional fluorite-type layer-block is inserted between two CuO₂ planes, the observation revealed a disordered arrangement of the CoO₄chains along the layer-stacking direction. Fig. 2(d) shows an ED pattern taken along [1-20] and the corresponding HRTEM image for the Co-1232 phase.

Ordering of the two types of chains presumably occurs so that the total lattice energy would be reduced. For all the three phases, the intralayer interaction between the chains is strong because of shortness of the distance between them. This is the reason for the general phenomenon of intralayer ordering of the two types of chains. On the other hand, the distance across the SrO-CuO₂-(Y,Ce)-(O₂-(Y,Ce))_s_-1-CuO₂-SrO block is long and the interlayer interaction thus weak. For the Co-1222 and Co-1232 phases, the interaction is especially weak, which probably causes the lack of interlayer order of the CoO₄ chains.