Superconductors: a dimpled beauty.

Superconductors: A dimpled beauty

X-ray diffraction and neutron diffractionare two standard techniques crystallographers use to determine the structures of crystals. The first neutron diffraction studies of the new high-temperature superconducting materials are now being reported, one by a group working at Argonne (Ill.) National Laboratory, the other by scientists working at the Rutherford Appleton Laboratory in Didcot, England. According to Ivan K. Schuller of Argonne, neutron diffraction is important because it is more sensitive to the precise location of oxygen atoms, and oxygen is an important building block in these materials.

The Argonne neutron diffraction study,which involved the compound YBa2Cu3O7, found a structure that the scientists say is different in important respects from what X-ray diffraction had previously shown. Both the Rutherford Appleton work, which used La1.85Ba0.15CuO4, and an Argonne X-ray diffraction study of the yttrium-barium-copper oxide show that temperature variations can cause changes in the crystalline arrangement between orthorhombic (having three unequal axes at right angles to each other) and tetragonal structures.

Results of previous studies of the yttrium-bariummaterial by X-ray diffraction had differed over whether the structure, which belongs to the class of crystals called perovskites, had tetragonal or orthorhombic symmetry. The neutron diffraction study found an orthorhombic structure. This structure has "something for everybody,' Schuller says. "There are two-dimensional planes, one-dimensional chains--the planes may or may not be coupled depending on the point of view one wishes to adopt.' According to the report, the basic structure consists of planar layers of CuO2 that are joined together by "fencelike' chains of CuO3.

However, this study finds that the CuO2layers are not flat or "corrugated,' as previous reports have held, but "dimpled.' Also the locations of the yttrium and barium ions are different. Says Schuller, ". . . this unique structure is important for the superconductivity, and I would dare to say it has some esthetic value.'

The Argonne report opines that thisstructure could show highly anisotropic electronic properties--that is, properties dependent on direction. In fact, as this work was being prepared for publication, scientists at IBM reported very anisotropic conductivity in related materials (SN: 5/16/87, p.308).

The Argonne X-ray diffraction studyreports that the yttrium-barium material undergoes a transition from orthorhombic to tetragonal structure as it is heated through a temperature of 750 kelvins. If this change is frozen in by quenching and the material cooled toward the superconducting temperature range, the transition temperature for superconductivity is depressed substantially below the 92.5 kelvins previously found for this material. The experimenters say this suggests that the one-dimensional copper oxide chains characteristic of the orthorhombic structure are necessary for high superconducting temperatures.

The Retherford Appleton researchersfound a transition for the lanthanum barium material from tetragonal to orthorhombic at 180 K. They also found "subtle, anomalous structural instabilities,' at lower temperatures. They say their experiments demonstrate a relation between the structural anomalies and changes in electrical resistivity.

The Argonne neutron diffraction study,done by scientists from Argonne, Illinois Institute of Technology in Chicago and Western Michigan University in Kalamazoo, will appear in APPLIED PHYSICS LETTERS. The Argonne X-ray diffraction work, by scientists from Argonne, the University of Leuven, Belgium, and Illinois Institute of Technology will appear in SOLID STATE COMMUNICATIONS. The Rutherford Appleton work, by D. McK. Paul of the University of Warwick in Coventry, England, and others appears in the May 11 PHYSICAL REVIEW LETTERS.