Structural phase transformations and high-T/sub c/ superconductivity

The recent discoveries of T/sub c/ ≅ 40K (much higher T/sub c/'s are rumored) in the La2CuO4-type systems raise questions concerning the importance of lattice instabilities to superconductivity in these systems. High resolution synchrotron x-ray powder diffraction measurements were performed on the superconductor La/sub 1.8/Ba/sub 0.2/CuO4 which show unusual temperature dependent broadening of certain Bragg peaks. This pattern of peak broadening can be consistently explained as an unresolved splitting due to a spontaneous reduction of the tetragonal symmetry of the high temperature structure to a lower monoclinic (or possibly triclinic) form. In order to confirm this interpretation of the powder diffraction data, the size of the highly collimated synchrotron x-ray beam was reduced and individual ≅ 70 μm single crystals were isolated and studied within the powder aggregate