Local structure of Y_1-xCa_xSr₂Cu₂GaO₇ superconductors

The variation in the average and local structure of the superconductor family Y_1-xCa_xSr₂Cu₂GaO₇ as calcium is substituted for yttrium (x = 0.20, 0.35, and 0.40) is presented and discussed. Electron microscopy and X-ray diffraction were the primary techniques employed. The local structure found by the use of electron microscopy and X-ray diffraction agrees with that previously derived from single crystal X-ray and neutron diffraction. The gallate can be confused with the three layer perovskite structure of YBCO (YBa₂Cu₃O₇) because of the similarity of [011]_Ga≈[100]_YBCO and [010]_Ga≈[110]_YBCO zones. However, the composition of all crystals examined were homogeneous on a 10-100 nm scale. It was also found that the twin structure varied as a function of the calcium content.