Orthorhombic-to-tetragonal transition in R1+xBa2-xCu3O7+ (R=Nd,Sm,and Eu)

The orthorhombic-to-tetragonal structural phase transition in the high-Tc superconducting oxides of the type R1+xBa2-xCu3O7+ [R (for rare earth) =Nd,Sm,and Eu] has been investigated using powder x-ray diffraction, dc resistivity, and thermogravimetric techniques. It was found that the orthorhombic-to-tetragonal transition occurs for samples whose nominal stoichiometric content of oxygen is greater than 7.0 (0<<0.3) as compared to less than 7.0 in YBa2Cu3O7-. With increasing [RBa] ratio in R1+xBa2-xCu3O7+, a clear convergence of multiple orthorhombic peaks to a well-defined single tetragonal peak was observed in the x-ray diffraction pattern. The presence of orthorhombic distortion in this system appears to be essential for achieving 90-K superconductivity.