Phase Transformations in the High-Tc Superconducting Compounds, Ba2RCu3O7-δ (R = Nd, Sm, Gd, Y, Ho, and Er)

The phase transformation between the orthorhombic and tetragonal structures of six high-T c superconductors, Ba2RCu3O7- δ , where R = Nd, Sm, Gd, Y, Ho, and Er, and δ = 0 to 1, has been investigated using techniques of x-ray diffraction, differential thermal analysis/thermogravimetric analysis (DTA/TGA) and electron diffraction. The transformation from the oxygen-rich orthorhombic phase to the oxygen-deficient tetragonal phase involves two orthorhombic phases. A superlattice cell caused by oxygen ordering, with a' = 2a, was observed for materials with smaller ionic radius (Y, Ho, and Er). For the larger lanthanide samples (Nd, Sm, and Gd), the a' = 2a type superlattice cell was not observed. The structural phase transition temperatures, oxygen stoichiometry and characteristics of the T c plateaus appear to correlate with the ionic radius, which varies based on the number of f electrons. Lanthanide elements with a smaller ionic radius stabilize the orthorhombic phase to higher temperatures and lower oxygen content. Also, the superconducting temperature is less sensitive to the oxygen content for materials with smaller ionic radius. The trend of dependence of the phase transformation temperature on ionic radius across the lanthanide series can be explained using a quasi-chemical approximation (QCA) whereby the strain effect plays an important role on the order-disorder transition due to the effect of oxygen content on the CuO chain sites.