A Thermogravimetric Study of the Influence of Internal Stresses on Oxygen Variations in Ln_2-xCe_xCuO₄

The variation with temperature of oxygen content in the electron-doped superconductors Ln_2-xCe_xCuO₄ (Ln = lanthanide) have been investigated systematically by thermogravimetric analysis. The tendency to lose oxygen generally decreases with increasing Ce content or decreasing Ln³⁺ size due to a decreasing degree of internal stresses resulting from an improved bond length matching between the (Ln, Ce)-O and Cu-O bonds; this is supported by bond valence sum calculations. With smaller a lattice parameters as in Gd_2-xCe_xCuO₄, however, an increasing electrostatic repulsion between the interstitial octahedral-site and the normal tetrahedral-site oxygen atoms forces the excess interstitial oxygen atoms to be lost at lower temperatures. It also causes the oxygen loss to occur in two steps, one corresponding to the interstitial oxygen and the other to the tetrahedral-site oxygen atoms. In contrast, the oxygen loss occurs smoothly in one step for larger a lattice parameters as in LaNd_1-xCe_xCuO₄. © 1995 Academic Press. All rights reserved.