NONSTOICHIOMETRY AND PHASE TRANSITION IN RBa2Cu3O7 - delta (R: Y, Sm, Eu, Gd, Dy, Ho, Er and Yb).

The orthorhombic (O) to tetragonal (T) phase transition in RBa//2Cu//3O//7// minus // delta (R: Y, Sm, Eu, Gd, Dy, Ho, Er and Yb) was investigated by high temperature X-ray powder diffraction and thermogravimetry in air and O//2 gases. The ortho-I phase gradually transforms to the tetragonal phase through the ortho-II phase with increasing temperature, accompanied with increase of oxygen vacancies. No crystallographic distinction between the ortho-I and the ortho-II phase was found. The O-T transition in all systems is of second order and the transition temperature increases with increasing atomic number of R elements.